Skip to main content
NCBI home page
NIHPA Author Manuscripts logoLink to NIHPA Author Manuscripts
. Author manuscript; available in PMC: 2019 Dec 1.
Published in final edited form as: Curr Opin Neurobiol. 2018 Oct 10;53:192–197. doi: 10.1016/j.conb.2018.09.005

Molecular mechanisms underlying sexual differentiation of the nervous system

Joseph R Knoedler 1, Nirao M Shah 1,2,*
PMCID: PMC6347421  NIHMSID: NIHMS1507837  PMID: 30316066

Abstract

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.

Introduction

Sexually reproducing species exhibit sex differences in social interactions, presumably to enhance reproductive success and survival of progeny. Many of these differences are acquired traits that depend on experience. Nevertheless, a core set of sex-typical behavioral displays such as mating, territorial aggression, and parental care, although modifiable by experience, are innate in the sense that they can be displayed without prior training. These sexual dimorphisms in innate behavior reflect the action of a sexually differentiated nervous system [1–3]. While the chromosomal and endocrine mechanisms mediating sexual differentiation can vary across taxa, such mechanisms converge to regulate sexually dimorphic gene expression that in turn guides sex-typical developmental trajectories and adult function of neural pathways [4]. The molecular mechanisms underlying these sexually dimorphic gene expression programs in the central nervous system remain poorly understood. Because sexual differentiation in mammals depends on the long-lasting actions of early life exposure to steroid hormones, it is likely that epigenetic mechanisms play an important role [5,6]. In this review, we highlight recent advances in understanding the transcriptional and epigenetic mechanisms of sexual differentiation of the central nervous system. We illustrate these advances with examples from flies, worms, and mice, and we discuss how modern molecular and genetic approaches should enable unprecedented insights into sexual differentiation of the nervous system.

Development and function of sex-specific circuits in C. elegans

The nematode worm Caenorhabditis elegans (C. elegans) is unique in allowing investigation of sexual differentiation at the level of individual identified neurons, as its nervous system is numerically simpler and mapped in great detail [7,8], with 294 neurons shared between both sexes, 8 neurons that are hermaphrodite-specific, and 91 neurons that are male-specific. These sex-divergent neurons enable molecular dissection of sex-specific neural pathways that mediate specific adaptive behavioral programs in this organism. Hermaphrodites have two X chromosomes whereas males have one (XO), and the number of X chromosomes (or X-to-autosome ratio) determines the sex of the worm [9]. Sexual differentiation occurs in a largely cell-autonomous manner under control of the zinc-finger transcription factor TRA-1 [10–12]. In hermaphroditic cells, TRA-1 represses male-specific genes. In the male, TRA-1 is degraded by proteolysis, allowing development of the male phenotype [13,14]. Importantly, as a result of this system sexually mosaic animals can be generated by regulating TRA-1 expression in specific cells [15]. This allows the generation of ‘male’ neurons in hermaphrodites, or vice-versa, allowing elegant dissection of sex-specific circuits.

Remarkably, in addition to sex-specific neurons, male and hermaphrodite C. elegans also possess shared neurons that nevertheless form sex-specific projections and synapses. Recent studies by Oliver Hobert’s group have made progress in describing the cellular and transcriptional mechanisms that mediate the development of sex-specific connections of shared neurons [16••]. The shared PHB sensory neuron forms synapses on to three downstream interneurons in hermaphrodites, while in males it forms synapses on to a different shared interneuron as well as male-specific motor, sensory and inter- neurons. These sexually dimorphic connections appear to arise by at least two distinct mechanisms. PHB efferent synapses develop in both males and hermaphrodites at early larval stages but connections to different shared interneurons are selectively lost in the two sexes (‘selective pruning’). In some other neurons, connections are formed only in larval males and these persist in adult males, providing evidence for a ‘pre-patterning’ mechanism for circuit wiring. PHB neurons mediate sex-specific functions: disruption of the PHB neuron altered locomotion and the behavioral response to noxious chemicals in hermaphrodites, whereas it reduced the ability of males to mate. Sex-specific synapse pruning of PHB connections is cell-autonomous since masculinization or feminization of this neuron elicited pruning of the connections appropriate to the sex of the PHB neuron and not to the sex of the post-synaptic cells. Remarkably, the authors found that the sex of the post-synaptic neurons was also sufficient to lead to rewiring of PHB connectivity such that it was now appropriate to the sex of the post-synaptic neuron. In this latter instance, the authors showed that such rewiring was dependent on the expression of DMD-5 and DMD-11, members of the DM family of transcription factors that govern sex determination or differentiation across metazoans [4,16••]. It will be interesting to understand the mechanisms whereby such selective sexual differentiation of either a sensory neuron or its post-synaptic partner leads to re-wiring of the pathway.

Work by Hobert’s group on the sex-shared PHC neurons provides new molecular insights into repurposing of shared neurons [17••]. PHC neurons appear to be typical sensory neurons in hermaphrodites, with connections to a limited number of shared neurons, that are required for response to harsh touch. By contrast, PHC neurons resemble hub neurons in males, with extensive connectivity with male-specific neurons and to different shared neurons, and they are required for specific aspects of the male mating ritual. This sex-specific connectivity is controlled to a large extent cell-autonomously because altering sexual identity of PHC by manipulating expression of TRA-1, the master regulator of sexual differentiation in worms, rewires connectivity of this neuron accordingly. In keeping with the greater connectivity of male PHC neurons, there is a corresponding transcriptional scaling of many components of the synaptic machinery, including the vesicular glutamate transporter EAT-4. Expression of EAT-4 was dependent on shared transcriptional factors as well as sexually dimorphic utilization of cis-regulatory elements that repressed expression in hermaphrodites and boosted it in males. Remarkably, the neuritic arbor of as well as transcriptional scaling in male PHC neurons was regulated cell autonomously by DMD-3, yet another member of the DM family of transcriptional regulators. Whether DMD-3 acts directly by binding to the regulatory elements of the eat-4 locus is unknown.

Elegant studies in C. elegans point the way to a systems-level understanding of how internal states such as hunger and age interact with shared circuitry in a sex-specific manner to enable presumptively adaptive male or hermaphrodite-specific responses [18••]. Adult hermaphrodites show a stronger attraction toward the compound diacetyl (a food cue) than do males. This attraction is mediated by two pairs of sex-shared neurons (AWA and AWC) in hermaphrodites but by AWC in males. This is associated with sex-specific expression of the odorant receptor ODR-10 in hermaphrodite AWA neurons. This sex difference only exists in well-fed adults. Larval AWA neurons of both sexes express this chemoreceptor, and both males and hermaphrodites show equivalent attraction to diacetyl. Food deprivation upregulates odr-10 in male AWA neurons and increases food-seeking behavior at the expense of seeking mates. Genetically specified circuits can thus be modulated dynamically by external cues to regulate sex differences in behavior; it will be interesting to identify the signaling cascades that modulate chemoreceptor expression in an age and nutritional status dependent manner. Sex-shared sensory neurons can also guide sexually dimorphic responses to pheromones in worms. Males but not hermaphrodites are attracted to ascarosides, components of the worm cuticle that can act as pheromones [19]. This attraction is mediated by a pair of sex-shared sensory neurons (ADF), and feminization of these neurons results in males being repelled by rather than attracted to ascarosides [20•]. The ability of ADF to detect and mediate attraction to ascarosides was dependent on the transcriptional regulator MAB-3, a founding member of the DM family of transcription factors. How MAB-3 regulates ADF function is presently unknown.

Together, these pioneering studies in worms show how sexually dimorphic wiring and behaviors can emerge from neurons present in both sexes. Moreover, they reveal a profound sexually dimorphic reconfiguration of morphology and function of shared sensory neurons (PHA, PHB, ADF, AWA). Whether such sexually dimorphic reconfiguration is present in other worm sensory neurons, and more generally, is common to sensory neurons in flies or mammals is an open question.

Sexual differentiation in Drosophila: the search for isoform- and sex-specific targets of master transcriptional regulators

In the fruit fly Drosophila melanogaster, sexual determination is chromosomal as in the worm, such that the number of X chromosomes (or the ratio of X chromosomes to autosomes) determines the sex of the animal [9,21,22]. Sexual differentiation of the nervous system is mediated by two transcription factors, Fruitless (Fru) and Doublesex (Dsx) both of which exhibit sex-specific splice variants in the nervous system (FruM/F; DsxM/F) [23]. Strikingly, FruM is required for male Drosophila to exhibit courtship behavior, and ectopic expression of FruM in female neurons is sufficient to induce most components of male courtship behaviors [24–26]. The molecular mechanisms whereby Fru drives sexual differentiation are now beginning to be elucidated. FruM exhibits three splice variants (FruMA,MB,MC), all of which are expressed in the male nervous system. Fru is a BTB-domain-containing zinc finger transcription factor, and each of the A, B and C isoforms contains an alternative DNA-binding domain. Mutations restricted to specific splice isoforms of FruM demonstrated separable functions in the development and execution of male courtship behavior [25,27••]. Isoforms B and C are widely expressed in overlapping populations whereas isoform A is expressed in a more restricted set of neurons. Males mutant for isoforms B and C recapitulated some of the behavioral deficits shown by males mutant for all FruM isoforms, including male-male courtship, whereas no behavioral phenotypes were discernible in males mutant for isoform A. In addition, there were differences between the phenotypes of males mutant for isoforms B and C, particularly in courtship song production. Together, the findings from this study are consistent with the notion that these isoforms of FruM perform distinct as well as redundant functions. Indeed, Goodwin’s group demonstrated overlapping and distinct genomic binding sites for these isoforms of FruM [27••]. While all three isoforms regulated genes related to the development of the nervous system and regulated many targets in common, they showed differences in consensus motif binding and some non-overlapping targets. This implies some degree of functional differentiation of FruM isoforms. Of the 19 potential target genes tested for sexually dimorphic expression, 14 showed higher expression in males, and upon testing, 2 showed male courtship defects following RNAi-mediated knockdown. These findings represent a significant advance in our understanding of how FruM regulates various components of the courtship ritual. They suggest a model in which FruM, acting via different isoforms, regulates sexually dimorphic expression of target genes that, individually, regulate different aspects of male courtship in a modular manner. This model is supported by a recent study from Yamamoto’s group in which they showed that the B isoform of FruM represses robo1 in a specific cluster of neurons through a cis-regulatory sequence located in the promoter of robo1 [28•]. This results in a regulatory switch that guides these neurons down a male or female developmental pathway. This modular mode of FruM action is similar to that proposed for mice in which sex hormones drive sexually dimorphic expression of target genes that in turn regulate different components of male or female social behaviors in a modular manner [29•]. FruM can also exert diverse effects through differential coupling with other transcriptional regulators. Another study by Yamamoto’s group showed that the effects of FruM on development of sex-specific neuronal morphology depended partially on its interaction with two chromatin regulators, HDAC1 and HP1a, that function antagonistically in driving male-typical differentiation of specific groups of neurons [30•]. It is remains to be determined whether this interaction with HDAC1 or HP1a is exclusive to a particular FruM isoform.

Dsx, which is the other founding member of the DM family of transcription factors (Mab3 in worms being the other), is required for sexual differentiation of peripheral tissues as well as a subset of neurons. While to date no genome-wide studies appear to have identified its targets in the fly brain, a genome-wide study of DsxM and DsxF targets in gonad and fat tissue by Oliver’s group may be instructive in understanding the sex-specificity of this transcription factor [31••]. They found that the vast majority of Dsx targets were bound by both male and female isoforms in multiple tissues. Furthermore, only a minority of genes with Dsx association in their genomic regions showed expression changes upon dsx knockdown or upon switching isoforms. However, many of the most highly-occupied Dsx target genes showed sex-specific defects upon knockdown. These were sometimes strikingly tissue- and sex-specific; for example, knockdown of the Dsx target lola caused almost total loss of external genitalia in females and deformation of the testes in males. Thus, shared targets of Dsx mediate distinct developmental programs in the two sexes. This work is intriguing in that it suggests that although males and females express different isoforms of Dsx that drive different developmental programs, their sex-specific effects are not apparent from their occupancy in chromatin but presumably depend on downstream signaling events or differential interaction with other transcriptional regulators.

The requirement for both Dsx and FruM in masculinization of the fly nervous system raises the question of whether these transcription factors co-regulate target gene expression in neurons. Recent work by Baker’s group suggests that at least for Lgr3, a member of the relaxin receptor family of GPCRS, the answer is more complicated [32•]. They showed that the B isoform of FruM inhibits Lgr3 expression in median bundle neurons via an intronic cis-regulatory element whereas DsxF inhibits and activates Lgr3 expression via two distinct cis-regulatory elements in two different pools of abdominal ganglionic neurons. These findings demonstrate unexpected complexity in the regulation of sexually dimorphic expression of a single gene in different neuronal populations.

These recent studies have advanced our understanding of how master regulators drive sexual differentiation of the developing Drosophila nervous system. However, refinements to sex-specific circuits can also occur post-eclosion. Wang’s group elegantly elucidated mechanisms underlying the ability of older male flies to outcompete younger males in copulating with females [33••]. They showed that the success of older males was dependent on increased sensitivity of Or47b-expressing chemosensory neurons to the courtship-promoting cuticular pheromone palmitoleic acid. In turn, this increase in sensitivity of Or47b neurons is mediated by juvenile hormone acting via its cognate receptor Met in Or47b neurons. This later differentiation event underscores how the sexually dimorphic nervous system is fine-tuned to maximize the display of behaviors that promote reproductive success. This study is also interesting as it highlights a case in which the maturation of a circuit important for social behaviors in flies is not cell-autonomous but requires hormonal signaling, as is the case in mammals.

Sexual differentiation of the mouse brain: emerging evidence for a role of epigenetic modifiers

While sexual differentiation in worms and flies is largely cell-autonomous, in most mammals it proceeds by both chromosomal and endocrine mechanisms [1–3]. The Y-linked gene Sry is necessary and sufficient for differentiating the bipotential gonad in midgestation mouse embryos into testes. In the absence of Sry, the bipotential gonad is pre-patterned to differentiate into ovaries. During a critical perinatal window (the exact time can vary depending on the species) in mice, there is a transient surge of testosterone that is thought to irreversibly masculinize a bipotential brain (organizational action of sex hormones). Many of the actions of testosterone on the brain appear to be mediated by its estrogenic metabolites, with 17β-estradiol being the most biologically potent estrogen in vivo. Such conversion of testosterone into estrogens is catalyzed by the enzyme aromatase expressed in discrete regions of the mouse brain [34]. By contrast, the ovaries are quiescent at this stage, and therefore the male brain is selectively exposed to testosterone and estrogen during this critical organizational period. Following puberty, there is a rise in sex hormones in the circulation in both sexes, and these hormones modulate physiological functioning (activational action of sex hormones) of developmentally wired circuits to enable the display of sexually dimorphic social behaviors [1–3]. The sex steroids testosterone, estrogens, and progesterone act through specific nuclear receptors that are ligand-activated transcription factors and are also found in chromatin-modifying complexes [35,36]; in addition, they also exert rapid, non-transcriptional (traditionally referred to as “non-genomic”) effects by interacting with various signal transduction pathways (reviewed in [1]). Sex hormone receptors may therefore exert their organizational and activational effects on the brain through diverse mechanisms. Previous work by Xu and colleagues found genes in multiple regions of the mouse hypothalamus and amygdala that 1) showed sexually dimorphic expression, 2) were regulated by circulating steroid hormones, and 3) affected specific components of adult sexually dimorphic social behaviors [29•]. At least some of these genes showed evidence of epigenetic programming. A particularly striking example is the gene Cckar, which is required for the high wildtype level of female sexual behavior. This neuropeptide receptor is more highly expressed in the female ventromedial hypothalamus, ventrolateral region (VMHvl) than in males. Castration of adult females reduces its expression and eliminates the sex difference. Estrogen treatment of adult females restores its expression, but treatment of castrated adult males with estrogen does not elevate Cckar expression to female-typical levels. Thus, sex hormone-regulated genes in the mammalian brain are expressed in a sexually dimorphic manner, likely to be epigenetically regulated, and relevant for the display of social behaviors. The full extent of sex hormone regulated sexually dimorphic gene expression and the molecular mechanisms underlying these sex differences are presently unknown.

The complexity and cellular heterogeneity of the mammalian brain renders the search for epigenetic and transcriptional regulators of sexual dimorphism difficult. Nevertheless, progress has been made in establishing that epigenetic processes are at work in establishing sex differences in the developing and adult mammalian brain. Several groups have looked at histone modifications as candidate mechanisms for sexual differentiation. These studies find changes in methylation or acetylation at key lysine residues and show that experimental alteration of these modifications can alter sexual differentiation of cell number in particular brain regions or modify adult social behaviors [37•,38•,39•,40•].

Another candidate mechanism for epigenetic programming of sexually dimorphic gene expression is DNA methylation, which is typically associated with gene repression. Recent work by Forger’s group showed that pharmacological inhibition of DNA methyltransferase activity at birth feminized expression of Esr1, a nuclear hormone receptor for estrogen, in two hypothalamic nuclei [41•]. This suggests that DNA methylation may establish or maintain male-specific gene repression in these regions. Interestingly, the McCarthy lab found that neonatal female rats had higher DNA methyltransferase activity in the preoptic area (POA). Whereas Forger’s group found that inhibition of DNA methylation feminized specific gene expression patterns in male mice, McCarthy’s group found that such inhibition masculinized neural spine morphology in female rats [42•]. Thus, epigenetic regulation of sexual differentiation can modify different sexual differentiation events in a sex-specific manner. Regardless, such developmental effects of methylation not only have acute effects on sexual differentiation but also result in longer lasting effects that persist into adulthood, consistent with the notion that epigenetic processes have enduring effects [42•,43•].

Future directions

We are excited by the recent advances in unraveling the molecular mechanisms that drive sexual differentiation of the nervous system. Although the mechanisms that regulate sexual differentiation appear different between worms, flies, and mice, there are also commonalities. As we have discussed, the DM family of transcription factors plays an important role in sexual differentiation in both worms and flies; intriguingly, the mouse genome contains several loci that code for DM family related transcription factors (DMRTs) [44]. However, the function of these genes in sexual differentiation in mice remains unclear. Moreover, converging evidence from worms, flies, and mice suggests that such master regulators of sexual differentiation drive sex differences in gene expression that in turn control cellular changes in specific neurons and regulate discrete components of sexually dimorphic behaviors in a modular manner. While the scale of the problem of unraveling the molecular determinants of sexually dimorphic circuit development is greater in mammals than in worms or flies, recent work suggests that it should be feasible to not only identify sexually dimorphically expressed genes but also to elucidate their function and the function of the underlying neural circuits [1]. Together, the combined use of cell type targeted genome-wide studies and the powerful toolkit for manipulating neural circuits [45] should lead to exciting and novel insights into how an apparently similar brain generates distinct behavioral repertoires in the two sexes.

Highlights.

  • Master regulators of sex determination initiate program of sexually dimorphic gene expression that controls sexual differentiation of neural circuits and behavior in a modular manner

  • In worms, transcriptional and genomic mechanisms that regulate sexually dimorphic gene expression are beginning to emerge

  • In flies, sex-specific isoforms of Fru and Dsx regulate shared and sexually dimorphic gene expression patterns and neuronal morphology

  • In mice, molecular mechanisms that mediate sexually dimorphic gene expression and modulate behavior are being identified

Acknowledgments

We thank members of the Shah lab for comments on the manuscript. This work was supported by NIH (R01NS049488, R01NS083872, R01MH108319) and a Seed grant from Stanford WSDM Center to N.M.S.

Footnotes

Publisher's Disclaimer: This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final citable form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.

References

  • 1.Yang CF, Shah NM: Representing sex in the brain, one module at a time. Neuron 2014, 82:261–78. [DOI] [PMC free article] [PubMed] [Google Scholar]
  • 2.Arnold AP: The organizational-activational hypothesis as the foundation for a unified theory of sexual differentiation of all mammalian tissues. Horm Behav 2009, 55:570–578. [DOI] [PMC free article] [PubMed] [Google Scholar]
  • 3.McCarthy MM, Arnold AP: Reframing sexual differentiation of the brain. Nat Neurosci 2011, 14:677–683. [DOI] [PMC free article] [PubMed] [Google Scholar]
  • 4.Matson CK, Zarkower D: Sex and the singular DM domain: insights into sexual regulation, evolution and plasticity. Nat Rev Genet 2012, 13:163–174. [DOI] [PMC free article] [PubMed] [Google Scholar]
  • 5.Forger NG: Past, Present, and Future of Epigenetics in Brain Sexual Differentiation. J Neuroendocrinol 2017, doi: 10.1111/jne.12492. [DOI] [PubMed] [Google Scholar]
  • 6.McCarthy MM, Nugent BM, Lenz KM: Neuroimmunology and neuroepigenetics in the establishment of sex differences in the brain. Nat Rev Neurosci 2017, 18:471–484. [DOI] [PMC free article] [PubMed] [Google Scholar]
  • 7.White JG, Southgate E, Thomson JN, Brenner S: The structure of the nervous system of the nematode Caenorhabditis elegans. Philos Trans R Soc Lond B Biol Sci 1986, 314:1–340. [DOI] [PubMed] [Google Scholar]
  • 8.Jarrell TA, Wang Y, Bloniarz AE, Brittin CA, Xu M, Thomson JN, Albertson DG, Hall DH, Emmons SW: The Connectome of a Decision-Making Neural Network. Science 2012, 337:437. [DOI] [PubMed] [Google Scholar]
  • 9.Cline TW, Meyer BJ: Vive la difference: males vs females in flies vs worms. Annu Rev Genet 1996, 30:637–702. [DOI] [PubMed] [Google Scholar]
  • 10.Hodgkin J: A genetic analysis of the sex-determining gene, tra-1, in the nematode Caenorhabditis elegans. Genes Dev 1987, 1:731–745. [DOI] [PubMed] [Google Scholar]
  • 11.Hunter CP, Wood WB: The tra-1 gene determines sexual phenotype cell-autonomously in C. elegans. Cell 1990, 63:1193–1204. [DOI] [PubMed] [Google Scholar]
  • 12.Zarkower D, Hodgkin J: Molecular analysis of the C. elegans sex-determining gene tra-1: a gene encoding two zinc finger proteins. Cell 1992, 70:237–249. [DOI] [PubMed] [Google Scholar]
  • 13.Schvarzstein M, Spence AM: The C. elegans sex-determining GLI protein TRA-1A is regulated by sex-specific proteolysis. Dev Cell 2006, 11:733–740. [DOI] [PubMed] [Google Scholar]
  • 14.Starostina NG, Lim J, Schvarzstein M, Wells L, Spence AM, Kipreos ET: A CUL-2 ubiquitin ligase containing three FEM proteins degrades TRA-1 to regulate C. elegans sex determination. Dev Cell 2007, 13:127–139. [DOI] [PMC free article] [PubMed] [Google Scholar]
  • 15.Portman DS: Sexual modulation of sex-shared neurons and circuits in Caenorhabditis elegans. J Neurosci Res 2017, 95:527–538. [DOI] [PMC free article] [PubMed] [Google Scholar]
  • 16.••.Oren-Suissa M, Bayer EA, Hobert O: Sex-specific pruning of neuronal synapses in Caenorhabditis elegans. Nature 2016, 533:206–211. [DOI] [PMC free article] [PubMed] [Google Scholar]; A shared sensory neuron PHB mediates sexually dimorphic functions and is shown to be wired in a sexually dimorphic manner. This sex-specific wiring is regulated autonomously by the sexual identity of PHB or, remarkably, its postsynaptic partner. The authors also show that sex-specific wiring can arise through selective pruning of synapses that initially form in both sexes or through pre-patterning of synapses that occur in just one sex.
  • 17.••.Serrano-Saiz E, Oren-Suissa M, Bayer EA, Hobert O: Sexually Dimorphic Differentiation of a C. elegans Hub Neuron Is Cell Autonomously Controlled by a Conserved Transcription Factor. Curr Biol CB 2017, 27:199–209. [DOI] [PMC free article] [PubMed] [Google Scholar]; A shared sensory neuron PHC mediates sexually dimorphic behaviors and is wired as a typical sensory neuron in hermaphrodites. By contrast, its wiring resembles that of a typical hub neuron in males; this extensive connectivity in males is accompanied by transcriptional scaling of much of the synaptic machinery. Both the transcriptional scaling and re-wiring are controlled cell-autonomously by DMD-3.
  • 18.••.Ryan DA, Miller RM, Lee K, Neal SJ, Fagan KA, Sengupta P, Portman DS: Sex, Age, and Hunger Regulate Behavioral Prioritization through Dynamic Modulation of Chemoreceptor Expression. Curr Biol 2014, 24:2509–2517. [DOI] [PMC free article] [PubMed] [Google Scholar]; Expression of the chemoreceptor ODR-10 is shown to be sexually dimorphic in shared sensory neurons, and this difference in expression mediates differential behavioral responses to food cues and ultimately to mates. Remarkably, this sex difference in ODR-10 expression is age dependent (apparent in adults) and can be modulated by nutritional status.
  • 19.Srinivasan J, Kaplan F, Ajredini R, Zachariah C, Alborn HT, Teal PEA, Malik RU, Edison AS, Sternberg PW, Schroeder FC: A blend of small molecules regulates both mating and development in Caenorhabditis elegans. Nature 2008, 454:1115–1118. [DOI] [PMC free article] [PubMed] [Google Scholar]
  • 20.•.Fagan KA, Luo J, Lagoy RC, Schroeder FC, Albrecht DR, Portman DS: A Single-Neuron Chemosensory Switch Determines the Valence of a Sexually Dimorphic Sensory Behavior. Curr Biol 2018, 28:902–914.e5. [DOI] [PMC free article] [PubMed] [Google Scholar]
  • 21.Burtis KC: The regulation of sex determination and sexually dimorphic differentiation in Drosophila. Curr Opin Cell Biol 1993, 5:1006–1014. [DOI] [PubMed] [Google Scholar]
  • 22.Marin I, Baker BS: The evolutionary dynamics of sex determination. Science 1998, 281:1990–1994. [DOI] [PubMed] [Google Scholar]
  • 23.Manoli DS, Fan P, Fraser EJ, Shah NM: Neural control of sexually dimorphic behaviors. Curr Opin Neurobiol 2013, 23:330–338. [DOI] [PMC free article] [PubMed] [Google Scholar]
  • 24.Manoli DS, Foss M, Villella A, Taylor BJ, Hall JC, Baker BS: Male-specific fruitless specifies the neural substrates of Drosophila courtship behaviour. Nature 2005, 436:395–400. [DOI] [PubMed] [Google Scholar]
  • 25.Billeter J-C, Villella A, Allendorfer JB, Dornan AJ, Richardson M, Gailey DA, Goodwin SF: Isoform-specific control of male neuronal differentiation and behavior in Drosophila by the fruitless gene. Curr Biol CB 2006, 16:1063–1076. [DOI] [PubMed] [Google Scholar]
  • 26.Demir E, Dickson BJ: fruitless splicing specifies male courtship behavior in Drosophila. Cell 2005, 121:785–794. [DOI] [PubMed] [Google Scholar]
  • 27.••.Neville MC, Nojima T, Ashley E, Parker DJ, Walker J, Southall T, Van de Sande B, Marques AC, Fischer B, Brand AH, et al. : Male-specific fruitless isoforms target neurodevelopmental genes to specify a sexually dimorphic nervous system. Curr Biol CB 2014, 24:229–241. [DOI] [PMC free article] [PubMed] [Google Scholar]; Different splice isoforms of FruM are shown to regulate different aspects of male courtship, and using genome-wide approaches, are shown to regulate shared as well as non-overlapping target genes. Thus, different splice isoforms of FruM perform distinct as well as redundant functions
  • 28.•.Ito H, Sato K, Kondo S, Ueda R, Yamamoto D: Fruitless Represses robo1 Transcription to Shape Male-Specific Neural Morphology and Behavior in Drosophila. Curr Biol CB 2016, 26:1532–1542. [DOI] [PubMed] [Google Scholar]
  • 29.•.Xu X, Coats JK, Yang CF, Wang A, Ahmed OM, Alvarado M, Izumi T, Shah NM: Modular genetic control of sexually dimorphic behaviors. Cell 2012, 148:596–607. [DOI] [PMC free article] [PubMed] [Google Scholar]
  • 30.•.Ito H, Sato K, Koganezawa M, Ote M, Matsumoto K, Hama C, Yamamoto D: Fruitless recruits two antagonistic chromatin factors to establish single-neuron sexual dimorphism. Cell 2012, 149:1327–1338. [DOI] [PubMed] [Google Scholar]
  • 31.••.Clough E, Jimenez E, Kim Y-A, Whitworth C, Neville MC, Hempel LU, Pavlou HJ, Chen Z-X, Sturgill D, Dale RK, et al. : Sex- and tissue-specific functions of Drosophila doublesex transcription factor target genes. Dev Cell 2014, 31:761–773. [DOI] [PMC free article] [PubMed] [Google Scholar]; The authors use genome-wide methods to identify genomic targets of sex-specific variants of Dsx. This is an important step in understanding how Dsx can have sexually dimorphic effects on development.
  • 32.•.Meissner GW, Luo SD, Dias BG, Texada MJ, Baker BS: Sex-specific regulation of Lgr3 in Drosophila neurons. Proc Natl Acad Sci U S A 2016, 113:E1256–1265. [DOI] [PMC free article] [PubMed] [Google Scholar]
  • 33.••.Lin H-H, Cao D-S, Sethi S, Zeng Z, Chin JSR, Chakraborty TS, Shepherd AK, Nguyen CA, Yew JY, Su C-Y, et al. : Hormonal Modulation of Pheromone Detection Enhances Male Courtship Success. Neuron 2016, 90:1272–1285. [DOI] [PMC free article] [PubMed] [Google Scholar]; In this paper the authors demonstrate that the reproductive advantage of older male flies is at least partially dependent on enhanced sensitivity to female cuticular pheromones. Further, they show that this sensitivity is dependent on upregulation of a specific olfactory receptor, which is in turn mediated by a hormone signaling through its cognate receptor.
  • 34.Wu MV, Manoli DS, Fraser EJ, Coats JK, Tollkuhn J, Honda S-I, Harada N, Shah NM: Estrogen masculinizes neural pathways and sex-specific behaviors. Cell 2009, 139:61–72. [DOI] [PMC free article] [PubMed] [Google Scholar]
  • 35.Liu XF, Bagchi MK: Recruitment of distinct chromatin-modifying complexes by tamoxifen-complexed estrogen receptor at natural target gene promoters in vivo. J Biol Chem 2004, 279:15050–8. [DOI] [PubMed] [Google Scholar]
  • 36.Mangelsdorf DJ, Thummel C, Beato M, Herrlich P, Schutz G, Umesono K, Blumberg B, Kastner P, Mark M, Chambon P, et al. : The nuclear receptor superfamily: the second decade. Cell 1995, 83:835–9. [DOI] [PMC free article] [PubMed] [Google Scholar]
  • 37.•.Matsuda KI, Mori H, Nugent BM, Pfaff DW, McCarthy MM, Kawata M: Histone deacetylation during brain development is essential for permanent masculinization of sexual behavior. Endocrinology 2011, 152:2760–2767. [DOI] [PMC free article] [PubMed] [Google Scholar]
  • 38.•.Shen EY, Ahern TH, Cheung I, Straubhaar J, Dincer A, Houston I, de Vries GJ, Akbarian S, Forger NG: Epigenetics and sex differences in the brain: A genome-wide comparison of histone-3 lysine-4 trimethylation (H3K4me3) in male and female mice. Exp Neurol 2015, 268:21–9. [DOI] [PMC free article] [PubMed] [Google Scholar]
  • 39.•.Tsai HW, Grant PA, Rissman EF: Sex differences in histone modifications in the neonatal mouse brain. Epigenetics 2009, 4:47–53. [DOI] [PMC free article] [PubMed] [Google Scholar]
  • 40.•.Murray EK, Hien A, de Vries GJ, Forger NG: Epigenetic control of sexual differentiation of the bed nucleus of the stria terminalis. Endocrinology 2009, 150:4241–4247. [DOI] [PMC free article] [PubMed] [Google Scholar]
  • 41.•.Mosley M, Weathington J, Cortes LR, Bruggeman E, Castillo-Ruiz A, Xue B, Forger NG: Neonatal Inhibition of DNA Methylation Alters Cell Phenotype in Sexually Dimorphic Regions of the Mouse Brain. Endocrinology 2017, 158:1838–1848. [DOI] [PMC free article] [PubMed] [Google Scholar]
  • 42.•.Nugent BM, Wright CL, Shetty AC, Hodes GE, Lenz KM, Mahurkar A, Russo SJ, Devine SE, McCarthy MM: Brain feminization requires active repression of masculinization via DNA methylation. Nat Neurosci 2015, 18:690–7. [DOI] [PMC free article] [PubMed] [Google Scholar]
  • 43.•.Ghahramani NM, Ngun TC, Chen PY, Tian Y, Krishnan S, Muir S, Rubbi L, Arnold AP, de Vries GJ, Forger NG, et al. : The effects of perinatal testosterone exposure on the DNA methylome of the mouse brain are late-emerging. Biol Sex Differ 2014, 5:8. [DOI] [PMC free article] [PubMed] [Google Scholar]
  • 44.Zhang T, Zarkower D. DMRT Proteins and coordination of mammalian spermatogenesis. Stem Cell Research 2017, 24:195–202 [DOI] [PMC free article] [PubMed] [Google Scholar]
  • 45.Luo L, Callaway EM, Svoboda K: Genetic Dissection of Neural Circuits: A Decade of Progress. Neuron 2018, 98:256–281. [DOI] [PMC free article] [PubMed] [Google Scholar]

RESOURCES