Molecular analysis of the C. elegans sex-determining gene tra-1: a gene encoding two zinc finger proteins

Chronic thermal stress on Octopus maya embryos down-regulates epigenome-related genes and those involved in the nervous system development and morphogenesis

Red Octopus maya is strongly influenced by temperature. Recent studies have reported negative reproduction effects on males and females when exposed to temperatures higher than 27 °C. Embryos under thermal stress show morphological and physiological alterations; similar phenotypes have been reported in embryos from stressed females, evidencing transgenerational consequences. Transcriptomic profiles were characterized along embryo development during normal-under thermal stress and epigenetic alterations through DNA methylation and damage quantification. Total RNA in organogenesis, activation, and growth stages in control and thermal stress were sequenced with Illumina RNA-Seq. Similarly, total DNA was used for DNA methylation and damage quantification between temperatures and embryo stages. Differential gene expression analyses showed that embryos express genes associated with oxygen transport, morphogenesis, nervous system, neuroendocrine cell differentiation, spermatogenesis, and male sex differentiation. Conversely, embryos turn off genes involved mainly in nervous system development, morphogenesis, and gene expression regulation when exposed to thermal stress - consistent with O. maya embryo phenotypes showing abnormal arms, eyes, and body development. No significant differences were observed in quantifying DNA methylation between temperatures but they were for DNA damage quantification. Epigenetic alterations are hypothesized to occur since several genes found downregulated belong to the epigenetic machinery but at histone tail level.

Rewiring the Sex-Determination Pathway During the Evolution of Self-Fertility

Although evolution is driven by changes in how regulatory pathways control development, we know little about the molecular details underlying these transitions. The TRA-2 domain that mediates contact with TRA-1 is conserved in Caenorhabditis. By comparing the interaction of these proteins in two species, we identified a striking change in how sexual development is controlled. Identical mutations in this domain promote oogenesis in Caenorhabditis elegans but promote spermatogenesis in Caenorhabditis briggsae. Furthermore, the effects of these mutations involve the male-promoting gene fem-3 in C. elegans but are independent of fem-3 in C. briggsae. Finally, reciprocal mutations in these genes show that C. briggsae TRA-2 binds TRA-1 to prevent expression of spermatogenesis regulators. By contrast, in C. elegans TRA-1 sequesters TRA-2 in the germ line, allowing FEM-3 to initiate spermatogenesis. Thus, we propose that the flow of information within the sex determination pathway has switched directions during evolution. This result has important implications for how evolutionary change can occur.

Let's talk about sex: Mechanisms of neural sexual differentiation in Bilateria

In multicellular organisms, sexed gonads have evolved that facilitate release of sperm versus eggs, and bilaterian animals purposefully combine their gametes via mating behaviors. Distinct neural circuits have evolved that control these physically different mating events for animals producing eggs from ovaries versus sperm from testis. In this review, we will describe the developmental mechanisms that sexually differentiate neural circuits across three major clades of bilaterian animals-Ecdysozoa, Deuterosomia, and Lophotrochozoa. While many of the mechanisms inducing somatic and neuronal sex differentiation across these diverse organisms are clade-specific rather than evolutionarily conserved, we develop a common framework for considering the developmental logic of these events and the types of neuronal differences that produce sex-differentiated behaviors. This article is categorized under: Congenital Diseases > Stem Cells and Development Neurological Diseases > Stem Cells and Development.

CaMKII mediates sexually dimorphic synaptic transmission at neuromuscular junctions in C. elegans

Sexually dimorphic behaviors are ubiquitous throughout the animal kingdom. Although both sex-specific and sex-shared neurons have been functionally implicated in these diverse behaviors, less is known about the roles of sex-shared neurons. Here, we discovered sexually dimorphic cholinergic synaptic transmission in C. elegans occurring at neuromuscular junctions (NMJs), with males exhibiting increased release frequencies, which result in sexually dimorphic locomotion behaviors. Scanning electron microscopy revealed that males have significantly more synaptic vesicles (SVs) at their cholinergic synapses than hermaphrodites. Analysis of previously published transcriptome identified the male-enriched transcripts and focused our attention on UNC-43/CaMKII. We ultimately show that differential accumulation of UNC-43 at cholinergic neurons controls axonal SV abundance and synaptic transmission. Finally, we demonstrate that sex reversal of all neurons in hermaphrodites generates male-like cholinergic transmission and locomotion behaviors. Thus, beyond demonstrating UNC-43/CaMKII as an essential mediator of sex-specific synaptic transmission, our study provides molecular and cellular insights into how sex-shared neurons can generate sexually dimorphic locomotion behaviors.

Temporal transitions in the postembryonic nervous system of the nematode Caenorhabditis elegans: Recent insights and open questions

After the generation, differentiation and integration into functional circuitry, post-mitotic neurons continue to change certain phenotypic properties throughout postnatal juvenile stages until an animal has reached a fully mature state in adulthood. We will discuss such changes in the context of the nervous system of the nematode C. elegans, focusing on recent descriptions of anatomical and molecular changes that accompany postembryonic maturation of neurons. We summarize the characterization of genetic timer mechanisms that control these temporal transitions or maturational changes, and discuss that many but not all of these transitions relate to sexual maturation of the animal. We describe how temporal, spatial and sex-determination pathways are intertwined to sculpt the emergence of cell-type specific maturation events. Finally, we lay out several unresolved questions that should be addressed to move the field forward, both in C. elegans and in vertebrates.

X-chromosome target specificity diverged between dosage compensation mechanisms of two closely related Caenorhabditis species

An evolutionary perspective enhances our understanding of biological mechanisms. Comparison of sex determination and X-chromosome dosage compensation mechanisms between the closely related nematode species Caenorhabditis briggsae (Cbr) and Caenorhabditis elegans (Cel) revealed that the genetic regulatory hierarchy controlling both processes is conserved, but the X-chromosome target specificity and mode of binding for the specialized condensin dosage compensation complex (DCC) controlling X expression have diverged. We identified two motifs within Cbr DCC recruitment sites that are highly enriched on X: 13 bp MEX and 30 bp MEX II. Mutating either MEX or MEX II in an endogenous recruitment site with multiple copies of one or both motifs reduced binding, but only removing all motifs eliminated binding in vivo. Hence, DCC binding to Cbr recruitment sites appears additive. In contrast, DCC binding to Cel recruitment sites is synergistic: mutating even one motif in vivo eliminated binding. Although all X-chromosome motifs share the sequence CAGGG, they have otherwise diverged so that a motif from one species cannot function in the other. Functional divergence was demonstrated in vivo and in vitro. A single nucleotide position in Cbr MEX can determine whether Cel DCC binds. This rapid divergence of DCC target specificity could have been an important factor in establishing reproductive isolation between nematode species and contrasts dramatically with the conservation of target specificity for X-chromosome dosage compensation across Drosophila species and for transcription factors controlling developmental processes such as body-plan specification from fruit flies to mice.

The great small organisms of developmental genetics: Caenorhabditis elegans and Drosophila melanogaster

Experimental embryologists working at the turn of the 19th century suggested fundamental mechanisms of development, such as localized cytoplasmic determinants and tissue induction. However, the molecular basis underlying these processes proved intractable for a long time, despite concerted efforts in many developmental systems to isolate factors with a biological role. That road block was overcome by combining developmental biology with genetics. This powerful approach used unbiased genome-wide screens to isolate mutants with developmental defects and to thereby identify genes encoding key determinants and regulatory pathways that govern development. Two small invertebrates were the pioneers: the fruit fly Drosophila melanogaster and the nematode Caenorhabditis elegans. Their modes of development differ in many ways, but the two together led the way to unraveling the molecular mechanisms of many fundamental developmental processes. The discovery of the grand homologies between key players in development throughout the animal kingdom underscored the usefulness of studying these small invertebrate models for animal development and even human disease. We describe developmental genetics in Drosophila and C. elegans up to the rise of genomics at the beginning of the 21st Century. Finally, we discuss themes that emerge from the histories of such distinct organisms and prospects of this approach for the future.

The transcriptional landscape of the giant freshwater prawn: Embryonic development and early sexual differentiation mechanisms

The giant freshwater prawn pjMacrobrachium rosenbergii is one of the best studied species in aquaculture. However, the transcriptional changes associated with embryonic development and the sexual differentiation mechanism of M. rosenbergii remain to be elucidated. To characterize the embryonic development of this prawn and to determine whether differential expression and differential splicing play roles in the early sexual differentiation of M. rosenbergii, we profiled five developmental days of male and female embryos by RNA sequencing. We identified modules of co-expressed genes representing waves of transcription that correspond to physiological processes in early embryonic development (such as the maternal-to-zygotic transition) up to preparation for life outside the egg (development of muscles, cuticle etc.). Additionally, we found that hundreds of genes are differentially expressed between sexes, most of them uncharacterized, suggesting that the sex differentiation mechanism of M. rosenbergii might contain clade-specific elements. The resulting first-of-a-kind transcriptional map of embryonic development of male and female M. rosenbergii will guide future studies to reveal the roles of specific genes and splicing isoforms in the embryonic development and sexual differentiation process of M. rosenbergii.

Sulforaphane Targets TRA-1/GLI Upstream of DAF-16/FOXO to Promote C. elegans Longevity and Healthspan

Broccoli-derived isothiocyanate sulforaphane inhibits inflammation and cancer. Sulforaphane may support healthy aging, but the underlying detailed mechanisms are unclear. We used the C. elegans nematode model to address this question. Wild-type and 4 mutant C. elegans worm strains were fed in the presence or absence of sulforaphane and E. coli food bacteria transfected with RNA interference gene constructs. Kaplan-Meier survival analysis, live imaging of mobility and pharyngeal pumping, fluorescence microscopy, RT-qPCR, and Western blotting were performed. In the wild type, sulforaphane prolonged lifespan and increased mobility and food intake because of sulforaphane-induced upregulation of the sex-determination transcription factor TRA-1, which is the ortholog of the human GLI mediator of sonic hedgehog signaling. In turn, the tra-1 target gene daf-16, which is the ortholog of human FOXO and the major mediator of insulin/IGF-1 and aging signaling, was induced. By contrast, sulforaphane did not prolong lifespan and healthspan when tra-1 or daf-16 was inhibited by RNA interference or when worms with a loss-of-function mutation of the tra-1 or daf-16 genes were used. Conversely, the average lifespan of C. elegans with hyperactive TRA-1 increased by 8.9%, but this longer survival was abolished by RNAi-mediated inhibition of daf-16. Our data suggest the involvement of sulforaphane in regulating healthy aging and prolonging lifespan by inducing the expression and nuclear translocation of TRA-1/GLI and its downstream target DAF-16/FOXO.

Molecular Mechanisms of Sexually Dimorphic Nervous System Patterning in Flies and Worms

Male and female brains display anatomical and functional differences. Such differences are observed in species across the animal kingdom, including humans, but have been particularly well-studied in two classic animal model systems, the fruit fly Drosophila melanogaster and the nematode Caenorhabditis elegans. Here we summarize recent advances in understanding how the worm and fly brain acquire sexually dimorphic features during development. We highlight the advantages of each system, illustrating how the precise anatomical delineation of sexual dimorphisms in worms has enabled recent analysis into how these dimorphisms become specified during development, and how focusing on sexually dimorphic neurons in the fly has enabled an increasingly detailed understanding of sex-specific behaviors.

Sex Determination: TRA-1 Is a Non-binary Regulator of Sexual Identity

In nematodes, TRA-1 represses the transcription of genes involved in male differentiation, allowing XX animals to undergo normal hermaphrodite development. New reports show that this transcription factor also acts in XO males, to control the differentiation of many neurons.

Transcriptional profiles in Strongyloides stercoralis males reveal deviations from the Caenorhabditis sex determination model

The human and canine parasitic nematode Strongyloides stercoralis utilizes an XX/XO sex determination system, with parasitic females reproducing by mitotic parthenogenesis and free-living males and females reproducing sexually. However, the genes controlling S. stercoralis sex determination and male development are unknown. We observed precocious development of rhabditiform males in permissive hosts treated with corticosteroids, suggesting that steroid hormones can regulate male development. To examine differences in transcript abundance between free-living adult males and other developmental stages, we utilized RNA-Seq. We found two clusters of S. stercoralis-specific genes encoding predicted transmembrane proteins that are only expressed in free-living males. We additionally identified homologs of several genes important for sex determination in Caenorhabditis species, including mab-3, tra-1, fem-2, and sex-1, which may have similar functions. However, we identified three paralogs of gld-1; Ss-qki-1 transcripts were highly abundant in adult males, while Ss-qki-2 and Ss-qki-3 transcripts were highly abundant in adult females. We also identified paralogs of pumilio domain-containing proteins with sex-specific transcripts. Intriguingly, her-1 appears to have been lost in several parasite lineages, and we were unable to identify homologs of tra-2 outside of Caenorhabditis species. Together, our data suggest that different mechanisms control male development in S. stercoralis and Caenorhabditis species.

The development of sex differences in the nervous system and behavior of flies, worms, and rodents

Understanding how sex differences in innate animal behaviors arise has long fascinated biologists. As a general rule, the potential for sex differences in behavior is built by the developmental actions of sex-specific hormones or regulatory proteins that direct the sexual differentiation of the nervous system. In the last decade, studies in several animal systems have uncovered neural circuit mechanisms underlying discrete sexually dimorphic behaviors. Moreover, how certain hormones and regulatory proteins implement the sexual differentiation of these neural circuits has been illuminated in tremendous detail. Here, we discuss some of these mechanisms with three case-studies-mate recognition in flies, maturation of mating behavior in worms, and play-fighting behavior in young rodents. These studies illustrate general and unique developmental mechanisms to establish sex differences in neuroanatomy and behavior and highlight future challenges for the field.

The conserved molting/circadian rhythm regulator NHR-23/NR1F1 serves as an essential co-regulator of C. elegans spermatogenesis

In sexually reproducing metazoans, spermatogenesis is the process by which uncommitted germ cells give rise to haploid sperm. Work in model systems has revealed mechanisms controlling commitment to the sperm fate, but how this fate is subsequently executed remains less clear. While studying the well-established role of the conserved nuclear hormone receptor transcription factor, NHR-23/NR1F1, in regulating C. elegans molting, we discovered that NHR-23/NR1F1 is also constitutively expressed in developing primary spermatocytes and is a critical regulator of spermatogenesis. In this novel role, NHR-23/NR1F1 functions downstream of the canonical sex-determination pathway. Degron-mediated depletion of NHR-23/NR1F1 within hermaphrodite or male germlines causes sterility due to an absence of functional sperm, as depleted animals produce arrested primary spermatocytes rather than haploid sperm. These spermatocytes arrest in prometaphase I and fail to either progress to anaphase or attempt spermatid-residual body partitioning. They make sperm-specific membranous organelles but fail to assemble their major sperm protein into fibrous bodies. NHR-23/NR1F1 appears to function independently of the known SPE-44 gene regulatory network, revealing the existence of an NHR-23/NR1F1-mediated module that regulates the spermatogenesis program.

Ubiquitin-dependent regulation of a conserved DMRT protein controls sexually dimorphic synaptic connectivity and behavior

Sex-specific synaptic connectivity is beginning to emerge as a remarkable, but little explored feature of animal brains. We describe here a novel mechanism that promotes sexually dimorphic neuronal function and synaptic connectivity in the nervous system of the nematode Caenorhabditis elegans. We demonstrate that a phylogenetically conserved, but previously uncharacterized Doublesex/Mab-3 related transcription factor (DMRT), dmd-4, is expressed in two classes of sex-shared phasmid neurons specifically in hermaphrodites but not in males. We find dmd-4 to promote hermaphrodite-specific synaptic connectivity and neuronal function of phasmid sensory neurons. Sex-specificity of DMD-4 function is conferred by a novel mode of posttranslational regulation that involves sex-specific protein stabilization through ubiquitin binding to a phylogenetically conserved but previously unstudied protein domain, the DMA domain. A human DMRT homolog of DMD-4 is controlled in a similar manner, indicating that our findings may have implications for the control of sexual differentiation in other animals as well.

What about the males? the C. elegans sexually dimorphic nervous system and a CRISPR-based tool to study males in a hermaphroditic species

Sexual dimorphism is a device that supports genetic diversity while providing selective pressure against speciation. This phenomenon is at the core of sexually reproducing organisms. Caenorhabditis elegans provides a unique experimental system where males exist in a primarily hermaphroditic species. Early works of John Sulston, Robert Horvitz, and John White provided a complete map of the hermaphrodite nervous system, and recently the male nervous system was added. This addition completely realized the vision of C. elegans pioneer Sydney Brenner: a model organism with an entirely mapped nervous system. With this 'connectome' of information available, great strides have been made toward understanding concepts such as how a sex-shared nervous system (in hermaphrodites and males) can give rise to sex-specific functions, how neural plasticity plays a role in developing a dimorphic nervous system, and how a shared nervous system receives and processes external cues in a sexually-dimorphic manner to generate sex-specific behaviors. In C. elegans, the intricacies of male-mating behavior have been crucial for studying the function and circuitry of the male-specific nervous system and used as a model for studying human autosomal dominant polycystic kidney disease (ADPKD). With the emergence of CRISPR, a seemingly limitless tool for generating genomic mutations with pinpoint precision, the C. elegans model system will continue to be a useful instrument for pioneering research in the fields of behavior, reproductive biology, and neurogenetics.

Dynamic, Non-binary Specification of Sexual State in the C. elegans Nervous System

Biological sex in animals is often considered a fixed, individual-level characteristic. However, not all sex-specific features are static: for example, C. elegans males (XO) can sometimes exhibit hermaphrodite (XX)-like feeding behavior [1, 2]. (C. elegans hermaphrodites are somatic females that transiently produce self-sperm.) Essentially all somatic sex differences in C. elegans are governed by the master regulator tra-1, whose activity is controlled by chromosomal sex and is necessary and sufficient to specify the hermaphrodite state [3]. One aspect of this state is high expression of the chemoreceptor odr-10. In hermaphrodites, high odr-10 expression promotes feeding, but in males, low odr-10 expression facilitates exploration [4]. However, males suppress this sex difference in two contexts: juvenile males exhibit high odr-10 expression and food deprivation activates odr-10 in adult males [4-6]. Remarkably, we find that both of these phenomena require tra-1. In juvenile (L3) males, tra-1 is expressed in numerous neurons; this expression diminishes as individuals mature into adulthood, a process that requires conserved regulators of sexual maturation. tra-1 remains expressed in a small number of neurons in adult males, where it likely has a permissive role in odr-10 activation. Thus, the neuronal functions of tra-1 are not limited to hermaphrodites; rather, tra-1 also acts in the male nervous system to transiently suppress a sexual dimorphism, developmentally and in response to nutritional stress. Our results show that the molecular and functional representation of sexual state in C. elegans is neither static nor homogeneous, challenging traditional notions about the nature of biological sex.

Sex-specific regulation of neuronal functions in Caenorhabditis elegans: the sex-determining protein TRA-1 represses goa-1/Gα(i/o)

Females and males differ substantially in various neuronal functions in divergent, sexually dimorphic animal species, including humans. Despite its developmental, physiological and medical significance, understanding the molecular mechanisms by which sex-specific differences in the anatomy and operation of the nervous system are established remains a fundamental problem in biology. Here, we show that in Caenorhabditis elegans (nematodes), the global sex-determining factor TRA-1 regulates food leaving (mate searching), male mating and adaptation to odorants in a sex-specific manner by repressing the expression of goa-1 gene, which encodes the Gα(i/o) subunit of heterotrimeric G (guanine–nucleotide binding) proteins triggering physiological responses elicited by diverse neurotransmitters and sensory stimuli. Mutations in tra-1 and goa-1 decouple behavioural patterns from the number of X chromosomes. TRA-1 binds to a conserved binding site located in the goa-1 coding region, and downregulates goa-1 expression in hermaphrodites, particularly during embryogenesis when neuronal development largely occurs. These data suggest that the sex-determination machinery is an important modulator of heterotrimeric G protein-mediated signalling and thereby various neuronal functions in this organism and perhaps in other animal phyla.

A natural antisense transcript of the fem-1 gene was found expressed in female gonads during the characterization, expression profile, and cellular localization of the fem-1 gene in Pacific white shrimp Penaeus vannamei

The fem-1 gene in Caenorhabditis elegans is involved in sex differentiation; it is specifically required for all aspects of male development. In this study, the full-length cDNA of the fem-1 (Pvfem-1) gene was isolated from the Pacific whiteleg shrimp Penaeus vannamei. The Pvfem-1 transcript is 3778 nt long and encodes a putative protein (PvFEM-1) of 638 amino acids that presented eight ankyrin repeats. The translated protein showed a significant (P < 0.05) structural similitude by superposition with C. elegans FEM-1 protein. Pvfem-1 expression was evaluated by qPCR and in situ hybridization (ISH) during embryogenesis, larval development, and gonads of both genders in subadult and adult life stages. Pvfem-1 was found expressed in brain, intestine, hepatopancreas, and in the gonads of both genders in subadults and adults when quantified by RT-qPCR. A significant finding was the discovery of a natural antisense transcript (NAT) of Pvfem-1 by ISH. It was present in the oocyte nucleus of subadult female shrimp gonads but was not seen within oocytes from adult females, although it was detected in follicular cells, suggesting a possible post-transcriptional regulation of Pvfem-1 in female gonad. Conversely, in males, no NAT was observed, and Pvfem-1 was found expressed in spermatogonia of both, subadult and adult shrimps indicating a function in male sexual differentiation and gametes generation. This study represents the first step for future functional analysis that is expected to contribute to clarifying the role of Pvfem-1 in sex differentiation and determination.

Chromosome-Wide Evolution and Sex Determination in the Three-Sexed Nematode Auanema rhodensis

Trioecy, a mating system in which males, females and hermaphrodites co-exist, is a useful system to investigate the origin and maintenance of alternative mating strategies. In the trioecious nematode Auanema rhodensis, males have one X chromosome (XO), whereas females and hermaphrodites have two (XX). The female vs. hermaphrodite sex determination mechanisms have remained elusive. In this study, RNA-seq analyses show a 20% difference between the L2 hermaphrodite and female gene expression profiles. RNAi experiments targeting the DM (doublesex/mab-3) domain transcription factor dmd-10/11 suggest that the hermaphrodite sexual fate requires the upregulation of this gene. The genetic linkage map (GLM) shows that there is chromosome-wide heterozygosity for the X chromosome in F2 hermaphrodite-derived lines originated from crosses between two parental inbred strains. These results confirm the lack of recombination of the X chromosome in hermaphrodites, as previously reported. We also describe conserved chromosome elements (Nigon elements), which have been mostly maintained throughout the evolution of Rhabditina nematodes. The seven-chromosome karyotype of A. rhodensis, instead of the typical six found in other rhabditine species, derives from fusion/rearrangements events involving three Nigon elements. The A. rhodensis X chromosome is the smallest and most polymorphic with the least proportion of conserved genes. This may reflect its atypical mode of father-to-son transmission and its lack of recombination in hermaphrodites and males. In conclusion, this study provides a framework for studying the evolution of chromosomes in rhabditine nematodes, as well as possible mechanisms for the sex determination in a three-sexed species.

Construction of a Genomic Bacterial Artificial Chromosome (BAC) Library for the Prawn Macrobrachium rosenbergii and Initial Analysis of ZW Chromosome-Derived BAC Inserts

Knowledge on sex determination has proven valuable for commercial production of the prawn Macrobrachium rosenbergii due to sex dimorphism of the male and female individuals. Previous studies indicated that prawn sex is determined by a ZW-ZZ chromosomal system, but no genomic information is available for the sex chromosome. Herein, we constructed a genomic bacterial artificial chromosome (BAC) library and identified the ZW-derived BAC clones for initial analysis of the sex chromosomal DNA sequence. The arrayed BAC library contains 200,448 clones with average insert size of 115.4 kb, corresponding to ∼ 4× coverage of the estimated 5.38 Gb genome. Based on a short female-specific marker, a Z- and a W-fragment were retrieved with the genomic walking method. Screening the BAC library using a ZW-specific marker as probe resulted in 12 positive clones. From these, a Z-derived (P331M17) and a W-derived (P122G2) BAC clones were randomly selected and sequenced by PacBio method. We report the construction of a large insert, deep-coverage, and high-quality BAC library for M. rosenbergii that provides a useful resource for positional cloning of target genes, genomic organization, and comparative genomics analysis. Our study not only confirmed the ZW/ZZ system but also discovered sex-linked genes on ZW chromosomes for the first time, contributing to a comprehensive understanding of the genomic structure of sex chromosomes in M. rosenbergii.

Molecular mechanisms underlying sexual differentiation of the nervous system

A long-standing goal in developmental neuroscience is to understand the mechanisms by which steroid sex hormones pattern the mammalian central nervous system along male or female pathways to enable subsequent displays of sexually dimorphic behaviors. In this article, we review recent advances in understanding the epigenetic and transcriptional mechanisms mediating sexual differentiation of the brain in mammals, flies, and worms. These studies suggest a model of sexual differentiation wherein master regulators of sex determination initiate a cascade of sexually dimorphic gene expression that controls development of neural pathways and behavioral displays in a strikingly modular manner. With these advances in molecular genetics, it is now feasible to disassemble different components of sexually dimorphic social behaviors without disrupting other behavioral interactions. Such experimental tractability promises rapid advances in this exciting field.

Functional Interactions Between rsks-1/S6K, glp-1/Notch, and Regulators of Caenorhabditis elegans Fertility and Germline Stem Cell Maintenance

The proper accumulation and maintenance of stem cells is critical for organ development and homeostasis. The Notch signaling pathway maintains stem cells in diverse organisms and organ systems. In Caenorhabditis elegans, GLP-1/Notch activity prevents germline stem cell (GSC) differentiation. Other signaling mechanisms also influence the maintenance of GSCs, including the highly-conserved TOR substrate ribosomal protein S6 kinase (S6K). Although C. elegans bearing either a null mutation in rsks-1/S6K or a reduction-of-function (rf) mutation in glp-1/Notch produce half the normal number of adult germline progenitors, virtually all these single mutant animals are fertile. However, glp-1(rf) rsks-1(null) double mutant animals are all sterile, and in about half of their gonads, all GSCs differentiate, a distinctive phenotype associated with a significant reduction or loss of GLP-1 signaling. How rsks-1/S6K promotes GSC fate is unknown. Here, we determine that rsks-1/S6K acts germline-autonomously to maintain GSCs, and that it does not act through Cyclin-E or MAP kinase in this role. We found that interfering with translation also enhances glp-1(rf), but that regulation through rsks-1 cannot fully account for this effect. In a genome-scale RNAi screen for genes that act similarly to rsks-1/S6K, we identified 56 RNAi enhancers of glp-1(rf) sterility, many of which were previously not known to interact functionally with Notch. Further investigation revealed at least six candidates that, by genetic criteria, act linearly with rsks-1/S6K. These include genes encoding translation-related proteins, cacn-1/Cactin, an RNA exosome component, and a Hedgehog-related ligand. We found that additional Hedgehog-related ligands may share functional relationships with glp-1/Notch and rsks-1/S6K in maintaining germline progenitors.

Decoding Sex Differences in the Brain, One Worm at a Time

Sex differences in the brain are prominent features across the animal kingdom. Understanding the anatomical and regulatory mechanisms behind these differences is critical for both explaining sexually dimorphic behaviors and developing sex-targeted treatments for neurological disorders. Clinical studies considering sex biases and basic research on animal models have provided much evidence for the existence of sex differences in the brain and, in a larger sense, sexual dimorphisms in the nervous system. However, due to the complexity of structure and dimorphic behaviors, it is yet unclear precisely how neuronal sexual dimorphisms are regulated on a molecular or cellular level. This commentary reviews available tools for investigating sexual dimorphisms using a simple model organism, the roundworm Caenorhabditis elegans ( C. elegans), which enables one to study gene regulation at single-cell resolution with a number of cutting-edge molecular and genetic technologies. I highlight the doublesex/mab-3 family of transcription factors, first discovered in invertebrates, and their roles in a potentially universal regulatory mechanism underlying neuronal sexual dimorphisms. Studies of these transcription factors using C. elegans, fruit flies, and vertebrates will promote our understanding of fundamental mechanisms behind sex differences in the brain.

Sex-specific regulation of aging in Caenorhabditis elegans

A fascinating aspect of sexual dimorphism in various animal species is that the two sexes differ substantially in lifespan. In humans, for example, women's life expectancy exceeds that of men by 3-7 years. Whether this trait can be attributed to dissimilar lifestyles or genetic (regulatory) factors remains to be elucidated. Herein, we demonstrate that in the nematode Caenorhabditis elegans, the significantly longer lifespan of hermaphrodites-which are essentially females capable of sperm production-over males is established by TRA-1, the terminal effector of the sex-determination pathway. This transcription factor directly controls the expression of daf-16/FOXO, which functions as a major target of insulin/IGF-1 signaling (IIS) and key modulator of aging across diverse animal phyla. TRA-1 extends hermaphrodite lifespan through promoting daf-16 activity. Furthermore, TRA-1 also influences reproductive growth in a DAF-16-dependent manner. Thus, the sex-determination machinery is an important regulator of IIS in this organism. These findings provide a mechanistic insight into how longevity and development are specified unequally in the two genders. As TRA-1 is orthologous to mammalian GLI (glioma-associated) proteins, a similar sex-specific mechanism may also operate in humans to determine lifespan.

Sexual Dimorphism and Sex Differences in Caenorhabditis elegans Neuronal Development and Behavior

As fundamental features of nearly all animal species, sexual dimorphisms and sex differences have particular relevance for the development and function of the nervous system. The unique advantages of the nematode Caenorhabditis elegans have allowed the neurobiology of sex to be studied at unprecedented scale, linking ultrastructure, molecular genetics, cell biology, development, neural circuit function, and behavior. Sex differences in the C. elegans nervous system encompass prominent anatomical dimorphisms as well as differences in physiology and connectivity. The influence of sex on behavior is just as diverse, with biological sex programming innate sex-specific behaviors and modifying many other aspects of neural circuit function. The study of these differences has provided important insights into mechanisms of neurogenesis, cell fate specification, and differentiation; synaptogenesis and connectivity; principles of circuit function, plasticity, and behavior; social communication; and many other areas of modern neurobiology.

Sexually Dimorphic unc-6/Netrin Expression Controls Sex-Specific Maintenance of Synaptic Connectivity

Nervous systems display intriguing patterns of sexual dimorphisms across the animal kingdom, but the mechanisms that generate such dimorphisms remain poorly characterized. In the nematode Caenorhabditis elegans, a number of neurons present in both sexes are synaptically connected to one another in a sexually dimorphic manner as a result of sex-specific synaptic pruning and maintenance [1-3]. We define here a mechanism for the male-specific maintenance of the synaptic connections of the phasmid sensory neuron PHB and its male-specific target, the sex-shared AVG interneuron. We show that the C. elegans Netrin ortholog UNC-6, signaling through its cognate receptor UNC-40/DCC and the CED-5/DOCK180 guanine nucleotide exchange factor, is both required and sufficient for male-specific synaptic maintenance. The dimorphism of unc-6 activity is brought about by sex-specific regulation of unc-6 transcription. Although unc-6 is transcribed in the AVG neuron of males and hermaphrodites during juvenile stages, unc-6 expression is downregulated in AVG in hermaphrodites during sexual maturation but is maintained during sexual maturation of males. unc-6 downregulation in hermaphrodites is conferred by the master regulator of hermaphrodite sexual identity, the Gli/CI homolog TRA-1, which antagonizes the non-sex-specific function of the LIM homeobox gene lin-11, a terminal selector and activator of unc-6 in AVG. Preventing the downregulation of unc-6 in AVG of hermaphrodites through ectopic expression of unc-6 in transgenic animals results in the maintenance of the PHB>AVG synapses in hermaphrodites. Taken together, intersectional transcriptional regulation of unc-6/Netrin is required and sufficient to cell autonomously pattern sexually dimorphic synapses.

Sexual modulation of sex-shared neurons and circuits in Caenorhabditis elegans

Studies using the nematode C. elegans have provided unique insights into the development and function of sex differences in the nervous system. Enabled by the relative simplicity of this species, comprehensive studies have solved the complete cellular neuroanatomy of both sexes as well as the complete neural connectomes of the entire adult hermaphrodite and the adult male tail. This work, together with detailed behavioral studies, has revealed three aspects of sex differences in the nervous system: sex-specific neurons and circuits; circuits with sexually dimorphic synaptic connectivity; and sex differences in the physiology and functions of shared neurons and circuits. At all of these levels, biological sex influences neural development and function through the activity of a well-defined genetic hierarchy that acts throughout the body to translate chromosomal sex into the state of a master autosomal regulator of sexual differentiation, the transcription factor TRA-1A. This Review focuses on the role of genetic sex in implementing sex differences in shared neurons and circuits, with an emphasis on linking the sexual modulation of specific neural properties to the specification and optimization of sexually divergent and dimorphic behaviors. An important and unexpected finding from these studies is that chemosensory neurons are a primary focus of sexual modulation, with genetic sex adaptively shaping chemosensory repertoire to guide behavioral choice. Importantly, hormone-independent functions of genetic sex are the principal drivers of all of these sex differences, making nematodes an excellent model for understanding similar but poorly understood mechanisms that likely act throughout the animal kingdom. © 2016 Wiley Periodicals, Inc.

Genetic Intersection of Tsix and Hedgehog Signaling during the Initiation of X-Chromosome Inactivation

X-chromosome inactivation (XCI) silences one X chromosome in the female mammal and is essential to peri-implantation development. XCI is thought to be cell autonomous, with all factors required being produced within each cell. Nevertheless, external cues may exist. Here, we search for such developmental signals by combining bioinformatic, biochemical, and genetic approaches. Using ex vivo and in vivo models, we identify the Hedgehog (HH) paracrine system as a candidate signaling cascade. HH signaling keeps XCI in check in pluripotent cells and is transduced by GLI transcription factors to binding sites in Tsix, the antisense repressor of XCI. GLI potentiates Tsix expression and impedes XCI. In vivo, mutating Indian Hedgehog results in a sex ratio bias against females, and the female lethality is rescued by a second-site mutation in Tsix. These data demonstrate a genetic and functional intersection between HH and XCI and support a role for intercellular signaling during XCI.

Programmed Cell Death During Caenorhabditis elegans Development

Programmed cell death is an integral component of Caenorhabditis elegans development. Genetic and reverse genetic studies in C. elegans have led to the identification of many genes and conserved cell death pathways that are important for the specification of which cells should live or die, the activation of the suicide program, and the dismantling and removal of dying cells. Molecular, cell biological, and biochemical studies have revealed the underlying mechanisms that control these three phases of programmed cell death. In particular, the interplay of transcriptional regulatory cascades and networks involving multiple transcriptional regulators is crucial in activating the expression of the key death-inducing gene egl-1 and, in some cases, the ced-3 gene in cells destined to die. A protein interaction cascade involving EGL-1, CED-9, CED-4, and CED-3 results in the activation of the key cell death protease CED-3, which is tightly controlled by multiple positive and negative regulators. The activation of the CED-3 caspase then initiates the cell disassembly process by cleaving and activating or inactivating crucial CED-3 substrates; leading to activation of multiple cell death execution events, including nuclear DNA fragmentation, mitochondrial elimination, phosphatidylserine externalization, inactivation of survival signals, and clearance of apoptotic cells. Further studies of programmed cell death in C. elegans will continue to advance our understanding of how programmed cell death is regulated, activated, and executed in general.

Sexually dimorphic control of gene expression in sensory neurons regulates decision-making behavior in C. elegans

Animal behavior is directed by the integration of sensory information from internal states and the environment. Neuroendocrine regulation of diverse behaviors of Caenorhabditis elegans is under the control of the DAF-7/TGF-β ligand that is secreted from sensory neurons. Here, we show that C. elegans males exhibit an altered, male-specific expression pattern of daf-7 in the ASJ sensory neuron pair with the onset of reproductive maturity, which functions to promote male-specific mate-searching behavior. Molecular genetic analysis of the switch-like regulation of daf-7 expression in the ASJ neuron pair reveals a hierarchy of regulation among multiple inputs—sex, age, nutritional status, and microbial environment—which function in the modulation of behavior. Our results suggest that regulation of gene expression in sensory neurons can function in the integration of a wide array of sensory information and facilitate decision-making behaviors in C. elegans.

DOI:http://dx.doi.org/10.7554/eLife.21166.001

For almost all species of animal, males and females will often behave differently in similar situations. Little is known about how these sex-specific differences are generated or, for example, how different the nervous system of a male is to that of a female. Moreover, it is also poorly understood how these underlying differences based on the biological sex of an animal are integrated with and influenced by its experiences and environment.

The roundworm Caenorhabditis elegans has two sexes, hermaphrodites and males. The male worms behave differently to the hermaphrodites in a number of situations. This means that these animals offer the opportunity to explore and understand sex-specific differences in behavior. It is also possible to analyze the underlying factors that contribute to behavior in C. elegans, because it has a relatively simple and well-defined nervous system.

Now, Hilbert and Kim show that a signal that influences how C. elegans explores in response to chemicals in its environment is expressed differently in male and hermaphrodite worms. The signal in question is molecule called DAF-7, which is released by several sensory neurons—nerve cells that are used for detecting cues from the environment. The sensory neurons that release DAF-7 are found in both sexes of C. elegans but the specific way that the male worms express this signal encourages them to search for mates. Hermaphrodites, on the other hand, do not need to search for mates because they can fertilize their own eggs.

Hilbert and Kim showed that the biological sex in combination with multiple other inputs – including the animal’s past diet and age – regulate how the DAF-7 signal is expressed in C. elegans. These inputs all converge onto a single pair of sensory neurons, which integrate the inputs and enable the worm to assess its current and past experiences and alter its behavior accordingly.

Moving forward the next challenge is to understand how information about both external environment and internal states, such as hunger, are communicated to and integrated by these sensory neurons. Decoding the signals behind this process may illuminate how biological sex and internal states influence behavior in other species of animals.

DOI:http://dx.doi.org/10.7554/eLife.21166.002

Sexually Dimorphic Differentiation of a C. elegans Hub Neuron Is Cell Autonomously Controlled by a Conserved Transcription Factor

Functional and anatomical sexual dimorphisms in the brain are either the result of cells that are generated only in one sex or a manifestation of sex-specific differentiation of neurons present in both sexes. The PHC neuron pair of the nematode C. elegans differentiates in a strikingly sex-specific manner. In hermaphrodites the PHC neurons display a canonical pattern of synaptic connectivity similar to that of other sensory neurons, but in males PHC differentiates into a densely connected hub sensory neuron/interneuron, integrating a large number of male-specific synaptic inputs and conveying them to both male-specific and sex-shared circuitry. We show that the differentiation into such a hub neuron involves the sex-specific scaling of several components of the synaptic vesicle machinery, including the vesicular glutamate transporter eat-4/VGLUT, induction of neuropeptide expression, changes in axonal projection morphology, and a switch in neuronal function. We demonstrate that these molecular and anatomical remodeling events are controlled cell autonomously by the phylogenetically conserved Doublesex homolog dmd-3, which is both required and sufficient for sex-specific PHC differentiation. Cellular specificity of dmd-3 action is ensured by its collaboration with non-sex-specific terminal selector-type transcription factors, whereas the sex specificity of dmd-3 action is ensured by the hermaphrodite-specific transcriptional master regulator of hermaphroditic cell identity tra-1, which represses the transcription of dmd-3 in hermaphrodite PHC. Taken together, our studies provide mechanistic insights into how neurons are specified in a sexually dimorphic manner.

"The persistence of memory"-Hermaphroditism in nematodes

Self-fertility has evolved many times in nematodes. This transition often produces an androdioecious species, with XX hermaphrodites and XO males. Although these hermaphrodites resemble females in most respects, early germ cells differentiate as sperm, and late ones as oocytes. The sperm then receive an activation signal, populate the spermathecae, and are stored for later use in self-fertilization. These traits are controlled by complex modifications to the sex-determination and sperm activation pathways, which have arisen independently during the evolution of each hermaphroditic species. This transformation in reproductive strategy then promotes other major changes in the development, evolution, and population structure of these animals. Mol. Reprod. Dev. 84: 144-157, 2017. © 2016 Wiley Periodicals, Inc.

Charactering the ZFAND3 gene mapped in the sex-determining locus in hybrid tilapia (Oreochromis spp.)

Zinc finger AN1-type domain 3 (ZFAND3) is essential for spermatogenesis in mice. However, its function in teleosts remains unclear. In this study, we characterized the ZFAND3 gene (termed as OsZFAND3) in an important food fish, tilapia. The OsZFAND3 cDNA sequence is 1,050 bp in length, containing an ORF of 615 bp, which encodes a putative peptide of 204 amino acid residues. Quantitative real-time PCR revealed that the OsZFAND3 transcripts were exclusively expressed in the testis and ovary. In situ hybridization showed that the high expression of OsZFAND3 transcripts was predominantly localized in the spermatocyte and spermatid. These results suggest that OsZFAND3 is involved in male germ cell maturation. Three single nucleotide polymorphisms (SNPs) were detected in the introns of OsZFAND3. The OsZFAND3 gene was mapped in the sex-determining locus on linkage group 1 (LG1). The three SNPs in the OsZFAND3 gene were strictly associated with sex phenotype, suggesting that the OsZFAND3 gene is tightly linked to the sex-determining locus. Our study provides new insights into the functions of the OsZFAND3 gene in tilapia and a foundation for further detailed analysis of the OsZFAND3 gene in sex determination and differentiation.

Genomic Analyses of Sperm Fate Regulator Targets Reveal a Common Set of Oogenic mRNAs in Caenorhabditis elegans

Germ cell specification as sperm or oocyte is an ancient cell fate decision, but its molecular regulation is poorly understood. In Caenorhabditis elegans, the FOG-1 and FOG-3 proteins behave genetically as terminal regulators of sperm fate specification. Both are homologous to well-established RNA regulators, suggesting that FOG-1 and FOG-3 specify the sperm fate post-transcriptionally. We predicted that FOG-1 and FOG-3, as terminal regulators of the sperm fate, might regulate a battery of gamete-specific differentiation genes. Here we test that prediction by exploring on a genomic scale the messenger RNAs (mRNAs) associated with FOG-1 and FOG-3. Immunoprecipitation of the proteins and their associated mRNAs from spermatogenic germlines identifies 81 FOG-1 and 722 FOG-3 putative targets. Importantly, almost all FOG-1 targets are also FOG-3 targets, and these common targets are strongly biased for oogenic mRNAs. The discovery of common target mRNAs suggested that FOG-1 and FOG-3 work together. Consistent with that idea, we find that FOG-1 and FOG-3 proteins co-immunoprecipitate from both intact nematodes and mammalian tissue culture cells and that they colocalize in germ cells. Taking our results together, we propose a model in which FOG-1 and FOG-3 work in a complex to repress oogenic transcripts and thereby promote the sperm fate.

Cell-Specific mRNA Profiling of the Caenorhabditis elegans Somatic Gonadal Precursor Cells Identifies Suites of Sex-Biased and Gonad-Enriched Transcripts

The Caenorhabditis elegans somatic gonad differs greatly between the two sexes in its pattern of cell divisions, migration, and differentiation. Despite decades of study, the genetic pathways directing early gonadal development and establishing sexual dimorphism in the gonad remain largely unknown. To help define the genetic networks that regulate gonadal development, we employed cell-specific RNA-seq. We identified transcripts present in the somatic gonadal precursor cells and their daughter cells of each sex at the onset of sexual differentiation. We identified several hundred gonad-enriched transcripts, including the majority of known regulators of early gonadal development, and transgenic reporter analysis confirmed the effectiveness of this approach. Before the division of the somatic gonad precursors, few sex-biased gonadal transcripts were detectable; less than 6 hr later, after their division, we identified more than 250 sex-biased transcripts, of which about a third were enriched in the somatic gonad compared to the whole animal. This indicates that a robust sex-biased developmental program, some of it gonad-specific, initiates in the somatic gonadal precursor cells around the time of their first division. About 10% of male-biased transcripts had orthologs with male-biased expression in the early mouse gonad, suggesting possible conservation of gonad sex differentiation. Cell-specific analysis also identified approximately 70 previously unannotated mRNA isoforms that are enriched in the somatic gonad. Our data illustrate the power of cell-specific transcriptome analysis and suggest that early sex differentiation in the gonad is controlled by a relatively small suite of differentially expressed genes, even after dimorphism has become apparent.

Molecular characterization and expression profile of three Fem-1 genes in Eriocheir sinensis provide a new insight into crab sex-determining mechanism

The FEM-1 protein of Caenorhabditis elegans plays a crucial role in the nematode sex-determination pathway. Here, we reported the characterization of three members of Fem-1 gene family in Eriocheir sinensis (designated EsFem-1a, EsFem-1b, and EsFem-1c), which were homologs of the nematode FEM-1 protein. The amino acid sequences of EsFem-1a, EsFem-1b, and EsFem-1c contained eight, nine, and eight ankyrin repeats, respectively. None of the ankyrin repeats had its own specific signature, and the evolution of ankyrin repeat was not completely independent. The predicted three-dimensional structure of EsFem-1 proteins exhibited highly similar superhelical conformation, especially the N-terminal six contiguous ankyrin repeats, which provided a binding surface for the protein-protein interaction. Phylogenetic tree based on the amino acid sequences revealed that EsFem-1a, EsFem-1b, and EsFem-1c were divided into three obvious separated clades. EsFem-1 genes were highly expressed in fertilized egg, 2-4 cell and blastula stage comparing with larval stage (P<0.01), which suggested they might be maternal genes. They also showed a certain degree of sexually dimorphic expression in some tissues. Notably, the highest expression of EsFem-1a was in the hepatopancreas, with EsFem-1b in testes and EsFem-1c in muscle (P<0.05), which indicated their potential role in a broad array of tissues. In addition, the genes initially involved in sex differentiation were not limited to those specifically expressed in the developing gonad. Taken together, these results suggested that EsFem-1 might function in crab early sex determination and late gonad development. The identification of Fem-1 gene family in E. sinensis provides a new insight into crab sex-determination mechanism.

Molecular signatures discriminating the male and the female sexual pathways in the pearl oyster Pinctada margaritifera

The genomics of economically important marine bivalves is studied to provide better understanding of the molecular mechanisms underlying their different reproductive strategies. The recently available gonad transcriptome of the black-lip pearl oyster Pinctada margaritifera is a novel and powerful resource to study these mechanisms in marine mollusks displaying hermaphroditic features. In this study, RNAseq quantification data of the P. margaritifera gonad transcriptome were analyzed to identify candidate genes in histologically-characterized gonad samples to provide molecular signatures of the female and male sexual pathway in this pearl oyster. Based on the RNAseq data set, stringent expression analysis identified 1,937 contigs that were differentially expressed between the gonad histological categories. From the hierarchical clustering analysis, a new reproduction model is proposed, based on a dual histo-molecular analytical approach. Nine candidate genes were identified as markers of the sexual pathway: 7 for the female pathway and 2 for the male one. Their mRNA levels were assayed by real-time PCR on a new set of gonadic samples. A clustering method revealed four principal expression patterns based on the relative gene expression ratio. A multivariate regression tree realized on these new samples and validated on the previously analyzed RNAseq samples showed that the sexual pathway of P. margaritifera can be predicted by a 3-gene-pair expression ratio model of 4 different genes: pmarg-43476, pmarg-foxl2, pmarg-54338 and pmarg-fem1-like. This 3-gene-pair expression ratio model strongly suggests only the implication of pmarg-foxl2 and pmarg-fem1-like in the sex inversion of P. margaritifera. This work provides the first histo-molecular model of P. margaritifera reproduction and a gene expression signature of its sexual pathway discriminating the male and female pathways. These represent useful tools for understanding and studying sex inversion, sex differentiation and sex determinism in this species and other related species for aquaculture purposes such as genetic selection programs.

Sex, age, and hunger regulate behavioral prioritization through dynamic modulation of chemoreceptor expression

BACKGROUND

Adaptive behavioral prioritization requires flexible outputs from fixed neural circuits. In C. elegans, the prioritization of feeding versus mate searching depends on biological sex (males will abandon food to search for mates, whereas hermaphrodites will not) as well as developmental stage and feeding status. Previously, we found that males are less attracted than hermaphrodites to the food-associated odorant diacetyl, suggesting that sensory modulation may contribute to behavioral prioritization.

RESULTS

We show that somatic sex acts cell autonomously to reconfigure the olfactory circuit by regulating a key chemoreceptor, odr-10, in the AWA neurons. Moreover, we find that odr-10 has a significant role in food detection, the regulation of which contributes to sex differences in behavioral prioritization. Overexpression of odr-10 increases male food attraction and decreases off-food exploration; conversely, loss of odr-10 impairs food taxis in both sexes. In larvae, both sexes prioritize feeding over exploration; correspondingly, the sexes have equal odr-10 expression and food attraction. Food deprivation, which transiently favors feeding over exploration in adult males, increases male food attraction by activating odr-10 expression. Furthermore, the weak expression of odr-10 in well-fed adult males has important adaptive value, allowing males to efficiently locate mates in a patchy food environment.

CONCLUSIONS

We find that modulated expression of a single chemoreceptor plays a key role in naturally occurring variation in the prioritization of feeding and exploration. The convergence of three independent regulatory inputs--somatic sex, age, and feeding status--on chemoreceptor expression highlights sensory function as a key source of plasticity in neural circuits.

The evolutionary origins and consequences of self-fertility in nematodes

Self-fertile hermaphrodites have evolved from male/female ancestors in many nematode species, and this transition occurred on three independent occasions in the genus Caenorhabditis. Genetic analyses in Caenorhabditis show that the origin of hermaphrodites required two types of changes: alterations to the sex-determination pathway that allowed otherwise female animals to make sperm during larval development, and the production of signals from the gonad that caused these sperm to activate and fertilize oocytes. Comparisons of C. elegans and C. briggsae hermaphrodites show that the ancestral sex-determination pathway has been altered in multiple unique ways. Some of these changes must have precipitated the production of sperm in XX animals, and others were modifying mutations that increased the efficiency of hermaphroditic reproduction. Reverse genetic experiments show that XX animals acquired the ability to activate sperm by co-opting one of the two redundant pathways that normally work in males. Finally, the adoption of a hermaphroditic lifestyle had profound effects on ecological and sexual interactions and genomic organization. Thus, nematode mating systems are ideal for elucidating the origin of novel traits, and studying the influence of developmental processes on evolutionary change.

The development of sexual dimorphism: studies of the Caenorhabditis elegans male

Studies of the development of the Caenorhabditis elegans male have been carried out with the aim of understanding the basis of sexual dimorphism. Postembryonic development of the two C. elegans sexes differs extensively. Development along either the hermaphrodite or male pathway is specified initially by the X to autosome ratio. The regulatory events initiated by this ratio include a male-determining paracrine intercellular signal. Expression of this signal leads to different consequences in three regions of the body: the nongonadal soma, the somatic parts of the gonad, and the germ line. In the nongonadal soma, activity of the key Zn-finger transcription factor TRA-1 determines hermaphrodite development; in its absence, the male pathway is followed. Only a few genes directly regulated by TRA-1 are currently known, including members of the evolutionarily conserved, male-determining DM domain Zn-finger transcription factors. In the somatic parts of the gonad and germ line, absence of TRA-1 activity is not sufficient for full expression of the male pathway. Several additional transcription factors involved have been identified. In the germ line, regulatory genes for sperm development that act at the level of RNA in the cytoplasm play a prominent role.

Dependence of the sperm/oocyte decision on the nucleosome remodeling factor complex was acquired during recent Caenorhabditis briggsae evolution

The major families of chromatin remodelers have been conserved throughout eukaryotic evolution. Because they play broad, pleiotropic roles in gene regulation, it was not known if their functions could change rapidly. Here, we show that major alterations in the use of chromatin remodelers are possible, because the nucleosome remodeling factor (NURF) complex has acquired a unique role in the sperm/oocyte decision of the nematode Caenorhabditis briggsae. First, lowering the activity of C. briggsae NURF-1 or ISW-1, the core components of the NURF complex, causes germ cells to become oocytes rather than sperm. This observation is based on the analysis of weak alleles and null mutations that were induced with TALENs and on RNA interference. Second, qRT-polymerase chain reaction data show that the C. briggsae NURF complex promotes the expression of Cbr-fog-1 and Cbr-fog-3, two genes that control the sperm/oocyte decision. This regulation occurs in the third larval stage and affects the expression of later spermatogenesis genes. Third, double mutants reveal that the NURF complex and the transcription factor TRA-1 act independently on Cbr-fog-1 and Cbr-fog-3. TRA-1 binds both promoters, and computer analyses predict that these binding sites are buried in nucleosomes, so we suggest that the NURF complex alters chromatin structure to allow TRA-1 access to Cbr-fog-1 and Cbr-fog-3. Finally, lowering NURF activity by mutation or RNA interference does not affect this trait in other nematodes, including the sister species C. nigoni, so it must have evolved recently. We conclude that altered chromatin remodeling could play an important role in evolutionary change.

Sexual modulation of neural circuits and behavior in Caenorhabditis elegans

Sex differences in behavior-both sex-specific and shared behaviors-are fundamental to nearly all animal species. One often overlooked mechanism by which these behavioral differences can be generated is through sex-specific modulation of shared circuitry (i.e., circuits present in both sexes). In vertebrates this modulation is likely regulated by hormone-dependent mechanisms as well as by somatic sex itself; invertebrate models have particular promise for understanding the latter of these. Here we review molecular and behavioral evidence of sexual modulation of shared circuitry in the nematode Caenorhabditis elegans. Multiple behaviors in this species, both copulatory and not, are modulated by the genetic sex of shared neurons and circuit. These studies are close to uncovering the molecular mechanisms by which somatic sex modulates neural function in the worm, mechanisms which may be well conserved in more complex organisms. Improving our understanding of the modulation of neural circuit development and function by somatic sex may lend important insight into sex differences in the mammalian nervous system which, in turn, may have important implications for sex biases in disease.

The regulation of spermatogenesis and sperm function in nematodes

In the nematode C. elegans, both males and self-fertile hermaphrodites produce sperm. As a result, researchers have been able to use a broad range of genetic and genomic techniques to dissect all aspects of sperm development and function. Their results show that the early stages of spermatogenesis are controlled by transcriptional and translational processes, but later stages are dominated by protein kinases and phosphatases. Once spermatids are produced, they participate in many interactions with other cells - signals from the somatic gonad determine when sperm activate and begin to crawl, signals from the female reproductive tissues guide the sperm, and signals from sperm stimulate oocytes to mature and be ovulated. The sperm also show strong competitive interactions with other sperm and oocytes. Some of the molecules that mediate these processes have conserved functions in animal sperm, others are conserved proteins that have been adapted for new roles in nematode sperm, and some are novel proteins that provide insights into evolutionary change. The advent of new techniques should keep this system on the cutting edge of research in cellular and reproductive biology.

A role for WDR5 in TRA-1/Gli mediated transcriptional control of the sperm/oocyte switch in C. elegans

The hermaphrodite germline of Caenorhabditis elegans initially engages in spermatogenesis and then switches to oogenesis during late stages of larval development. TRA-1, a member of the Ci/Gli family of transcriptional repressors, plays an essential role in this switch by repressing genes that promote spermatogenesis. WDR5 proteins are conserved components of histone methyltransferase complexes normally associated with gene activation. However, two C. elegans WDR5 homologs, wdr-5.1 and wdr-5.2 are redundantly required for normal TRA-1 dependent repression, and this function is independent of their roles in histone methylation. Animals lacking wdr-5.1/wdr-5.2 function fail to switch to oogenesis at 25°C, resulting in a masculinization of germline (Mog) phenotype. The Mog phenotype is caused by ectopic expression of fog-3, a direct target of TRA-1 repression. WDR-5.1 associates with the fog-3 promoter and is required for TRA-1 to bind to fog-3 promoter. Other direct targets of TRA-1 are similarly derepressed in the double mutant. These results show that WDR5 plays a novel and important role in stabilizing transcriptional repression during C. elegans sex determination, and provide evidence that this important protein may operate independently of its established role in histone methyltransferase complexes.

Noncanonical cell death in the nematode Caenorhabditis elegans

The nematode Caenorhabditis elegans has served as a fruitful setting for cell death research for over three decades. A conserved pathway of four genes, egl-1/BH3-only, ced-9/Bcl-2, ced-4/Apaf-1, and ced-3/caspase, coordinates most developmental cell deaths in C. elegans. However, other cell death forms, programmed and pathological, have also been described in this animal. Some of these share morphological and/or molecular similarities with the canonical apoptotic pathway, while others do not. Indeed, recent studies suggest the existence of an entirely novel mode of programmed developmental cell destruction that may also be conserved beyond nematodes. Here, we review evidence for these noncanonical pathways. We propose that different cell death modalities can function as backup mechanisms for apoptosis, or as tailor-made programs that allow specific dying cells to be efficiently cleared from the animal.

Evolutionary change within a bipotential switch shaped the sperm/oocyte decision in hermaphroditic nematodes

A subset of transcription factors like Gli2 and Oct1 are bipotential--they can activate or repress the same target, in response to changing signals from upstream genes. Some previous studies implied that the sex-determination protein TRA-1 might also be bipotential; here we confirm this hypothesis by identifying a co-factor, and use it to explore how the structure of a bipotential switch changes during evolution. First, null mutants reveal that C. briggsae TRR-1 is required for spermatogenesis, RNA interference implies that it works as part of the Tip60 Histone Acetyl Transferase complex, and RT-PCR data show that it promotes the expression of Cbr-fog-3, a gene needed for spermatogenesis. Second, epistasis tests reveal that TRR-1 works through TRA-1, both to activate Cbr-fog-3 and to control the sperm/oocyte decision. Since previous studies showed that TRA-1 can repress fog-3 as well, these observations demonstrate that it is bipotential. Third, TRR-1 also regulates the development of the male tail. Since Cbr-tra-2 Cbr-trr-1 double mutants resemble Cbr-tra-1 null mutants, these two regulatory branches control all tra-1 activity. Fourth, striking differences in the relationship between these two branches of the switch have arisen during recent evolution. C. briggsae trr-1 null mutants prevent hermaphrodite spermatogenesis, but not Cbr-fem null mutants, which disrupt the other half of the switch. On the other hand, C. elegans fem null mutants prevent spermatogenesis, but not Cel-trr-1 mutants. However, synthetic interactions confirm that both halves of the switch exist in each species. Thus, the relationship between the two halves of a bipotential switch can shift rapidly during evolution, so that the same phenotype is produce by alternative, complementary mechanisms.

Fem1b promotes ubiquitylation and suppresses transcriptional activity of Gli1

The mammalian Fem1b gene encodes a homolog of FEM-1, a protein in the sex-determination pathway of the nematode Caenorhabditis elegans. Fem1b and FEM-1 proteins each contain a VHL-box motif that mediates their interaction with certain E3 ubiquitin ligase complexes. In C. elegans, FEM-1 negatively regulates the transcription factor TRA-1, and functions as an E3 ubiquitin ligase substrate recognition subunit to target TRA-1 for ubiquitylation. TRA-1 is homologous to the mammalian Gli1 protein, a transcription factor that mediates Hedgehog signaling as well as having Hedgehog-independent functions. Whether the interaction between nematode FEM-1 and TRA-1 proteins is conserved, between corresponding mammalian homologs, has not been reported. Herein, we show that Fem1b interacts with Gli1 within cells, and directly binds Gli1. Fem1b also promotes ubiquitylation of Gli1, suppresses transcriptional activation by Gli1, and attenuates an oncogenic Gli1 autoregulatory loop in cancer cells, all dependent on the VHL-box of Fem1b. These findings have implications for understanding the cellular functions of Fem1b, and the regulation of Gli1 oncoprotein activity.