
==== Front
Proc Natl Acad Sci U S A
Proc Natl Acad Sci U S A
PNAS
Proceedings of the National Academy of Sciences of the United States of America
0027-8424
1091-6490
National Academy of Sciences

202414869
10.1073/pnas.2414869121
retroRetrospective97
Retrospective
Igor B. Dawid (1935–2024): A pioneer of developmental and molecular biology
Taira Masanori a
Ozato Keiko b https://orcid.org/0000-0001-7013-2728

Harland Richard M. c https://orcid.org/0000-0001-8247-4880

Moody Sally A. d https://orcid.org/0000-0003-4192-1087

Saint-Jeannet Jean-Pierre jsj4@nyu.edu
e 1 https://orcid.org/0000-0003-3259-2103

aDepartment of Biological Sciences, Faculty of Science and Engineering, Chuo University, Tokyo 112-8551, Japan
bDivision of Developmental Biology, National Institute of Child Health and Human Development, NIH, Bethesda, MD 20892
cMolecular and Cell Biology Department, Genetics, Genomics and Development Division, University of California, Berkeley, CA 94720
dDepartment of Anatomy and Cell Biology, School of Medicine and Health Sciences, The George Washington University, Washington, DC 20037
eDepartment of Molecular Pathobiology, New York University College of Dentistry, New York, NY 10010
1To whom correspondence may be addressed. Email: jsj4@nyu.edu.
28 8 2024
10 9 2024
28 8 2024
121 37 e2414869121Copyright © 2024 the Author(s). Published by PNAS.
2024
https://creativecommons.org/licenses/by-nc-nd/4.0/ This article is distributed under Creative Commons Attribution-NonCommercial-NoDerivatives License 4.0 (CC BY-NC-ND).

access-typefree
==== Body
pmcIgor Dawid was born in 1935 in Chernivtsi (now in Ukraine). Because of its geographic location, this city changed country several times over the years. The city then had a substantial Jewish population which included Igor’s father who ran a business. Igor was a holocaust survivor; he and his parents miraculously escaped from not one but several Nazi round-up operations. His childhood experiences had a decisive impact on shaping his later life, as a scientist and a person. He trusted science to be worthy of a genuine effort to find objective truth. He also loved music perhaps as much as science, his favorite composers being Wolfgang Amadeus Mozart and Franz Schubert. As a child, Igor did not have a regular school education. However, he had no difficulty being accepted into the University of Vienna, Austria. There, he trained as a chemist studying sphingomyelin structure and received a Ph.D. in biochemistry in 1960, at the age of 25. He then did postdoctoral training in a hard-core biochemistry lab at the Massachusetts Institute of Technology (MIT) under John (Jack) Buchanan publishing one paper with him (1), but his interest quickly shifted to biochemical analysis in embryology (2, 3).

Igor Dawid in his office at the NIH in 1996.

1. Scientific Contributions

Igor’s scientific career as a developmental biologist began in 1963 when he moved from MIT to the Department of Embryology of the Carnegie Institution of Washington, in Baltimore, Maryland. The story of his subsequent career illustrates the very history of developmental and molecular biology using the African clawed frog, Xenopus laevis.

1.1. From Biochemist to Molecular Biologist.

In Baltimore, Igor joined Don Brown and others who were using the frog X. laevis and its large eggs to study genes and their RNA products. Igor had previously used equilibrium density centrifugation with cesium chloride to separate vaccinia DNA from host cell DNA (4), and this technique became the essential tool to purify genes before the advent of recombinant DNA. He started with the paradoxical observation that the frog egg, though a single cell, contains an unexpectedly large amount of DNA. He tracked down the source to the mitochondria, which had only recently been shown to have DNA and significantly identified the maternal origin of mitochondrial DNA (5, 6). Continuing his interest in DNA, he showed with Brown that ribosomal genes are amplified in oocytes and that the oocyte nucleus contains this extra DNA in its many nucleoli (7). Dawid and Brown exploited this amplification phenomenon to isolate DNA for early structure and function work, provided it to John Morrow at Stanford for the first cloning of animal DNA, and studied the structure of the repeated genes, which Igor continued with Ron Reeder and Peter Wellauer (8, 9). Much later, when the X. laevis genome project was underway and its genome size was estimated to be 3.1 Gbp by modern techniques, he said he was relieved at the accuracy of the X. laevis genome size he had biochemically quantified using erythrocytes, which do not have mitochondria (5). This truly speaks volumes of the precision of his experimentation.

1.2. From Molecular Biologist to Developmental Biologist.

Igor joined the National Cancer Institute (NCI) at the NIH, Bethesda, MD, in 1978. He became head of the Section on Developmental Biology at the National Institute of Child Health and Human Development (NICHD) in 1982 until his retirement in December 2016, transitioning to NICHD Scientist Emeritus after his retirement. He served as chief of the NICHD’s Laboratory of Molecular Genetics from 1982 to 2010, head of the Program in Genomics of Differentiation from 2007 to 2010, and was acting scientific director of NICHD from 1998 to 2000.

During his Carnegie years (1978–1989), many other molecular biologists, like Igor, dealt with abundant and easily isolated material such as mtDNA and rDNA. However, as technology developed, they gradually shifted to studying protein-coding genes that are highly expressed in specific tissues. Around the time he moved to the NIH, Igor, together with Walter Wahli and his colleagues, cloned the cDNA for vitellogenin, a protein abundantly synthesized in the liver and deposited into oocytes, and characterized the vitellogenin gene family (e.g., refs. 10–12). In the early 1980s, Tom Sargent and Igor developed for the first time the subtracted cDNA library technique and constructed a “differentially expressed in gastrula embryos (DG)” library (13). This new approach inspired many developmental biologists, but unfortunately, there were too many unknown genes in DG clones for complete analysis at that time. Igor, Tom, and their colleagues did go on to study several DG clones encoding epidermal cytokeratin genes (e.g., ref. 14) and one of them, XK81, was found to be larva-specific (15). Igor and colleagues also constructed tissue- or stage-specific cDNA libraries and performed differential screening with different probes, leading to the identification of genes specifically expressed in the notochord, neural tissue, and other tissue (e.g., refs. 16 and 17). As in other laboratories, this era of “gene cloning” lasted for many years in Igor’s lab. However, most genes identified during this time were not developmental regulators, which have relatively low expression levels, but rather genes that became useful markers for tissue differentiation.

One of the main topics in developmental biology around that time was the quest for factors that induce the formation of mesoderm in the blastula stage embryo. Igor and Frederic Rosa in collaboration with Michael Sporn’s lab found that TGF-beta 2 had mesoderm-inducing activity in animal cap explants (18); this was shortly after a paper in 1987 by another lab showing that FGF2 had mesoderm-inducing activity. Igor and Tom wrote an excellent review (19) that summarized the impressive achievements of X. laevis research in basic molecular biology at that time and emphasized X. laevis as a prime system for the study of vertebrate embryogenesis. They predicted the next coming climax of Xenopus research, which was the identification of genes that regulate how the embryo develops. In fact, findings from Xenopus research since 1988 have probably far exceeded their expectation; by enabling the functional analysis of genes cloned using various techniques (homology/degenerate PCR-based, differential screening, or functional cloning), many important development pathways were discovered or analyzed using X. laevis, including those driven by signaling factor components related to Wnt, activin, BMP, and FGF, and various transcription factors including those containing a homeobox, T-box, or Forkhead domain.

1.3. Gene Function Analysis in Xenopus Development.

For the next decade (1990–2008), Igor’s work focused on the role of the LIM homeodomain protein Lim1/Lhx1 and canonical Wnt signaling in the “organizer”—a small, localized region of the blastula embryo that involutes and induces the formation of the body axis including mesoderm and neural tissue, The nature of the organizer had been one of the most fascinating and long-standing questions in developmental biology since its discovery in 1924 by Hans Spemann and his graduate student, Hilde Mangold. In 1990, Igor and Masanori Taira launched a project on a newly identified LIM homeobox gene family. They adopted degenerate PCR cloning, a new technique at that time and successfully identified three Xenopus genes, named lim1/lhx1, lim3/lhx3, lim5/lhx5, which are homologs of Caenorhabditis elegans LIM homeobox genes, lin-11, mec-3, and Ceh-14. Among them, lim1 turned out to be expressed in the Spemann–Mangold organizer (20). Gain-of-function analysis by mRNA injection revealed that LIM domain mutants of lim1 exhibit organizer activity, and hence also for the first time suggested the function of LIM domains (21). This finding led to the identification of a LIM domain-binding protein, Ldb1 (22). Since lim1, lim3, and lim5 show noteworthy expression patterns in specific types of neurons (all three), the pituitary anlagen (lim3), and the developing kidney (lim1), these genes were also analyzed in zebrafish (23) and in mice (other labs); indeed, mice in which the genes are deleted (knocked out) exhibited remarkable phenotypes. Based on such studies in the 1990s, the Xenopus embryo emerged as a powerful system for the identification and functional analysis of genes regulating early development. Xenopus embryos were also important for testing the function of developmental genes first identified in mice, in combination with rigorous genetic analysis in that organism. The power of such an approach was recognized by Harold Varmus, the director of the NIH at the time (1993–1999), and one of his postdoctoral fellows, Xi He. They contacted Igor with interest in using the Xenopus axis duplication assay to investigate Wnt signal transduction during embryonic patterning in collaboration with Jean-Pierre Saint Jeannet, a postdoctoral fellow in Igor’s lab. This was the start of a very rich collaboration. One of these studies described the function in Xenopus of glycogen synthase kinase-3 (GSK-3), the vertebrate homolog of the Drosophila shaggy (zeste-white 3) gene, which is required for Wnt signaling in flies. The work demonstrated the critical role of GSK-3 activity in ventral differentiation and the need to suppress its activity by a Wnt signal to elicit dorsal development (24). The same axis duplication assay was used to show that the inability of Xwnt5A to promote axis formation in Xenopus as compared to other Wnt ligands, such as Wnt1, Wnt3a, or Wnt8, was due to the lack of expression of a functional Xwnt5A receptor in the embryo. The coexpression of Xwnt5A with hFz5, a member of the Frizzled family of receptors, induced extensive dorsal axis duplication and this activity was mediated by inhibition of GSK3—thereby identifying hFz5 as a canonical signaling receptor for Wnt5A. The importance of this finding was that it showed that different receptors change the activity of a previously presumed “non-canonical” ligand (25). This collaboration also revealed a key role for Wnt signaling in patterning the dorsal neural tube by providing instructional cues to specify neural crest cells in the embryo and neuralized ectoderm explants (26).

Igor’s lab further contributed to the analysis of many other regulatory genes in various developmental processes in Xenopus and later in zebrafish by adopting new technologies of the time. For example, microarray was adopted for differential gene expression screening using dissected Xenopus embryos. This approach and gene function analysis identified and characterized WGEF/Ahrgef19, a Rho guanine nucleotide exchanging factor, which was found to function in planar cell polarity in the posterior mesoderm (27) and Lrig3 in neural crest formation (28).

1.4. Gene Function Analysis in Zebrafish Development.

In the 1990s, zebrafish (Danio rerio) became a popular model organism following the two large genetic mutant screens performed by Christiane Nusslein-Volhard in Tubingen and Wolfgang Driever and Mark Fishman in Boston. Igor saw the enormous potential of this model organism to investigate the molecular bases of vertebrate embryogenesis. He immediately embarked on zebrafish research around 1992, gradually devoting more effort on this program. Tetsuhiro Kudoh, Michael Tsang, Neil Hukriede, and others carried out a whole mount in situ hybridization (WISH) gene expression screen using a normalized cDNA library prepared from somitogenesis stage embryos, focusing on genes with tissue-specific expression (29). The screen was extremely fruitful with the identification and characterization of novel modulators of key signaling pathways, including SEF/Il17rd and MKP3/Dusp6 as two negative regulators of FGF signaling (30, 31). Differential gene expression screening combined with WISH and other approaches also identified genes encoding important factors regulating embryonic patterning and differentiation across a broad range of tissues. These findings were reported in a series of publications throughout the 2000s (e.g., refs. 29 and 32–36). Igor also spearheaded efforts to map the zebrafish genome using radiation hybrid technology (37, 38), a tool widely used by the zebrafish community to identify candidate genes disrupted in zebrafish mutants.

2. Support to the Xenopus Research Community

In addition to Igor’s many scientific contributions, it is important to acknowledge his strong support for and influence on advancing the impact of the Xenopus research community. The first gathering of the leaders in the Xenopus “molecular development” field was organized by Igor and John Gurdon at Airlie House (Warrington, VA) in April 1984. About 25 researchers from the United States, United Kingdom, Europe, and Japan, representing expertise in molecular biology, biochemistry, embryogenesis, oogenesis, and neurobiology, were invited to present their current work and future goals. This culminated in a lively discussion of whether there was sufficient enthusiasm to band together to launch a biannual international conference. Thankfully there was, and from 1986 to 2023 there have been 19 International Xenopus Conferences (IXC), rotating between North America and Europe (and once in Japan). Igor and John organized the funding and speakers for at least the first 10 meetings before turning its oversight over to the next generation. Besides founding and organizing the IXC, Igor was a major influence on the distribution of resources to the broader community. For example, he was an influential supporter of the creation of Xenbase, a critical database for all aspects of Xenopus biology, and of the Affymetrix microarray GeneChips for both Xenopus and zebrafish, a technology that preceded RNAseq and led to numerous discoveries of genes involved in a variety of developmental processes. Igor’s lab also contributed several of the stage-and tissue-specific cDNA libraries that provided the many thousands of EST and full-length sequences deposited in NCBI’s GenBank (go to: Xenopus cDNA libraries) that continue to support many kinds of studies across developmental biology, regeneration, and human disease.

Igor not only contributed greatly to science but he also trained a large cohort of scientists. He attracted researchers not only from the United States but also from countries across continents, from Western and Eastern Europe to Middle East and Asia including India, Japan, China, and South Korea. His laboratory was thus very international, and furthermore, the atmosphere was almost entirely devoid of futile conflict or competition, providing an environment where everyone could devote themselves to their research and thrive. A weekly one-on-one discussion in his office was very fruitful and inspiring for all lab members. Igor was highly respected by his peers, and an inspiration for many young scientists. Everyone around him appreciated his kindness, thoughtfulness, and fairness. He was straightforward and open-minded, and treated everyone equally regardless of the circumstances, and his thinking was logical and organized, which made him very trustworthy. We will miss Igor deeply.

Author contributions

M.T., K.O., R.M.H., S.A.M., and J.-P.S.-J. wrote the paper.

Competing interests

The authors declare no competing interest.
==== Refs
1 I. B. Dawid, T. C. French, J. M. Buchanan, Azaserine-reactive sulfhydryl group of 2-formamido-N-ribosylacetamide 5’-phosphate: L-glutamine amido-ligase (adenosine diphosphate). II. Degradation of azaserine-C-14-labeled enzyme. J. Biol. Chem. 238 , 2178–2185 (1963).14025584
2 I. Dawid, Q & A: Igor Dawid. Curr. Biol. 16 , R391–R392 (2006).16791937
3 I. B. Dawid, REFLECTIONS: Differential gene expression in vertebrate embryos. J. Biol. Chem. 284 , 13277–13283 (2009).19158075
4 C. Jungwirth, I. B. Dawid, Vaccinia DNA: Separation of viral from host cell DNA. Arch. Gesamte Virusforsch. 20 , 464–468 (1967), 10.1007/BF01275228.5598023
5 I. B. Dawid, Evidence for the mitochondrial origin of frog egg cytoplasmic DNA. Proc. Natl. Acad. Sci. U.S.A. 56 , 269–276 (1966), 10.1073/pnas.56.1.269.5229852
6 I. B. Dawid, A. W. Blackler, Maternal and cytoplasmic inheritance of mitochondrial DNA in Xenopus. Dev Biol. 29 , 152–161 (1972), 10.1016/0012-1606(72)90052-8.4672727
7 D. D. Brown, I. B. Dawid, Specific gene amplification in oocytes. Oocyte nuclei contain extrachromosomal replicas of the genes for ribosomal RNA. Science 160 , 272–280 (1968), 10.1126/science.160.3825.272.4867987
8 P. K. Wellauer, I. B. Dawid, Secondary structure maps of ribosomal RNA and its precursors as determined by electron microscopy. Cold Spring Harb. Symp. Quant. Biol. 38 , 525–535 (1974). 10.1101/sqb.1974.038.01.057.4524773
9 R. H. Reeder, D. D. Brown, P. K. Wellauer, I. B. Dawid, Patterns of ribosomal DNA spacer lengths are inherited. J. Mol. Biol. 105 , 507–516 (1976), 10.1016/0022-2836(76)90231-x.972393
10 W. Wahli, I. B. Dawid, Isolation of two closely related vitellogenin genes, including their flanking regions, from a Xenopus laevis gene library. Proc. Natl. Acad. Sci. U.S.A. 77 , 1437–1441 (1980).6445556
11 W. Wahli , Vitellogenin in Xenopus laevis is encoded in a small family of genes. Cell 16 , 535–549 (1979).455441
12 W. Wahli, I. B. Dawid, G. U. Ryffel, R. Weber, Vitellogenesis and the vitellogenin gene family. Science 212 , 298–304 (1981).7209528
13 T. D. Sargent, I. B. Dawid, Differential gene expression in the gastrula of Xenopus laevis. Science 222 , 135–139 (1983).6688681
14 M. Jamrich, T. D. Sargent, I. B. Dawid, Cell-type-specific expression of epidermal cytokeratin genes during gastrulation of Xenopus laevis. Genes Dev. 1 , 124–132 (1987).2445625
15 S. Miyatani, J. A. Winkles, T. D. Sargent, I. B. Dawid, Stage-specific keratins in Xenopus laevis embryos and tadpoles: The XK81 gene family. J. Cell Biol. 103 , 1957–1965 (1986).2430981
16 S. E. LaFlamme, M. Jamrich, K. Richter, T. D. Sargent, I. B. Dawid, Xenopus endo B is a keratin preferentially expressed in the embryonic notochord. Genes Dev. 2 , 853–862 (1988).2463213
17 K. Richter, H. Grunz, I. B. Dawid, Gene expression in the embryonic nervous system of Xenopus laevis. Proc. Natl. Acad. Sci. U.S.A. 85 , 8086–8090 (1988).3186710
18 F. Rosa , Mesoderm induction in amphibians: The role of TGF-beta 2-like factors. Science 239 , 783–785 (1988).3422517
19 I. B. Dawid, T. D. Sargent, Xenopus laevis in developmental and molecular biology. Science. 240 , 1443–1448 (1988).3287620
20 M. Taira, J.-P. Saint-Jeannet, I. B. Dawid, Role of Xlim-1 and Xbra genes in anteroposterior patterning of the neural tissue by the head and trunk organizer. Proc. Natl. Acad. Sci. U.S.A. 94 , 895–900 (1997).9023353
21 M. Taira, H. Otani, J.-P. Saint-Jeannet, I. B. Dawid, Role of the LIM class homeodomain protein Xlim-1 in neural and muscle induction by the Spemann organizer in Xenopus. Nature 372 , 677–679 (1994).7990959
22 A. D. Agulnick , Interactions of the LIM-domain-binding factor Ldb1 with LIM homeodomain proteins. Nature 384 , 270–272 (1996).8918878
23 R. Toyama , The LIM class homeobox gene lim5: Implied role in CNS patterning in Xenopus and Zebrafish. Dev. Biol. 170 , 583–593 (1995).7649385
24 X. He, J.-P. Saint-Jeannet, J. Woodgett, H. E. Varmus, I. B. Dawid, Glycogen synthase kinase 3 and dorsoventral patterning in Xenopus embryos. Nature 374 , 617–622 (1995).7715701
25 X. He , A member of the Frizzled protein family mediating axis induction by Wnt-5A. Science 275 , 1652–1654 (1997).9054360
26 J.-P. Saint-Jeannet, X. H. E. Varmus, I. B. Dawid, Regulation of dorsal fate in the neuraxis by Wnt-1 and Wnt-3a. Proc. Natl. Acad. Sci. U.S.A. 94 , 13713–13718 (1997).9391091
27 K. Tanegashima, H. Zhao, I. B. Dawid, WGEF activates Rho in the Wnt-PCP pathway and controls convergent extension in Xenopus gastrulation. EMBO J. 27 , 606–617 (2008).18256687
28 H. Zhao, K. Tanegashima, H. Ro, I. B. Dawid, Lrig3 regulates neural crest formation in Xenopus by modulating Fgf and Wnt signaling pathways. Development 135 , 1283–1293 (2008).18287203
29 T. Kudoh, I. B. Dawid, Role of Iroquois homeobox gene in organizer formation. Proc. Natl. Acad. Sci. U.S.A. 98 , 7852–7857 (2001).11438735
30 M. Tsang, R. Friesel, T. Kudoh, I. B. Dawid, Identification of Sef, a novel modulator of FGF signalling. Nat. Cell Biol. 4 , 165–169 (2002).11802164
31 M. Tsang , A role of MKP3 in axial patterning of the zebrafish embryo. Development 131 , 2769–2779 (2002).
32 A. Kawahara, I. B. Dawid, Critical role of biklf in erythroid cell differentiation in zebrafish. Curr. Biol. 11 , 1353–1357 (2001).11553329
33 T. Kudoh, W. W. Wilson, I. B. Dawid, Distinct roles for Fgf, Wnt and retinoic acid in posteriorizing the neural ectoderm. Development 129 , 4335–4346 (2002).12183385
34 S.-K. Hong, M. Tsang, I. B. Dawid, The mych gene is required for neural crest survival during zebrafish development. PLoS ONE 3 , e2029 (2008).18446220
35 E. Aamar, I. B. Dawid, Protocadherin-18a has a role in cell adhesion, behavior and migration in zebrafish development. Dev. Biol. 318 , 335–346 (2008).18468594
36 H. Ro, I. B. Dawid, Organizer restriction through modulation of Bozozok stability by the E3 ubiquitin ligase Lnx-like. Nat. Cell Biol. 11 , 1121–1127 (2009).19668196
37 N. Hukriede , Radiation hybrid mapping of the zebrafish genome. Proc. Natl. Acad. Sci. U.S.A. 96 , 9745–9750 (1999).10449765
38 N. Hukriede , The LN54 radiation hybrid map of zebrafish expressed sequences. Genome Res. 11 , 2127–2132 (2001).11731504
