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10.1371/journal.pone.0308141
PONE-D-24-08235
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No evidence for population-level benefits of polyandry in sharks and rays
evolution of polyandry in elasmobranchs
https://orcid.org/0000-0002-0839-3940
Gayford Joel H. Conceptualization Data curation Formal analysis Investigation Methodology Writing – original draft Writing – review & editing 1 2 *
Flores-Flores Estefanía M. Data curation Writing – original draft Writing – review & editing 2
1 Department of Life Sciences, Imperial College London, London, United Kingdom
2 Shark Measurements, London, United Kingdom
Schlatt Stefan Editor
University Hospital of Münster, GERMANY
Competing Interests: The authors have declared that no competing interests exist.

* E-mail: jhg19@ic.ac.uk
4 9 2024
2024
19 9 e03081411 3 2024
17 7 2024
© 2024 Gayford, Flores-Flores
2024
Gayford, Flores-Flores
https://creativecommons.org/licenses/by/4.0/ This is an open access article distributed under the terms of the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited.

Mating system variation refers to the spectrum between genetic monogamy and polyandry, and has important consequences for sexual conflict, sexual selection and individual fitness in animals. Theoretically this variation could also have substantial population-level effects, influencing population viability and extinction risk. Evidence for these effects is mixed, in part due to the fact that substantial environmental change is thought to be required for them to have visible demographic consequences. In this study we test for the presence of relationships between polyandry and population status in Elasmobranchii (sharks and rays). Elasmobranchii is a large vertebrate clade that exhibits substantial interspecific variation in both genetic mating system and population status, as well as being subject to intense anthropogenically-mediated environmental change. We also predict past macroevolutionary shifts in genetic mating system through elasmobranch phylogeny. Our results show that both genetic monogamy and polyandry have evolved multiple times independently within Elasmobranchii, and we suggest that both of these extremes represent alternative adaptive strategies that are favoured under discrete ecological and biological conditions. Nevertheless, there is no evidence of population-level consequences of mating system variation in elasmobranchs. These results are significant as they suggest that mating system variation in this clade is unlikely to be a major determinant of extinction vulnerability. Ultimately additional work will be required, however this study improves our understanding of the evolutionary dynamics underlying mating system variation in elasmobranchs, and the potential for resultant population-level consequences.

The author(s) received no specific funding for this work. Data AvailabilityAll relevant data are within the manuscript and its Supporting Information files.
Data Availability

All relevant data are within the manuscript and its Supporting Information files.
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pmcIntroduction

A wide spectrum of genetic mating systems is observed in the sexually reproducing organisms, ranging from strict genetic monogamy (where only one male contributes genetic material to a brood) to widespread polyandry [1–3]. Polyandry refers to the scenario in which multiple males contribute genetic material to a brood and is observed to varying extents in most major vertebrate radiations [2, 4–6]. Whilst the genetic mating systems of diverse taxa have been studied in detail, there remains substantial controversy regarding both the causes and consequences of variation in vertebrate mating systems [7–9]. The elevated fitness cost of sexual reproduction for females, resulting from increased sexual conflict and selection for reduced male care is broadly accepted (reviewed in [10]). However, increasingly studies suggest that polyandry may convey direct or indirect fitness benefits to females, and the conditions under which polyandry should incur net fitness costs or benefits to females remain poorly understood [10, 11]. Hypothesised benefits of polyandry to females include reduced risk of genetic incompatibility from selfish genetic elements, increased fecundity, increased offspring genetic variation (and hence resilience to environmental change) and increased offspring viability [11].

The hypothesised consequences of monogamy and polyandry extend beyond individual fitness to population viability and extinction risk [10]. This is because polyandry results in greater sexual conflict between males and females [10, 11]. Under different conditions this elevated sexual conflict can either weaken or enhance sexual selection, with consequences for mutation load, the rate of adaptive evolution and the balance between natural selection and genetic drift, all of which are important determinants of extinction risk [10]. Consequently, there may be no universally superior genetic mating system, with polyandry and monogamy potentiall representing alternative adaptive strategies that are beneficial in different ecological contexts [12–14]. Presently, demographic consequences of mating system variation have been difficult to study as their detection requires some degree of environmental change that has a strong effect on birth and deaths rates within the population in question [10]. Consequently, improving our understanding of genetic mating system variation is not only of great eco-evolutionary interest, but provides material benefit from a conservation and management perspective.

Elasmobranchii (sharks and rays) is a large, ancient vertebrate radiation renowned for its high diversity of reproductive modes [15, 16]. Whilst most vertebrate clades exhibit either oviparity or placental viviparity, both of these strategies are abundant within Elasmobranchii, as well as various other forms of viviparity including histrotrophy and yolk-based viviparity [15, 17]. This diversity of reproductive modes has important consequences for sexual selection and sexual conflict, which in elasmobranchs can manifest as phenomena such as oophagy, coercive mating and intrauterine cannibalism [18–20]. Both genetic monogamy and polyandry have been reported in a number of elasmobranch taxa [21, 22], stimulating debate as to the underlying evolutionary mechanisms [21, 22]. Convenience polyandry (where the costs of resistance for females outweigh the costs of mating) is often assumed to underlie polyandry in this clade [22], whereas recent studies suggest that additional factors such as female choice and male-male competition are important [21]. However, to date only two studies have investigated polyandry in elasmobranchs in an evolutionary context [4, 23], modelling mating system variation as a single binary trait (multiple paternity or monogomy) which doesn’t represent the full extent of variation observed in nature. Moreover, the potential population-level consequences of mating system variation in elasmobranchs remains entirely unstudied, which is notable given the concerning conservation status of many shark and ray species [24, 25].

In this study, we utilise data drawn from the literature to investigate the evolutionary history of polyandry in elasmobranchs, and test for signals of potential population-level consequences of mating system variation in a phylogenetic context. Elasmobranchii represents an ideal case study in this context given the extent of variation in mating systems that is observed in extant taxa (representing a spectrum between strict genetic monogamy and polyandry). Thus our results will not only provide additional context regarding the origins, causes and consequences of polyandry and genetic monogamy in sharks and rays, but improve our understanding of mating system variation more broadly. This context is of critical importance given the uncertainty regarding relationships between genetic mating system and extinction risk, and the impending anthropogenic extinction crisis.

Methodology

Ethics statement

No experimental work of any kind, working with either live or dead animals was carried out during this study, and consequently no ethics approval was required.

Data collection

We collected data regarding polyandry (the frequency of multiple paternity, henceforth MP frequency) from existing literature, resulting in a dataset of 40 species (Table 1). MP frequency was defined as the percentage of litters fathered by more than one sire [2]. Incorporating a quantitative value of MP is hugely beneficial, particularly in evolutionary analyses, as it allows for species that exhibit intermediate mating systems and is less sensitive to error resulting from low sample sizes.

10.1371/journal.pone.0308141.t001 Table 1 MP frequency and presence/absence of polyandry in extant elasmobranch species.

Species	Genetic polyandry?	MP frequency (%; averaged)	Data source	
Raja clavata	Yes	100	Lamarca et al., 2020 [4]	
Pristis pectinata	No	0	Lamarca et al., 2020 [4]	
Urobatis halleri	Yes	90	Lamarca et al., 2020 [4]	
Potamotrygon leopoldi	Yes	49.7	Torres et al., 2022 [31]	
Aetobatus narinari	Yes	100	Lamarca et al., 2020 [4]	
Hexanchus griseus	Yes	100	Lamarca et al., 2020 [4]	
Pristiophorus nudipinnis	Yes	100	Nevatte et al., 2023 [32]	
Pristiophorus cirratus	Yes	100	Nevatte et al., 2023 [32]	
Squalus mitsukurii	Yes	11.1	Lamarca et al., 2020 [4]	
Squalus acanthias	Yes	20.5	Lamarca et al., 2020 [4]	
Etmopterus molleri	No	0	Lamarca et al., 2020 [4]	
Etmopterus spinax	Yes	6.5	Lamarca et al., 2020 [4]	
Rhincodon typus	No	0	Lamarca et al., 2020 [4]	
Ginglymostoma cirratum	Yes	66.7	Lamarca et al., 2020 [4]	
Scyliorhinus canicula	Yes	92.3	Lamarca et al., 2020 [4]	
Galeocerdo cuvier	No	0	Lamarca et al., 2020 [4]	
Sphyrna lewini	Yes	61.1	Lamarca et al. 2020 [4]	
Sphyrna tiburo	Yes	18.8	Lamarca et al. 2020 [4]	
Carcharhinus altimus	Yes	100	Lamarca et al. 2020 [4]	
Carcharhinus plumbeus	Yes	65.1	Lamarca et al. 2020 [4]	
Carcharhinus leucas	Yes	67.4	Lamarca et al. 2020 [4]	
Carcharhinus acronotus	Yes	74	Lamarca et al. 2020 [4]	
Carcharhinus isodon	Yes	83.8	Nash et al. 2021 [33]	
Carcharhinus amblyrhynchos	Yes	66.7	Lamarca et al. 2020 [4]	
Prionace glauca	Yes	91.5	Armada‐Tapia et al. 2023 [34]	
Carcharhinus obscurus	Yes	35.7	Lamarca et al. 2020 [4]	
Carcharhinus galapagensis	No	0	Lamarca et al. 2020 [4]	
Negaprion acutidens	Yes	77.8	Lamarca et al. 2020 [4]	
Negaprion brevirostris	Yes	84.4	Lamarca et al. 2020 [4]	
Galeorhinus galeus	Yes	40	Lamarca et al. 2020 [4]	
Triakis semifasciata	Yes	36.4	Lamarca et al. 2020 [4]	
Mustelus henlei	Yes	63.8	Lamarca et al. 2020; Rendón-Herrera et al. 2022 [4, 35]	
Mustelus californicus	No	0	Tárula-Marín and Saavedra-Sotelo. 2021 [36]	
Mustelus mustelus	Yes	50	Lamarca et al. 2020 [4]	
Mustelus punctulatus	Yes	53.8	Lamarca et al. 2020 [4]	
Mustelus asterias	Yes	58.3	Lamarca et al. 2020 [4]	
Mustelus antarcticus	Yes	24.1	Lamarca et al. 2020 [4]	
Mustelus lenticulatus	Yes	15.8	Lamarca et al. 2020 [4]	
Carcharias taurus	Yes	42.9	Lamarca et al. 2020 [4]	
Isurus oxyrinchus	Yes	83.3	Liu et al. 2020 [37]	

Where multiple studies have investigated polyandry in a single species (e.g. Mustelus henlei), MP frequency was averaged and weighted by sample size to provide a single measure per taxon. It is important to recognise that MP values could differ substantially between different populations of the same species, and thus averaged values may not be representative of the true genetic mating system of a given population. However in light of the available data, and the fact that our chosen proxies for extinction risk are also averaged across populations, we assert that this approach is suitable for making interspecific comparisons.

Quantifying individual evolutionary fitness or lifetime reproductive success in elasmobranchs (let alone averaging across populations) is notoriously challenging due to their size, life history parameters and ecology [26, 27]. For this reason, we collated readily available ecological/biological data that could serve as proxies for the average population status of elasmobranch species. Specifically we collected the following information: minimum and maximum litter size (Sharks of the World: a Complete Guide [28]), conservation status (IUCN [29]), ‘lifetime reproductive output’ [21], generation time (FishBase [30]), average fecundity (FishBase [30]), vulnerability to fishing (FishBase [30]), vulnerability to climate (FishBase [30]). If polyandry does indeed convey significant population-level benefits in elasmobranchs, we might (see discussion for caveats) expect these parameters to vary systematically with MP frequency. All data can be found in the supplementary materials associated with this article.

Phylogenetic data (a set of 10,000 phylogenies including branch lengths and interrelationships) were extracted from Stein et al. [25], and a pruned, maximum clade credibility (MCC) phylogeny was produced to match our dataset using the packages picante and phangorn [38, 39].

Data analysis

All data analysis was carried out in R4.0.4 [40]. Prior to data analysis all quantitative variables were scaled.

To provide insight into the evolutionary dynamics underlying interspecific variation in MP frequency, we fit 3 evolutionary models to our data using the default parameters of the function fitContinuous in the package geiger [41]. Each of these models assumed a different model of trait evolution (Brownian motion, Ornstein Uhlenbeck and Early burst), and the model of best fit was selected on the basis of AIC values, where ΔAIC>2 indicates significant model support.

To test for potential population-level benefits of polyandry, we fit a series of phylogenetic generalised linear models using the package phylolm [42], with MP frequency as the predictor variable. These models can incorporate both continuous and discrete variables, and account for phylogenetic non-independence between the data [42]. Each model included one potential covariate, and assumed the model of trait evolution best supported by the previous analysis (see results for details). Importantly not all covariate data could be collected for all species, and thus each of these models differs in sample size and cannot be directly compared on the basis of AIC values.

Finally, to estimate evolutionary shifts in MP frequency through elasmobranch phylogeny, we performed ancestral state reconstruction of MP frequency using the package phytools [43].

Results

Comparison of Brownian motion (BM), Ornstein Uhlenbeck (OU) and Early burst (EB) models of trait evolution revealed that the phylogenetic distribution of MP frequency values is best explained by an OU model (Table 2). The EB model was the worst-performing of the three models (Table 2).

10.1371/journal.pone.0308141.t002 Table 2 AIC values for models of trait evolution fit to MP frequency data.

Model of trait evolution	AIC	ΔAIC	
Brownian motion (BM)	137.8	-	
Ornstein Uhlenbeck (OU)	117.2	-20.6	
Early burst (EB)	139.8	2.00	

None of the nine phylogenetic linear models fitted produced evidence of significant correlation between MP frequency and biological/ecological proxies of population status (Table 3).

10.1371/journal.pone.0308141.t003 Table 3 Output of phylogenetic linear models demonstrates the absence of any significant relationships between MP frequency and potential covariates.

Note that AIC values should not be used to compare directly between these models as they differ in sample size.

Covariate	n	Gradient	α	σ2	P value	AIC	
Conservation status	41	-2.92	0.01	20.68	0.46	392.9	
Lifetime reproductive output	29	-0.12	0.06	0.08	0.55	89.45	
Generation time	19	-0.17	0.06	0.10	0.48	61.82	
Minimum litter size	35	0.29	0.01	0.02	0.12	102.4	
Maximum litter size	35	0.17	8.77e-03	0.01	0.30	103.9	
Fecundity	35	0.15	0.03	0.06	0.39	105.7	
Resilience	41	0.25	9.28e-03	0.01	0.08	116.0	
Fishing vulnerability	41	0.24	0.01	0.02	0.13	116.3	
Climate vulnerability	19	0.16	0.19	3.54e-06	0.51	62.39	

Ancestral state reconstruction of MP frequency suggests that basal elasmobranchs displayed intermediate levels of genetic polyandry (MP frequency at ancestral node was 59.6%), although support for this result was extremely weak and the confidence interval overlapped both 0 and 100. This dataset is likely of insufficient size to provide a robust estimate of the ancestral state. Nevertheless, this analysis did provide valuable insight into subsequent evolutionary shifts in polyandry, suggesting that both extremely high and low MP frequency values have evolved multiple times independently (Fig 1). There appear to be at least six independent transitions from polyandry to genetic monogamy, and even more cases of high MP frequency evolving from ancestrally intermediate MP frequency values (Fig 1).

10.1371/journal.pone.0308141.g001 Fig 1 Ancestral state reconstruction of MP frequency through elasmobranch phylogeny.

Red coloration represents genetic monogamy (low values of MP frequency), blue coloration represents polyandry (high MP frequency values) and green coloration represents intermediate MP frequency values.

Discussion

Population-level consequences of polyandry

Despite numerous predicted consequences for population fitness, we found no association between the extent of polyandry (MP frequency) and various measures of population status (Table 3). Assuming a positive relationship between polyandry and the strength of sexual selection (a link that is still somewhat controversial), polyandry could prove beneficial either by increasing genetic variation or by reducing the reproductive success of ‘inferior’ males, thus purging less favourable mutations from the population over time [10, 44, 45]. Moreover, if sexual selection happens to favour similar trait optima to natural selection, polyandry could enhance the rate of adaptive evolution [10]. However, the potential for sexual selection to oppose rather than enhance natural selection is well known [46, 47], and thus depending on the adaptive landscape of the male and female traits in question, polyandry could either increase or retard the pace of adaptation. Whilst there is some empirical evidence for population-level consequences of polyandry, most of these putative links do not have visible demographic consequences, as to have noticeable impact on genetic diversity or population size they require birth/death rates to be modified by environmental change [10, 48, 49].

This should provide no barrier to the detection of population level consequences of polyandry in elasmobranchs however–most of the species included in this study are subject to intense anthropogenic fishing pressure [24, 50], a relatively new selective pressure evolutionarily speaking that would undoubtedly provide the environmental change necessary to detect demographic effects. Even so, we failed to find any evidence of population-level consequences of genetic mating system variation (Table 3). Assuming the data used in this study are sufficient to detect hypothetical population-level consequences (see below for caveats associated with this), this leaves two potential explanations: either mating system variation in elasmobranchs genuinely has no noticeable effect on population status or extinction risk, or there appears to be no noticeable effect due to costs and benefits of polyandry that are approximately equal in magnitude. The first of these scenarios is unlikely–there is evidence to suggest that genetic monogamy in sharks may reduce genetic variation and effective population sizes [22, 51, 52], and negative fitness consequences of unwanted mating attempts have been documented in multiple species [20, 53, 54]. Reduced genetic variation is likely of great significance to elasmobranch taxa, which exhibit slower rates of molecular evolution than other vertebrates [55, 56]. Thus, we suggest that mongogamy/polyandry in sharks may not exhibit population-level consequences as both endpoints of the mating system spectrum convey costs and benefits that are broadly equivalent in terms of their impacts on extinction risk.

The evolution of mating system variation in elasmobranchs

Whilst we did not directly test any of the hypothesised adaptive explanations for mating system variation in elasmobranchs, our results do provide insight into the evolutionary dynamics underlying macroevolutionary shifts in MP frequency. Only one previous study has investigated polyandry in elasmobranchs in a phylogenetic context, finding that polyandry was likely ancestral to the clade [4]. Whilst this was an important initial step, the approach taken by Lamarca et al. [4] was limited by the use of a binary trait to describe the presence or absence of polyandry. This clearly represents an oversimplification of the true diversity of genetic mating systems observed in elasmobranchs, as evidenced by the variation in MP frequency in extant species (Table 1). Instead using MP frequency (a continuous variable, more representative of variation in genetic mating systems) we did not find statistically robust evidence for polyandry representing the ancestral state in elasmobranchs. This does not imply that genetic monogamy was the ancestral state, but simply that existing data are insufficient to provide robust estimation of the ancestral genetic mating system in elasmobranchs. Despite this uncertainty, there is clear evidence that both very high (>80%) and very low (<20%) levels of polyandry have evolved multiple times independently (Fig 1). This leads us to suggest that both genetic monogamy and polyandry represents alternative adaptive mating systems in Elasmobranchii (as previously suggested for other clades, see [14], each of which may be favoured under different biological and ecological conditions. Our finding that the phylogenetic distribution of MP frequency is best explained by an Ornstein-Uhlenbeck model of trait evolution (Table 2) provides further support for this, implying that MP frequency in elasmobranchs is evolving towards at least one adaptive peak [57]. As for what selective pressures could be underlying these adaptive peaks, further work is needed, as consensus is yet to be reached regarding the adaptive basis of either genetic monogamy or polyandry in elasmobranchs [4, 21, 22].

Limitations

We recognise that there are several fundamental limitations underlying the analyses presented in this study, and our understanding of mating system variation in elasmobranchs more broadly. The need for increased taxonomic coverage of mating system studies in elasmobranchs is clear, with several major clades entirely data deficient [4] and the potential for alternative mating strategies such as polygyny entirely overlooked. This paucity of data not only restricts our understanding of the mating system variation in extant species, but reduces the robustness of comparative phylogenetic analyses such as ancestral state reconstruction. Most pertinent to the topic of this study, we are unable to categorically rule out the possibility of population-level consequences of polyandry or monogamy despite finding no evidence of such consequences (Table 3). We incorporated only one set of data per species, when in reality there is substantial regional variation in both reproductive biology and population status in many elasmobranch species [29, 58, 59]. Furthermore, differences in average population status between species are driven by a number of additional factors including life history traits and applied fishing pressure [60, 61], meaning that even if relationships between mating system and extinction risk existed, they may be cryptic and difficult to detect. Nonetheless, we find that on the basis of current data there is no substantial evidence for such relationships (Table 3). Thus, even if cryptic relationships between mating system and population status do exist, they are unlikely to be significant determinants of extinction vulnerability in extant elasmobranch species.

Conclusions

Polyandry and monogamy clearly both have important consequences for individual fitness, in the case of both males and females [7, 62]. Despite theoretical predictions, it remains unclear whether mating system variation has substantial population-level effects however [10]. In Elasmobranchii, a large vertebrate radiation exhibiting substantial variation in genetic mating system, there is no evidence for such effects (Table 3). Both genetic monogamy and polyandry have evolved multiple times independently from an ancestor of unknown mating system, and there is evidence that the variation observed in extant elasmobranch species is adaptive (Fig 1 and Table 2). Nevertheless, the specific conditions under which genetic monogamy and polyandry are favoured remain uncertain and may be taxon specific.

Supporting information

S1 Table Data underlying the analyses and results of this article.

(CSV)

10.1371/journal.pone.0308141.r001
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Schlatt Stefan Academic Editor
© 2024 Stefan Schlatt
2024
Stefan Schlatt
https://creativecommons.org/licenses/by/4.0/ This is an open access article distributed under the terms of the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited.
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PONE-D-24-08235No evidence for population-level benefits of polyandry in sharks and raysPLOS ONE

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The authors have compiled an interesting study on the role of polyandry in sharks and rays. I agree with both reviewers that the study is of value but has also significant limitations. Both reviewers make constructive comments and recommendations for revision. I am optimistic that after point-by-point review and appropriate revision of the manuscript, it will be a useful reference and acceptable for publication.

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Reviewer #1: The authors present an MS where they evaluated the possible benefits of polyandry at the population level in elasmobranchs. In general, it is an interesting article, however, currently, there is a consensus that a mating system is not necessarily better than others and the costs and benefits they offer are circumstantial. I mentioned this because the authors seem to part from this idea. I recommend reading some articles on this matter: Karl 2008 (DOI 10.1111/j.1365-294X.2008.03902.x), Lotterhos 2011 (DOI 10.1111/j.1558-5646.2011.01249.x), and Stephens & Sutherland 2000 (Chapter 9 in the book Vertebrate Mating Systems).

I think that the methodology to abord this idea is adequate, in this sense, the article could give more emphasis to the multi-model approach they use to put this idea to test.

What I do consider worth exploring in the discussion in more depth is how the benefits of polyandry and monogamy together can positively influence populations.

To help the authors, I will try to give comments throughout the document in such a way that it can be useful to improve the MS.

Abstract

Lines 27-28. In this sentence, authors should specify this in the elasmobranch group, I suppose that’s what they mean.

Line 30. This is plural, those are evidence to each effect.

Lines 32-35. This sentence is so complex, that it has a justification and objective, this became too confusing for an abstract. I suggest restructuring in two sentences.

Lines 37-41. I suggest restructuring this sentence because it sounds contradictory to the objective in the abstract. Also, it sounds a little catastrophic when you said We fail! I recommend this…. Results showed that both genetic monogamy and polyandry have evolved multiple times independently….and in biological conditions.

Lines 39-41 and 42-43. I think that this idea has been discussed in some articles. Karl in 2008 debated this idea and he demonstrated that monogamy is not always a maladaptive strategy. For its part, Lotterhos 2011 also refuted this idea with a model, which showed that any adverse effect derived from polyandry is attenuated by long generation times. Kvarnemo's 2018 observations about monogamy suggest that this cannot be a maladaptive strategy since it is present in many groups, even if its benefits are not in sight.

Introduction

Line 56. “rampant polyandry” how can I interpret it?

Line 62. Add in parentheses, revised in 10. Because it is a single cite of information in this sentence and it is controversial.

Line 65. I have a conflict with these cites (10-11). Although one of them is proposed for the animal kingdom, in general, it is necessary to think about the peculiarities of the elasmobranch group. My main conflict is in the citation of Simmons 2001 since he bases his arguments on insects. Please, you need to consider this for vertebrates and, particularly in elasmobranchs.

Line 85. What do you mean by “unwanted mating attempts”?

Lines 91-94. You need to cite this sentence.

Lines 101-103. This sentence reflects that there are many mating systems and not just polyandry and monogamy, what would those be? The authors assume that readers understand the limitations of mating system studies in elasmobranchs, it seems to me not necessarily. By this, I mean that polygyny is very feasible in this group, although it is extremely difficult to verify.

Methodology

Lines 120-121. I agree that there is no best form to get a value of MP frequency for a species, however, it cannot reflect the actual mating system in species that have geographical variations, or seasonal variation. Also, the sample size is an important factor influencing the MP frequency in different studies for the same species, along with the resolution power offered by the different molecular tools employed. I suggest adding this in the limitations section, or better, stating those understandable limitations right after the methods.

Lines 123-125. Assuming this, authors need to discuss it in species with geographical variation in the MP.

Lines 131-133. I invite to authors to read the articles before mentioned. I consider that it is not reflected in the introduction.

Results

Table 2. This table is unnecessary. I suggest sending it to supplementary materials and citing it in this part.

Lines 187-189. I’m not familiar with the analysis, however, looking at the tree, I think the analyses give a lot of weight to the MP value. I’m concerned that in some cases, such as M. californicus, the sample size is too low to consider real genetic monogamy, since a particularity of this work is possibly collecting small females with a low probability of observing MP. In this case, It can be an effect of the sample size.

Discussion

Lines 206-209. These ideas should be expressed in the abstract and of course in the introduction.

Lines 220-222. I’m not sure if this is correct according to the abstract of Dibattista et al. 2008. In this abstract, they said…” We find that offspring from polyandrous litters did not have a greater genetic diversity or greater survival than did the offspring of monandrous litters”. This sentence is contrary evidence.

Lines 225-228. This idea is correct and is the best of all paper. This idea has been discussed. I suggest to the authors redirect the justification of their article considering their methodology.

Lines 239-241. Using the average MP in a species cannot reflect the real mating system.

Lines 246-248. Please, read to Stephens & Sutherland 2000, they had already discussed it and explained the Allee effect in this context.

Lines 248-251. I do not agree. Since the model was not significant, I think the recurrent MP is not necessarily one adaptive peak. In this sense, this sentence is contradictory to the last sentence. Also, we need to consider that there are few studies to evaluate the mating system in the Elasmobranchii clade, thus this apparently adaptive peak can be skewed due to limited publications.

Limitations

Lines 264-266. Excellent! Authors need to moderate some previous sentences according to this section.

Conclusions

Lines 282-283. I think this is not demonstrated with your results.

Reviewer #2: The potential link between multiple paternity, sexual selection and population viability is an important and general one, and I agree with the authors that this taxonomic group is a potentially useful model system in which to study it. That being said, I found it a little difficult to know what to conclude from this study. In principle I think it is important that negative findings are published, but I do have concerns as to whether the study really had sufficient power to test the hypothesis in question. To be fair to the authors, they are quite open about the limitations of the dataset in their discussion, so on balance I think I would be in favour, but first would like to see several more minor points addressed:

Line 117: The data are presented as coming form the existing literature, but ca. 80% come from a single source. For that reason, I think it would be appropriate to present already in the Introduction the potential overlaps and novel aspects relative to this previous study.

143: It’s unclear to me why conservation status was coded quantitatively, rather than adopt a different analysis approach for this variable? Linear regression doesn’t feel like the most appropriate approach here.

173-177: Given all of these “relationships” are non-significant, I don’t think it is very meaningful to discuss which slopes are positive and which are negative; all you can reliably conclude is that none could be distinguished from zero.

200: on the face of it, purging would tend to reduce genetic diversity rather than increasing it - maybe take a bit more space to explain this point?

219: I understand you wished to use these investigated variables as proxies for extinction risk, but given you only look at extant species in this analysis I think this might be slightly overstated here.

244: the argument that both very high and very low polyandry evolves frequently in this group seems to support the approach of Lamarca et al.? I do think there is a real risk that using only one estimate of MP per species gives a falsely precise impression, since this presumably varies a lot from population to population. To be fair, this point is acknowledged later in the discussion, but is there scope to quantify this to give an idea about the scale of the problem? for how many species did you average across multiple populations?

Table 3: please provide per analysis sample sizes, rather than just saying these varied. I also saw no sign that you accounted for multiple hypothesis testing here - these are essentially 9 different tests of the same underlying hypothesis, so there ought to be some control of the global false positive rate. Given all 9 are non-significant this doesn’t really matter in practice, but in principle it’s an issue that could at least be acknowledged.

A few small typos/errors to correct:

36: through not though

56: for me rampant has potentially negative connotations, consider replacing with something more neutral (widespread?)

88: cost *of* mating

113: consequently

118: reference formatting

130: reference formatting

153: series *of*

259: missing word?

**********

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Reviewer #1: Yes: Nancy C. Saavedra-Sotelo

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10.1371/journal.pone.0308141.r002
Author response to Decision Letter 0
Submission Version1
11 Jun 2024

We thank the editor and reviewer for their time spent on this manuscript. We are very grateful for both of your comments, and believe that they have greatly improved the quality of the paper. Please find detailed responses to comments below. We hope that you find the changes made sufficient to consider acceptance of the manuscript, as we have to the best of our abilities incorporated all changes suggested by the reviewer into the text.

Reviewer 1

Comment: The authors present an MS where they evaluated the possible benefits of polyandry at the population level in elasmobranchs. In general, it is an interesting article, however, currently, there is a consensus that a mating system is not necessarily better than others and the costs and benefits they offer are circumstantial. I mentioned this because the authors seem to part from this idea. I recommend reading some articles on this matter: Karl 2008 (DOI 10.1111/j.1365-294X.2008.03902.x), Lotterhos 2011 (DOI 10.1111/j.1558-5646.2011.01249.x), and Stephens & Sutherland 2000 (Chapter 9 in the book Vertebrate Mating Systems).

I think that the methodology to abord this idea is adequate, in this sense, the article could give more emphasis to the multi-model approach they use to put this idea to test.

What I do consider worth exploring in the discussion in more depth is how the benefits of polyandry and monogamy together can positively influence populations.

Response: Thank you for your comment. We have read the papers you suggest and cited them where relevant in the manuscript.

Comment: Lines 27-28. In this sentence, authors should specify this in the elasmobranch group, I suppose that’s what they mean.

Response: Thank you for your comment. This sentence is general, and is referring to the entire animal kingdom. Several papers (including reference 10 in our list) explain how the theoretical consequences of mating system variation are applicable across animal life, and that the biological contexts that could result in different benefits or costs to polyandry or monogamy can occur independent of phylogeny.

Comment: Line 30. This is plural, those are evidence to each effect.

Response: Thank you for your comment. The word ‘these’ used in the original manuscript is plural and correct in this context and thus we have decided to keep it.

Comment: Lines 32-35. This sentence is so complex, that it has a justification and objective, this became too confusing for an abstract. I suggest restructuring in two sentences.

Response: Thank you for your comment. The requested change has been made.

Comment: Lines 37-41. I suggest restructuring this sentence because it sounds contradictory to the objective in the abstract. Also, it sounds a little catastrophic when you said We fail! I recommend this…. Results showed that both genetic monogamy and polyandry have evolved multiple times independently….and in biological conditions.

Response: Thank you for your comment. The requested change has been made.

Comment: Lines 39-41 and 42-43. I think that this idea has been discussed in some articles. Karl in 2008 debated this idea and he demonstrated that monogamy is not always a maladaptive strategy. For its part, Lotterhos 2011 also refuted this idea with a model, which showed that any adverse effect derived from polyandry is attenuated by long generation times. Kvarnemo's 2018 observations about monogamy suggest that this cannot be a maladaptive strategy since it is present in many groups, even if its benefits are not in sight.

Response: Thank you for your comment. We have added additional text in lines 81-86 and cited the requested references.

Comment: Line 56. “rampant polyandry” how can I interpret it?

Response: Thank you for your response. We have replaced the word rampant with widespread.

Comment: Line 62. Add in parentheses, revised in 10. Because it is a single cite of information in this sentence and it is controversial.

Response: Thank you for your comment. The requested change has been made.

Comment: Line 65. I have a conflict with these cites (10-11). Although one of them is proposed for the animal kingdom, in general, it is necessary to think about the peculiarities of the elasmobranch group. My main conflict is in the citation of Simmons 2001 since he bases his arguments on insects. Please, you need to consider this for vertebrates and, particularly in elasmobranchs.

Response: Thank you for your comment. For the purpose of this sentence, we do not see that any specific reference to elasmobranchs is required. Our understanding of the net fitness consequences of polyandry are poor across the animal kingdom, and the specific sentence you query was not talking only about elasmobranchs, but about our understanding of polyandry across animal life. The studies we cite are some of the only empirical tests of hypotheses concerning net fitness consequences, so when making a general statement such as “understanding of the net fitness consequences of polyandry are poor across the animal kingdom” they provide ideal references. We do go on to discuss later specific knowledge regarding elasmobranchs, but we do not believe that there is any issue with including these studies given the general nature of the statement.

Comment: Line 85. What do you mean by “unwanted mating attempts”?

Response: Thank you for your comment. We have clarified this in the text by replacing unwanted mating attempts with coercive mating

Comment: Lines 91-94. You need to cite this sentence.

Response: Thank you for your comment. We have made the change as requested.

Comment: Lines 101-103. This sentence reflects that there are many mating systems and not just polyandry and monogamy, what would those be? The authors assume that readers understand the limitations of mating system studies in elasmobranchs, it seems to me not necessarily. By this, I mean that polygyny is very feasible in this group, although it is extremely difficult to verify.

Response: Thank you for your comment. We have included additional text in lines 129-130 to clarify for the reader. We also provide additional text mentioning polygyny in lines 348-349

Comment: Lines 120-121. I agree that there is no best form to get a value of MP frequency for a species, however, it cannot reflect the actual mating system in species that have geographical variations, or seasonal variation. Also, the sample size is an important factor influencing the MP frequency in different studies for the same species, along with the resolution power offered by the different molecular tools employed. I suggest adding this in the limitations section, or better, stating those understandable limitations right after the methods.

Response: Thank you for your comment. We have addressed this issue in the methodology in lines 149-158.

Comment: Lines 123-125. Assuming this, authors need to discuss it in species with geographical variation in the MP.

Response: Thank you for your comment. We have added extra text to acknowledge this limitation and justify our approach in the methodology, in lines 149-158

Comment: Lines 131-133. I invite to authors to read the articles before mentioned. I consider that it is not reflected in the introduction.

Response: Thank you for your comment. As mentioned in our response to your previous comment we have read the articles and added text to the introduction as requested.

Comment: Table 2. This table is unnecessary. I suggest sending it to supplementary materials and citing it in this part.

Response: Thank you for your comment. We have decided to keep this table as it reports an important result that is referenced throughout the discussion.

Comment: Lines 187-189. I’m not familiar with the analysis, however, looking at the tree, I think the analyses give a lot of weight to the MP value. I’m concerned that in some cases, such as M. californicus, the sample size is too low to consider real genetic monogamy, since a particularity of this work is possibly collecting small females with a low probability of observing MP. In this case, It can be an effect of the sample size.

Response: Thank you for your comment. This is true to some extent, however the issues affecting this analyses are the same as those affecting the other analyses, and thus have been addressed in the limitations section. Even excluding cases with very low sample size there are still multiple cases in which very low values of MP have evolved, and even in species with low sample size it is unlikely that additional studies would find values dramatically different from existing studies. That is the main benefit of using a continuous measure of MP as opposed to a binary variable, as it allows use of MP as an approximation rather than a discrete variable that must show either monogamy or polyandry. We have added additional text in lines 149-158 to explain this.

Comment: Lines 206-209. These ideas should be expressed in the abstract and of course in the introduction.

Response: Thank you for your comment. This idea is already mentioned in lines 30-32 of the original abstract. We have added additional text in lines 75-79 in the introduction to further expand.

Comment: Lines 220-222. I’m not sure if this is correct according to the abstract of Dibattista et al. 2008. In this abstract, they said…” We find that offspring from polyandrous litters did not have a greater genetic diversity or greater survival than did the offspring of monandrous litters”. This sentence is contrary evidence.

Response: Thank you for your comment. DiBattista et al. 2008 do not present any data regarding monogamy, so their findings are not relevant to the sentence. However in the main text they do state that monogamy in sharks may reduce genetic variation, and hence this is why we included the reference.

Comment: Lines 225-228. This idea is correct and is the best of all paper. This idea has been discussed. I suggest to the authors redirect the justification of their article considering their methodology.

Response: Thank you for your comment. We believe that framing the paper in terms of the empirical results is the most scientifically robust choice. If you disagree with this we would welcome additional explanation of how you would alter the article’s focus or justification.

Comment: Lines 239-241. Using the average MP in a species cannot reflect the real mating system.

Response: Thank you for your comment. We believe that when comparing across species the approximation of average MP is the best available measure, as explained in the new text additions to the methodology. However as described in our response to a previous comment we have added text to the methodology to acknowledge this limitation.

Comment: Lines 246-248. Please, read to Stephens & Sutherland 2000, they had already discussed it and explained the Allee effect in this context.

Response: Thank you for your comment. We have read and cited the reference as requested.

Comment: Lines 248-251. I do not agree. Since the model was not significant, I think the recurrent MP is not necessarily one adaptive peak. In this sense, this sentence is contradictory to the last sentence. Also, we need to consider that there are few studies to evaluate the mating system in the Elasmobranchii clade, thus this apparently adaptive peak can be skewed due to limited publications.

Response: Thank you for your comment. We believe you may have been looking at the wrong table. Table 2 shows the results of our evolutionary model test. These tests do not come with p values and hence do not have a strict ‘significance’. They are compared on the basis of AIC, where a difference of 2 or more is considered significant. The Ornstein-Uhlenbeck model received substantially more support than the null model of Brownian motion. Therefore our results suggest that MP in elasmobranchs follows an OU model of trait evolution, in which there are one or more adaptive peaks. Of course the paucity of data does affect this analysis like all of the other analyses, and the limitations section was included specifically to address this.

Comment: Lines 264-266. Excellent! Authors need to moderate some previous sentences according to this section.

Response: Thank you for your comment. See our response to other comments for details of how we have made the requested changes.

Comment: Lines 282-283. I think this is not demonstrated with your results.

Response: Please see the above comment regarding Table 2. We found substantial support for an OU model of trait evolution over the null model of Brownian motion, which implies that MP is evolving towards one or more adaptive peaks.

Reviewer 2

Comment: The potential link between multiple paternity, sexual selection and population viability is an important and general one, and I agree with the authors that this taxonomic group is a potentially useful model system in which to study it. That being said, I found it a little difficult to know what to conclude from this study. In principle I think it is important that negative findings are published, but I do have concerns as to whether the study really had sufficient power to test the hypothesis in question. To be fair to the authors, they are quite open about the limitations of the dataset in their discussion, so on balance I think I would be in favour, but first would like to see several more minor points addressed:

Response: Thank you for your comment. We agree with the sentiment you convey. Of course the necessary data to robustly prove the fitness consequences and population level consequences of mate system variation in elasmobranchs do not yet exist. However, we believe this study is valuable as it not only provides valuable insight into the evolution of mating system variation in elasmobranchs, but may serve as the impetus for future studies that build on the existing data. We have endeavoured to make the requested changes wherever possible and hope that the revised manuscript addresses the concerns raised below.

Comment: Line 117: The data are presented as coming form the existing literature, but ca. 80% come from a single source. For that reason, I think it would be appropriate to present already in the Introduction the potential overlaps and novel aspects relative to this previous study.

Response: Thank you for your comment. The cited paper did itself gather these values from existing literature. The novelty of our study relative to Lamarca et al is discussed in lines 92-97. Essentially, their analyses is very simplistic, and doesn’t at all address potential population-level consequences. Rather it is entirely focussed on addressing the ancestral state of genetic mating system in elasmobranchs, however their analyses is very basic in this regard compared to our ancestral state reconstruction (Figure 3).

Comment: 143: It’s unclear to me why conservation status was coded quantitatively, rather than adopt a different analysis approach for this variable? Linear regression doesn’t feel like the most appropriate approach here.

Response: Thank you for your comment. After reviewing the approach we used, we have modified the text slightly in the methodology to explain better. The models we fit were phylogenetic generalised least squares regression models, which is a form of linear regression that takes into account phylogenetic non-independence. These models can incorporate discrete and continuous variables, and therefore using conservation status is entirely valid. We removed the text referencing coding as this has no effect on the results.

Comment: 173-177: Given all of these “relationships” are non-significant, I don’t think it is very meaningful to discuss which slopes are positive and which are negative; all you can reliably conclude is that none could be distinguished from zero.

Response: Thank you for your comment. We have now deleted this portion of the text.

Comment: 200: on the face of it, purging would tend to reduce genetic diversity rather than increasing it - maybe take a bit more space to explain this point?

Response: Thank you for your comment. Indeed the references to purging and genetic variation were supposed to be separate, and as such we have altered the sentence order to explain that these are

Attachment Submitted filename: Response to reviewers.docx

10.1371/journal.pone.0308141.r003
Decision Letter 1
Schlatt Stefan Academic Editor
© 2024 Stefan Schlatt
2024
Stefan Schlatt
https://creativecommons.org/licenses/by/4.0/ This is an open access article distributed under the terms of the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited.
Submission Version1
3 Jul 2024

PONE-D-24-08235R1No evidence for population-level benefits of polyandry in sharks and raysPLOS ONE

Dear Dr. Gayford,

Thank you for submitting your manuscript to PLOS ONE. After careful consideration, we feel that it has merit but does not fully meet PLOS ONE’s publication criteria as it currently stands. Therefore, we invite you to submit a revised version of the manuscript that addresses the points raised during the review process. The revision of this paper has been a long journey. At this point the manuscript is almost appropriately revised and will not go back into re-review. However the reviewer mentions a few minor changes which I like to ask the authors to consider and implement. The paper will then be acceptable.

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Reviewer #1: (No Response)

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Reviewer #1: Partly

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Reviewer #1: Yes

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Reviewer #1: Yes

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Reviewer #1: I consider that the authors aboarded all my comments and questions, especially in the interpretation of results.

I agree with almost all editions that they made in the manuscript. However, I have some comments that I would like to see in the paper. In this sense, I suggest another round of revision.

Abstract

Lines 27-28. Please specify if this idea is for the elasmobranch clade or animal kingdom.

Lines 37-40. I feel that the authors handle this idea a lot, but the topic is not delved into in the discussion, they leave everything to the reader's imagination. In this sense, I have a comment on the discussion.

Lines 43-44. I insist that this idea is no longer under discussion currently, even in elasmobranchs. Also, it doesn't delve into it, I suggest eliminating it.

Discussion

Lines 239-242. I suggest exploring this idea a little further in the discussion, I can imagine what they are referring to, but I think it would be better to read it. Since the article is titled "No evidence for population-level benefits of polyandry in sharks and rays" I hope to see some examples in this regard to understand much better how the cancellation of both characteristics (benefits and costs) of the mating system does not necessarily impact the risk of extinction of a species.

**********

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Reviewer #1: No

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10.1371/journal.pone.0308141.r004
Author response to Decision Letter 1
Submission Version2
4 Jul 2024

Response to reviewers: No evidence for population-level benefits of polyandry in sharks and rays

We thank the editor and reviewer for their time spent on this manuscript. We are very grateful for both of your comments, and believe that they have greatly improved the quality of the paper. Please find detailed responses to comments below. We hope that you find the changes made sufficient to consider acceptance of the manuscript, as we have to the best of our abilities incorporated all changes suggested by the reviewer into the text.

Reviewer 1

Comment: Lines 27-28. Please specify if this idea is for the elasmobranch clade or animal kingdom.

Response: The sentence has been clarified as requested.

Comment: Lines 37-40. I feel that the authors handle this idea a lot, but the topic is not delved into in the discussion, they leave everything to the reader's imagination. In this sense, I have a comment on the discussion.

Response: Please see our response to the comment about the discussion.

Comment: Lines 43-44. I insist that this idea is no longer under discussion currently, even in elasmobranchs. Also, it doesn't delve into it, I suggest eliminating it.

Response: The statement has been eliminated as requested.

Comment: Lines 239-242. I suggest exploring this idea a little further in the discussion, I can imagine what they are referring to, but I think it would be better to read it. Since the article is titled "No evidence for population-level benefits of polyandry in sharks and rays" I hope to see some examples in this regard to understand much better how the cancellation of both characteristics (benefits and costs) of the mating system does not necessarily impact the risk of extinction of a species.

Response: The current version of the manuscript contains quite a lot of discussion of the costs and benefits of different mating systems. See lines 62-80 and lines 243-255. The fact is that all of these costs and benefits are highly context dependent (as we point out at several points in the text), so we cannot make detailed inferences about specific costs and benefits in specific shark or ray species as we lack the necessary contextual information. The current sections on costs and benefits address to the fullest extent possible how each of these factors could apply to sharks and rays, and provide references that delve deeper into the theoretical basis of those costs and benefits, should the reader be interested.

Attachment Submitted filename: Response to reviewers_2.docx

10.1371/journal.pone.0308141.r005
Decision Letter 2
Schlatt Stefan Academic Editor
© 2024 Stefan Schlatt
2024
Stefan Schlatt
https://creativecommons.org/licenses/by/4.0/ This is an open access article distributed under the terms of the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited.
Submission Version2
9 Jul 2024

PONE-D-24-08235R2No evidence for population-level benefits of polyandry in sharks and raysPLOS ONE

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10.1371/journal.pone.0308141.r006
Author response to Decision Letter 2
Submission Version3
9 Jul 2024

The reference list has been checked, and is fully complete. No additional references need to be removed or added to the list. We have made some minor formatting adjustments for consistency as can be seen in the tracked changes document.

Attachment Submitted filename: Response to reviewers_3.docx

10.1371/journal.pone.0308141.r007
Decision Letter 3
Schlatt Stefan Academic Editor
© 2024 Stefan Schlatt
2024
Stefan Schlatt
https://creativecommons.org/licenses/by/4.0/ This is an open access article distributed under the terms of the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited.
Submission Version3
18 Jul 2024

No evidence for population-level benefits of polyandry in sharks and rays

PONE-D-24-08235R3

Dear Dr. Gayford,

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Stefan Schlatt

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PLOS ONE

Additional Editor Comments (optional):

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10.1371/journal.pone.0308141.r008
Acceptance letter
Schlatt Stefan Academic Editor
© 2024 Stefan Schlatt
2024
Stefan Schlatt
https://creativecommons.org/licenses/by/4.0/ This is an open access article distributed under the terms of the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited.
19 Jul 2024

PONE-D-24-08235R3

PLOS ONE

Dear Dr. Gayford,

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on behalf of

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Academic Editor

PLOS ONE
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