John Postlethwait is fascinated by how zebrafish offspring depend on their mom’s genome to get things started. In a study published in the May issue of G3: Genes|Genomes|Genetics, Postlethwait and co-author Catherine Wilson delve into the unique features of the zebrafish sex chromosome, identifying a maternal-to-zygotic-transition (MZT) gene regulatory block.

Zebrafish females are either ZW or WW. ZZ zebrafish are always males, but some fish with a W can sex-reverse to become males. Although sex-biased gene selection is important for understanding characteristics such as sexual dimorphism, little is known about the distribution of sex-biased genes along fish chromosomes.

To learn more about zebrafish sex-biased genes, Wilson and Postlethwait harvested gonads from male and female Nadia-strain zebrafish at three months post-fertilization and performed RNA-seq to compare gene expression patterns in females versus males. Differentially expressed (DE) genes were evaluated with DESeq2 software.

They also analyzed gonads from laboratory strain AB. After several generations of gynogenesis, these fish lack sex chromosomes. Analyzing gonads from AB fish allowed the authors to rule out sex determination as a means of ovary-specific gene silencing.

As expected, substantially more genes showed testis-biased than ovary-biased expression (10,495 to 6,557, respectively). DE genes aligned with these proportions across the genome—with the exception of Chr4, the sex chromosome. Chr4’s long right arm, Chr4R, was cytogenetically and transcriptionally unique. In fact, about 80% of Chr4R genes have no human orthologs.

Importantly, their analysis revealed that a long block of sex chromosome Chr4 features not only the unique silencing of protein-coding genes in egg cells but also encodes RNA molecules for maternal-to-zygotic transfer necessary for making proteins and eliminating the mother’s transcripts.

All egg-laying animals—including, to some extent, humans—begin development using maternally-produced RNA and proteins, which means some embryonic phenotypes depend on the mother’s genotype rather than the embryo’s. The MZT marks the point when the embryo begins to rely on its own RNA and proteins, and it requires complex gene regulation changes.

In contrast to much of the rest of the genome, Chr4R is mainly heterochromatic. Surprisingly, transcription in ovaries was suppressed for nearly all protein-coding genes in this region but still occurred in testes.

This area of suppressed ovary-biased transcription is involved in the MZT transfer of components such as ribosomes and spliceosomes that kickstart embryonic development following fertilization. An adjacent genomic area removes maternal transcripts when the zygote reaches the 1,000-cell stage. 

The study found that gonads from the AB strain followed the same general expression pattern as those from Nadia fish, meaning ovary-specific gene silencing must be related to gonad development, function, or both—not sex determination.

Postlethwait explains that this study lays some groundwork for egg quality research. Pollutants may affect egg quality, and studying non-placental zebrafish makes toxicological effects easier to uncover. Extrapolating such effects to humans requires understanding, among other things, the balance and mechanisms of ovary- and testis-biased expression in developing and adult zebrafish.

Guest post by Charles H. Langley.

Dic, hospes, doctis caelebs animalculum obisse

hicque iacere physis legibus conveniens.

(Stranger, tell the learned that the celibate little animal has passed away, and lies here, conforming to the laws of nature.)

—John Rundin, with apologies to Simonides and Cicero

Thirty-five years ago, in a celebrated News & Views (1986) John Maynard Smith shared his well-developed thoughts about the evolutionary forces that may have maintained sex in the surviving lineages of eukaryotes. While parthenogenesis appears to be widespread, phylogenetic analyses were establishing that the overwhelming majority of the ancient and successful lineages retained sex in all its biological richness. Simple population genetic theory had long raised the question: what is the evolutionary mechanism(s) that overcomes that clear two-fold Malthusian advantage of an asexual mode of reproduction? After highlighting a number of interesting asexual species, Maynard Smith disposed of all of these as young and likely to be evolutionary dead ends. One hundred years earlier, before the rediscovery of Mendel and the development of population genetic foundations for natural selection Weismann (1887) had already surmised that natural selection favoring sexuality in multicellular organisms must be strong.

Maynard Smith cast the bdelloid rotifers as the exceptions, being perhaps 40 million years on the earth, and even after centuries of study these beloved and easily collected and cultured micro-animals had not yielded a male. Maynard Smith’s reference to the bdelloid as “something of an evolutionary scandal” energized efforts to address this assertion. Perhaps the early rotifers had evolved a creative evasion of the strong evolutionary forces maintaining sex in other eukaryotic lineages? Or were bdelloids simply ‘modest,’ pursuing sexual reproduction rarely and/or in the most obscure environments? Both these hypotheses appeared tenable, although the force of the first depends on negation of the latter.

Over the ensuing decades the natural history and systematics of bdelloids improved yet no evidence of sex emerged. Meanwhile molecular geneticist Matt Meselson and colleagues proceeded to look at the bdelloid genomes. Ancient duplications were discovered. And then Nowell et al. (2018) found evidence for the conservation in bdelloids of genes associated with meiosis throughout all eukaryotes. For unknown reasons, the simple inference of sex in the population genetics of bdelloid rotifer species was only recently addressed. Independently two research groups have now reported evidence of sex in the recent descent of distinct bdelloid species (Vakhrusheva et al. 2020) and (Laine et al. 2021). Variants at genomic locations far apart or on different chromosomes occur in combinations that strictly asexual reproduction precludes, but mere occasional sexual reproduction strongly predicts. They concluded that, despite the lack of observed mating or even of “males,” these bdelloid rotifers must have had recent sexual common ancestors. Thus the ‘scandal’ of evolutionary biology is transformed into the latest evidence driving what is perhaps the most significant and controversial question in evolutionary genetics, what force(s) maintains sex in eukaryotes.

The lifting of a scandal often engenders rectification of respect and the drawing of lessons, if not morals. The generality of the observed conservation of sex is, indeed, bolstered by the new evidence in bdelloid rotifer. It also raises other questions. Should the study of the reproductive biology of rotifers have been given tenfold more resources and talent? After all it took 35 years. Or can one suggest that this decisive evidence of bdelloid sexuality would have come in the same amount of time without the impetus of the ‘scandal?’

The real substance of Maynard Smith’s short discussion was, of course, the mechanism of the natural selection that maintained sex. The accepted competing hypotheses rest on the indirect selection induced by linkage among loci under selection. As R.A. Fisher and H.J. Muller noted in the 1930s, in the complete absence of sex, strongly favored rare variants (mutations) must arise and go to fixation in a sequential order. In contrast, sexual reproduction allows unlinked variants to respond to natural selection independently. Thus, sexual populations can more readily adapt to changing environments. And that’s what we observe, i.e., asexual lineages go extinct.

However, as H.J. Muller always wanted us to remember, the great majority of mutations are deleterious and much, if not most, of natural selection is committed to their eventual purging from the population. What Maynard Smith was at that time just beginning to appreciate and in the ensuing decades has become widely recognized, is that linkage also impedes this mode of natural selection. Thus, the selection on deleterious mutations can indirectly select in favor of sexual individuals and thereby maintain this most conserved of eukaryotic life history traits. Happily, the charming bdelloid now stands proudly at the center of inquiry into the evolutionary impact of natural selection.

Literature Cited 

Laine V. N., T. Sackton, and M. Meselson, 2021 Genomic Signature of Sexual Reproduction in the Bdelloid Rotifer Macrotrachella quadricornifera. GENETICS https://doi.org/10.1093/genetics/iyab221

Maynard Smith J., 1986 Evolution: Contemplating life without sex. Nature 324: 300–301. https://doi.org/10.1038/324300a0

Nowell R. W., P. Almeida, C. G. Wilson, T. P. Smith, D. Fontaneto, A. Crisp, G. Micklem, A. Tunnacliffe, C. Boschetti, and T. G. Barraclough, 2018 Comparative genomics of bdelloid rotifers: Insights from desiccating and nondesiccating species, (C. Tyler-Smith, Ed.). PLOS Biol. 16: e2004830. https://doi.org/10.1371/journal.pbio.2004830

Vakhrusheva O. A., E. A. Mnatsakanova, Y. R. Galimov, T. V. Neretina, E. S. Gerasimov, S. A. Naumenko, S. G. Ozerova, A. O. Zalevsky, I. A. Yushenova, F. Rodriguez, I. R. Arkhipova, A. A. Penin, M. D. Logacheva, G. A. Bazykin, and A. S. Kondrashov, 2020 Genomic signatures of recombination in a natural population of the bdelloid rotifer Adineta vaga. Nat. Commun. 11: 6421. https://doi.org/10.1038/s41467-020-19614-y

Weismann A., 1887 On the signification of the polar globules. Nature. 36:607–609. https://doi.org/10.1038/036607a0

About the author:

Charles H. Langley is Distinguished Professor of Genetics at the University of California, Davis.

A Wnt protein involved in the formation of the human ovary plays an important role in female zebrafish sex development.   

Even though zebrafish are a well-studied research model, how these fish develop into males or females remains rather obscure—in part because the sex of lab strains is not determined by sex chromosomes. Research published in GENETICS reveals that one of the genes critical for proper development of mammalian ovaries seems to play a related role in zebrafish, providing insight into this major difference between fish and mammals.

In humans and other mammals, Wnt4 encodes a signaling molecule that antagonizes the male-promoting signal FGF9 and is crucial to the proper development of the ovaries. Though they lack an Fgf9 ortholog, zebrafish have two Wnt4-like genes—wnt4a and wnt4b—so Kossack et al. investigated whether these genes might play a role in zebrafish sexual development.

The authors first performed a phylogenetic analysis to better understand how the Wnt4-like genes have changed over evolutionary time. Zebrafish have two such genes, while mammals only have one, so two evolutionary scenarios are possible: either fish gained an extra copy of the gene, or mammals lost a copy that was originally present. The authors found that the latter scenario is more likely; both reptiles and birds have two such genes, making it more likely that  mammals lost one of their copies. Further analysis revealed that wnt4a is likely the ortholog of mammalian Wnt4, whereas wnt4b was lost in mammals after they split from birds.

The authors next used RT-PCR to examine the expression patterns of the two genes. They detected wnt4a, but not wnt4b, in the ovaries of female zebrafish. In contrast, only wnt4b was detected in the testis of male fish. They also found that wnt4a was dynamically expressed in gonads during development in a non-sex-specific manner.

Fish mutant for wnt4a develop predominantly—though not exclusively—as males, which supports the idea that wnt4a is involved in either differentiation into a female or the maintenance of a female phenotype throughout development. Analysis of mutant embryos during development suggests that wnt4a likely promotes female development since most mutant embryos developed as males rather than “reverting” to males from an initially female phenotype.

Interestingly, wnt4a mutants were unable to produce progeny when mated to each other or to  wild-type. Closer inspection revealed that wnt4a-mutant fish of both sexes had malformed reproductive tracts. Even though viable eggs and sperm could be obtained from their gonads, mutant fish were unable to release their gametes, preventing them from reproducing.

These findings suggest that Wnt4-like genes have been involved in female development of a diverse array of animals—including our fishy ancestors that swam the oceans some 450 millions year ago.

CITATION:

Female Sex Development and Reproductive Duct Formation Depend on Wnt4a in Zebrafish

Michelle E. Kossack, Samantha K. High, Rachel E. Hopton, Yi-lin Yan, John H. Postlethwait, Bruce W. Draper

GENETICS January 2019 211: 219-233; https://doi.org/10.1534/genetics.118.301620

http://www.genetics.org/content/211/1/219

Sexual reproduction is scarce in skin infection culprit.

While some people love to feel the burn during a workout, we generally seek that sensation in our muscles—not our feet. Treading barefoot in damp, communal environments like gym showers and the perimeters of pools can expose us to the fungus Trichophyton rubrum, the most common cause of athlete’s foot. Despite its name, athlete’s foot isn’t found exclusively in fitness fanatics—it affects around 15% of people worldwide. New work published in GENETICS shows that across this global range, the T. rubrum genome varies surprisingly little.

T. rubrum is widespread and comes in many varieties called morphotypes that differ in characteristics such as which parts of the body they can infect and the appearance of their colonies. In this study, the researchers found that T. rubrum samples from around the world were remarkably genetically similar to one another despite representing many different morphotypes. The data also suggest T. rubrum rarely, if ever, sexually reproduces. Mating in many fungi occurs between cells of different mating types, but of the 135 samples tested, the mating types of all but a single Mediterranean strain were identical.

The researchers found no evidence of mating when they paired the Mediterranean strain with strains of the opposite mating type, which supports the idea that the fungi reproduce clonally. This result comes with some caveats, though: the lab conditions may not have favored mating, and it’s possible that mating does occur when the Mediterranean strain comes in contact with some of the other strains in the wild. Overall, the results are consistent with one hypothesis that has been put forth about the fungi, which that is the species recently experienced a sharp decrease in sexual reproduction. The authors suggest this might have occured when T. rubrum began specializing for growth on humans.

Given that pathogens must dodge the defenses of their constantly adapting hosts, it may seem strange that T. rubrum exhibits such low genetic diversity, but it’s not alone in this trait. For reasons that haven’t been fully established, bacteria that cause tuberculosis and Hansen’s disease (leprosy) also come in a variety of types despite being highly clonal.

Although T. rubrum infections are treatable and rarely progress to serious disease, they’re common and often extremely uncomfortable. Those of us who wear sandals in the gym shower would certainly agree it’s well worth it to learn more about how this pesky fungus operates.

CITATION:

Whole-Genome Analysis Illustrates Global Clonal Population Structure of the Ubiquitous Dermatophyte Pathogen Trichophyton rubrum
Gabriela F. Persinoti, Diego A. Martinez, Wenjun Li, Aylin Döğen, R. Blake Billmyre, Anna Averette, Jonathan M. Goldberg, Terrance Shea, Sarah Young, Qiandong Zeng, Brian G. Oliver, Richard Barton, Banu Metin, Süleyha Hilmioğlu-Polat, Macit Ilkit, Yvonne Gräser, Nilce M. Martinez-Rossi, Theodore C. White, Joseph Heitman, Christina A. Cuomo
GENETICS 2018 208: 1657-1669; https://doi.org/10.1534/genetics.117.300573
http://www.genetics.org/content/208/4/1657

[wysija_form id=”1″]

Choosing a mate is hardly random. During courtship in the diploid phase of our life cycle, we often employ elaborate rituals and biological signals to attract and assess potential mates. But after that, we usually assume that eggs and sperm choose each other randomly at fertilization. Or so says Mendel’s First Law. But sometimes, Joseph Nadeau suggests, some genetic variants defy this Law, with the combination of sperm and egg being based on their genetic content.

In his review published in GENETICS, Nadeau explores cases in which nonrandom pairing of sperm and egg at fertilization distorts the expected ratio of offspring genotypes. Although it seems logical that the fastest sperm with the best sense of direction would be the most likely to fertilize the egg, there appear to be mechanisms that cause some unions to be more probable than others are.

For example, even once a chosen few sperm reach the oviduct, it’s not a simple race to find the egg. In one study, researchers found that in pigs, sperm bearing X-chromosomes elicit different transcriptomic responses in the female oviduct than those that have Y-chromosomes do. Many of the genes identified as specifically upregulated by sperm carrying a Y-chromosome are immune genes, hinting that the female immune system may contribute to sex selection.

This is only one of several mechanisms Nadeau describes that could account for nonrandom joining of sperm and eggs. But biased fertilization is often overlooked as a potential cause of transmission ratio distortion. Instead, departures from Mendelian expectations are often assumed to be caused by problems in embryonic development—without considering litter sizes or doing other tests to support this conclusion. Perhaps a closer look at these exceptions would reveal more instances in which other factors are involved, leading us to a deeper understanding of fertilization and the molecular basis of Mendel’s First Law.

Nadeau, J. Do Gametes Woo? Evidence for Their Nonrandom Union at Fertilization.
GENETICS, 207(2), 369-387.
DOI: 10.1534/genetics.117.300109
http://www.genetics.org/content/207/2/369

Jonathan Hodgkin

The Genetics Society of America’s Edward Novitski Prize recognizes a single experimental accomplishment or a body of work in which an exceptional level of creativity and intellectual ingenuity has been used to design and execute scientific experiments to solve a difficult problem in genetics.

The 2017 winner, Jonathan Hodgkin, used elegant genetic studies to unravel the sex determination pathway in Caenorhabditis elegans. He inferred the order of genes in the pathway and their modes of regulation using epistasis analyses, a powerful tool that was quickly adopted by other researchers. He expanded the number and use of informational suppressor mutants in C. elegans, which are able to act on many genes. He also introduced the use of collections of wild C. elegans to study naturally occurring genetic variation, paving the way for SNP mapping and QTL analysis, as well as studies of hybrid incompatibilities between worm species. His current work focuses on nematode-bacterial interactions and innate immunity.

This interview was published in the December 2017 issue of GENETICS.

What inspired you to become a scientist?

I was exposed to research from a very early age and learned a great deal from my father and grandfather. My father was Alan Hodgkin who was a physiologist and neurobiologist, and my mother’s father was Peyton Rous, who was a virologist who discovered the Rous sarcoma virus.  I had various other scientific relatives —cousins and what have you—going further back. I wouldn’t say it was inevitable that I became a scientist, but it was the easiest career option! And of course, I was inspired by the people who taught me at university, from about second-year onwards, who were enormously influential.

Why did you choose C. elegans as your research system?

Very largely because of Sydney Brenner. He gave a lecture that I heard as an undergraduate, and I thought that here was a great system for investigating the things in biology that I thought were really, really interesting. So, when I graduated I went and persuaded Brenner take me on, and he —somewhat reluctantly—did. He was inspirational and brilliant in all ways, truly extraordinary. And it was indeed a great system. It has gone off in all sorts of directions and keeps on generating new lines of research and new amazing discoveries.

What’s the most memorable moment from your career so far?

Probably when I realized the spectacular effect of a particular mutation on sex determination in C. elegans. It was a dominant mutation that caused the animal to change sex completely, and I’d previously found mutations in the same gene that caused the absolutely opposite transformation. It was just astonishing to realize that with that one gene you control everything about the animal’s sex.  This was a very satisfying and elegant result, which came together in a fairly short time.

Who have been your most important mentors?

At university, I was very lucky to be taught directly by a superb yeast geneticist, Brian Cox, whom I much admired. He did a lot of things that were underappreciated; for example, he discovered one of the systems that turned out to involve a yeast prion. He made it clear to me how immensely powerful genetics is. Then as a grad student, Sydney Brenner obviously, but also other people at the MRC Laboratory in Cambridge, like molecular biologist Mark Bretscher, who was very influential and full of good advice, and also inspirational people like Francis Crick. Francis was the only person smarter than Sydney at the MRC Lab!!

What types of questions are you fascinated by?

The big questions in development. Looking back on it now, I think what got me into research —and what I thought C. elegans was particularly powerful for—is a very difficult question that remains in many ways completely unanswered. How do you specify complicated behavior genetically? It’s obvious from all sorts of examples of instinctive behavior that they must be genetically programmed. How on earth do you do that? We have no idea whatsoever! We’ve made enormous progress in understanding development and the basis of the nervous system, but how do you do genetically specify things like the behavior of crows that can make tools out of bits of leaf? They don’t need to learn that! If you take a New Caledonian crow and allow it to hatch and grow up in isolation, after a while it will start finding bits of leaves and turning them into tools for picking up insects. I’m baffled by how such behavior can be generated, and I’d love to know the mechanism.  C. elegans has such simple behavior that we may   be able to eventually understand how it’s specified, though the more we more we learn about the worm, the fancier its behavior becomes. I don’t work in this area anymore, but the people who do are making nice progress. The most sophisticated aspects of it are still very mysterious though, and big questions remain unanswered.

What are you currently working on?

I’m working on how worms and bacteria interact with each other, and how the worms are able to fight off disease, how they recognize that they’ve got a disease, how infection happens, how some bacteria are able to infect some worms and not other worms. That involves a lot of interesting questions that still haven’t been answered, but it’s also led to lots of unexpected things along the way. For example, finding a bacterium that kills worms by causing them to stick together by their tails, but the worms can sometimes escape by dividing themselves in two. Nobody knew that nematodes s could do autotomy, so that was really surprising!

Autotomy raises questions about how they manage to do it. Unfortunately, so far, the half-worms survive but won’t regenerate. If you cut Planaria in half they will regenerate, but nematodes do not. Or at least we can’t get them to do it, which doesn’t necessarily mean it doesn’t happen.

If you hadn’t been a scientist, what would you have liked to have become?

Probably an archaeologist. I spent a lot of time hanging out with archaeologists on excavations until about the end of 1980s. Archaeology is like doing biological research in that you’re always discovering things, and it’s like genetics in that it’s open ended. The trouble with being an archeologist is there’s a factor of maybe 100 in the difference in employability between a biologist and an archaeologist.

What’s the best advice you ever received? 

Perhaps what Mark Bretscher said to me, which was: everything depends on having a good assay. I think that this is true whatever kind of experimental science you do, and it’s something I’ve always kept in mind. Avoid assays that are too arduous. Sometimes you have no choice, but if you can, try to get a stringent assay and one that’s easy to do.

What advice would you give to younger scientists?

Hang in there. It has gotten harder. It would be hard to deny, both in terms of career prospects and in terms of some of the questions that are out there, that it’s not as easy to do research as once it was.  But on the other hand, science goes on being enormously rewarding and enjoyable, and amongst other things there is a very strong community. You find yourself working with a lot of people who are interested in the same things and are all very bright, very funny, and very agreeable. That was the great thing about working at the Molecular Biology Laboratory in Cambridge: you instantly realized that it was a wonderful environment, nonhierarchical and dedicated and excited about research. Many other occupations can involve more or less going around in circles, or trying to solve incredibly difficult social problems. There aren’t many careers that go on being satisfying and fascinating in quite the same way as this one—or where you actually get to move back the frontiers of knowledge.

Genes involved in male reproduction tend to evolve rapidly. This has been observed in many different species and is thought to be due to sexual selection as males compete over mating opportunities. But in the August issue of GENETICS, Whittle and Extavour present results that flip this paradigm upside down. They find that in the yellow-fever mosquito, female-biased genes expressed in the ovaries evolve faster than their male counterparts. This fascinating break from the trend could be due to increased competition between females for mates, adaptive evolution during egg-sperm attraction, and/or limited sperm competition in this species.

First, the authors identified genes more highly expressed in either male or female gonads using whole transcriptome data. They found that ovarian-biased expression was typically due to elevated expression in females, not just reduced expression in males as has been observed in other species. They then identified nucleotide changes that altered the protein composition in these genes to compare the rates of protein evolution. Although a small subset of testis-biased genes were evolving rapidly, on average transcripts with ovary-biased expression showed a significantly higher protein evolution rate than those with testis-biased expression. Genes expressed only in the ovaries had the fastest protein evolution rate of all. They determined that the rapid evolution of some of these genes is most likely due to positive selection using a phylogenetic analysis including two other mosquito species.   

Interestingly, members of this set of rapidly-evolving, ovary-specific genes have functions preferentially related to the mosquito’s olfactory system, including odor molecule binding and smell receptor activity. Olfactory signaling appears to be important for mosquito mating; groups of males will gather together and swarm females, who are lured over by their scent. These types of chemical cues may also be important for guiding the sperm to the egg or directing females to store sperm after mating. Like some other insects, female mosquitoes have special storage organs that allow them to keep enough sperm from a single mating to fertilize all their eggs throughout their entire lives. There may be strong selective pressure on proteins that drive evolution of these critical reproductive functions.

The yellow-fever mosquito’s mating system is likely behind its unusual rapid evolution of ovary-biased genes. There might be competition between females to attract males or male mate choice, which could result in strong sexual selection on ovary-expressed genes involved in chemical sensing. These females usually mate once, and the male deposits a “copulation plug” in the female’s reproductive tract. This physical and chemical barrier prevents the sperm of another male from passing through, ensuring the first male will father all her offspring. The plug cuts off nearly any chance of competition between the sperm of multiple males, in contrast to many other organisms where the rapid evolution of testis-biased genes could be due to the pressure of this sperm competition arms race.

A combination of these diverse factors likely influences the rapid protein evolution of ovary-biased genes in yellow-fever mosquitoes. These results offer a fascinating glimpse into how ecology and reproductive lifestyle can affect genome evolution and illustrate how there are notable exceptions for every trend observed in nature.

CITATION:

Rapid Evolution of Ovarian-Biased Genes in the Yellow Fever Mosquito (Aedes aegypti)

Carrie A. Whittle and Cassandra G. Extavour

Genetics August 2017 206: 2119-2137

https://doi.org/10.1534/genetics.117.201343

http://www.genetics.org/content/206/4/2119

Fascinating discoveries sometimes emerge from the most daunting of experimental roadblocks. Designed to generate over 1,000 recombinant inbred mice lines for genetic mapping, the Collaborative Cross (CC) project unearthed astounding variation in male fertility when nearly 95% of the highly inbred CC lines went extinct. As part of the Multiparental Populations series in the June issue of GENETICS, Shorter et al. use these fortuitous results to map the genetic variation underlying differences in male fertility and other reproductive traits. Their findings suggest the infertility in these lines is caused by genetic variants distributed across the genome, revealing incompatibilities between subspecies.

The CC project was designed as a powerful genetic mapping population consisting of thousands of highly inbred lines that are extremely genetically different from each other. The population founders came from several common varieties of lab mice, as well as wild-derived animals representing the three mouse subspecies. All of these lines were crossed to incorporate as much genetic variation in the population as possible. The hybrid offspring were then inbred to create high homozygosity within a line. As the lines became more and more inbred, something unexpected began to happen.

From the start, the collaborating research teams agreed they would not undertake “heroic” efforts to save lines that were struggling to persist due to high mortality or low reproduction. This policy changed as the CC lines began to go extinct at an alarming rate. In the end, 95% of the CC lines were lost despite the efforts of researchers to maintain them through between-line crosses and male fertility testing. Although some extinctions are expected as the hidden phenotypes of deleterious alleles are progressively revealed by inbreeding, the number observed far exceeded these expectations.

The culprit behind this perplexing mouse mass extinction was male infertility; nearly half of the failed lines included males that were unable to sire offspring. This gave the authors an opportunity to turn lemons into lemonade: they decided to map male reproductive traits to identify the underlying genetic basis of the problems in the extinct CC lines. They found that the contribution of the X-chromosome and the autosomes to the genomes of the extinct lines was different, with the extinct lines showing a deficit in X-linked haplotypes from the wild-derived founders. This suggests selection against retaining wild alleles at X-linked genes during the inbreeding process. Looking more closely at the extinct lines, they found very high variability in sperm count, sperm motility, and reproductive organ weights. They performed QTL mapping on fertility and reproductive traits using the extinct CC lines and identified several loci across the genome associated with variation in reproductive phenotypes. One identified locus on the X-chromosome contained a region previously identified as affecting hybrid incompatibility and speciation.

They also found that the majority of haplotypes associated with infertility and poor reproductive traits came from the wild-derived founders of different subspecies than common lab mice. It seems that genetic incompatibility between these distinct subspecies causes male infertility and reproductive isolation. Indeed, the surviving CC strains were found to have a deficit of genetic contributions from these founders across their entire genome.

The wild-derived mice were meant to provide as much genetic variation as possible to the CC lines, but this variation has turned out to be a double-edged sword. The crossing scheme between these diverged subspecies created new genetic combinations that disrupted male reproduction—often one of the first processes to be affected during speciation. Although the line extinction was unplanned and unwanted, it also provided a unique opportunity to dissect the genetics of male reproduction and the early stages of species isolation in mammals.

CITATION

Male Infertility Is Responsible for Nearly Half of the Extinction Observed in the Mouse Collaborative Cross  

John R. Shorter, Fanny Odet, David L. Aylor, Wenqi Pan, Chia-Yu Kao, Chen-Ping Fu, Andrew P. Morgan, Seth Greenstein, Timothy A. Bell, Alicia M. Stevans, Ryan W. Feathers, Sunny Patel, Sarah E. Cates, Ginger D. Shaw, Darla R. Miller, Elissa J. Chesler, Leonard McMillian, Deborah A. O’Brien, and Fernando Pardo-Manuel de Villena

Genetics June 2017 206: 557-572

http://www.genetics.org/content/206/2/557

https://doi.org/10.1534/genetics.116.199596

We are pleased to announce that Jonathan Hodgkin, PhD is the 2017 recipient of the Edward Novitski Prize in recognition of his extraordinary creativity and intellectual ingenuity in solving significant problems in genetics research. Hodgkin uncovered the sex determination pathway in Caenorhabditis elegans, an important and widely used model for animal development and genetics. His innovations and contributions to genetic analysis, including the use of suppressor screens and epistasis analyses, helped advance the field in many ways. Hodgkin is a Professor in the Department of Biochemistry at the University of Oxford.

Jonathan Hodgkin is the winner of the 2017 Novitski Prize.

“Jonathan Hodgkin is a major innovator in C. elegans genetics and has tirelessly served the model organism community for decades,” says Eric Haag, PhD (University of Maryland). When Hodgkin began working in the 1970s, the main model for sex determination was the mammalian XY system. Unlike mammals, most C. elegans individuals are hermaphrodites and males are rare. These males have an X0 genotype, and typically result from chromosomal nondisjunction events during meiosis. Hodgkin was the first to perform screens to find mutations that turned XX hermaphrodites into males. From this beginning, he used elegant genetic studies to identify important components of the C. elegans sex determination pathway. He inferred the order of genes in the pathway and their modes of regulation using epistasis analyses, a powerful tool that was quickly adopted by other researchers.

“Jonathan’s work on sexual fate specification demonstrated to model organism researchers the power of genetic analysis to dissect biological processes and helped to define principles in sex determination that we now take for granted,” says Tim Schedl, PhD (Washington University of St. Louis). Hodgkin’s work uncovering and manipulating the pathway in C. elegans helped reveal the amazing flexibility of genetic sex determination, which is achieved by mechanisms unique to different animal groups.

Hodgkin led the field in several other areas in addition to his work on sex determination. He expanded the number and use of informational suppressor mutants in C. elegans, which are able to act on many genes. He also introduced the use of collections of wild C. elegans to study naturally occurring genetic variation, paving the way for SNP mapping and QTL analysis, as well as studies of hybrid incompatibilities between worm species.

The Novitski Prize recognizes a single experimental accomplishment or a body of work in which an exceptional level of creativity and intellectual ingenuity that has been used to design and execute scientific experiments to solve a difficult problem in genetics. It recognizes the beautiful and intellectually ingenious experimental design and execution involved in genetics scientific discovery. The Prize, established by the Novitski family and GSA, honors the memory of Edward Novitski (1918-2006), a Drosophila geneticist and lifelong GSA member who specialized in chromosome mechanics and elucidating meiosis through the construction of modified chromosomes.

The Prize will be presented to Hodgkin at the 21st International C. elegans Conference, which will be held June 21-25 at the University of California, Los Angeles.

To learn more about the GSA awards, and to view a list of previous recipients, please see http://www.genetics-gsa.org/awards.

Sex chromosomes have evolved from autosomes hundreds of times across the tree of life. In mammals, sex is controlled by the Y chromosome-linked gene SRY, which triggers the development of male anatomy. Sex determination in most mammals is extremely conserved; essentially all marsupials and placental mammals share the same pair of X and Y chromosomes which originated millions of years ago in their common ancestor. Birds also share a very ancient pair of sex chromosomes, but use a ZW sex determination system, in which the W chromosome is found only in females.

But this evolutionary stability is not the case in all groups of vertebrates. In many reptiles and amphibians, sex chromosomes are extremely variable both between species and across groups. In the latest issue of G3, Furman and Evans show that new sex chromosomes have evolved rapidly in a group of five African clawed frog species in the genus Xenopus. These frogs have a ZW sex determination system just as birds do. Previous work has shown that a W-linked gene called DM-W is the trigger for female sexual development in some species this group, but one species, X. borealis, is missing this gene. Furman and Evans used a combination of publically available previously sequenced genomic DNA and newly sequenced transcriptomes to unearth how sex chromosomes and sex determination has evolved in this group.

First, the researchers built a robust phylogenetic tree that confirms that DM-W was present in the shared group ancestor and has since been lost in X. borealis. They also confirmed that DM-W is female specific in the species that is most closely related to X. borealis, indicating the gene was likely sex-linked in the shared ancestor of the whole group. That means the new sex determination system in X. borealis evolved recently.

So if DM-W is no longer the trigger for sex determination in X. borealis, then what is? To answer this question, Furman and Evans collected a large number of random genomic DNA sequences from an X. borealis mother and father and their male and female offspring. They sorted through this dataset to find SNPs with sex-biased inheritance patterns, finding a total of 24 SNPs that were heterozygous in the mother and the daughters, but homozygous in the father and sons. This pattern suggests these SNPs are present on the female-specific W chromosome. These fragments are homologous to regions on one autosome in the other Xenopus species. At some point in X. borealis, that ancestral autosome acquired a sex determination gene and a new sex chromosome was born.

In addition to being recently evolved, the X. borealis sex chromosome system has some surprising properties. It carries three genes that are also located on the mammalian X. Several of these genes are known to be sex-linked in other distantly related lineages, including some fish and lizards. So although autosomes can become sex chromosomes quite rapidly, these findings suggest that some regions of the genome might be predisposed to develop sex determination functions. Though genetic sex determination is relatively common in nature, it is only well understood in a handful of species. This study hints at broader evolutionary patterns that could explain how new sex chromosomes arise repeatedly within lineages without disrupting the critical process of becoming male or female.

Furman, B. L., & Evans, B. J. (2016). Sequential turnovers of sex chromosomes in African clawed frogs (Xenopus) suggest some genomic regions are good at sex determination. G3: Genes| Genomes| Genetics, g3-116. DOI: 10.1534/g3.116.033423

http://www.g3journal.org/content/6/11/3625.full