Cell death and sexual differentiation of behavior: worms, flies, and mammals

Deep conservation and co-option of programmed cell death facilitates evolution of alternative phenotypes at multiple biological levels

Alternative phenotypes, such as polyphenisms and sexual dimorphisms, are widespread in nature and appear at all levels of biological organization, from genes and cells to morphology and behavior. Yet, our understanding of the mechanisms through which alternative phenotypes develop and how they evolve remains understudied. In this review, we explore the association between alternative phenotypes and programmed cell death, a mechanism responsible for the elimination of superfluous cells during development. We discuss the ancient origins and deep conservation of programmed cell death (its function, forms and underlying core regulatory gene networks), and propose that it was co-opted repeatedly to generate alternative phenotypes at the level of cells, tissues, organs, external morphology, and even individuals. We review several examples from across the tree of life to explore the conditions under which programmed cell death is likely to facilitate the evolution of alternative phenotypes.

Developmental mechanisms of sex differences: from cells to organisms

Male-female differences in many developmental mechanisms lead to the formation of two morphologically and physiologically distinct sexes. Although this is expected for traits with prominent differences between the sexes, such as the gonads, sex-specific processes also contribute to traits without obvious male-female differences, such as the intestine. Here, we review sex differences in developmental mechanisms that operate at several levels of biological complexity - molecular, cellular, organ and organismal - and discuss how these differences influence organ formation, function and whole-body physiology. Together, the examples we highlight show that one simple way to gain a more accurate and comprehensive understanding of animal development is to include both sexes.

Genetic basis of susceptibility to low-dose paraquat and variation between the sexes in Drosophila melanogaster

Toxicant resistance is a complex trait, affected both by genetics and the environment. Like most complex traits, it can exhibit sexual dimorphism, yet sex is often overlooked as a factor in studies of toxicant resistance. Paraquat, one such toxicant, is a commonly used herbicide and is known to produce mitochondrial oxidative stress, decrease dopaminergic neurons and dopamine (DA) levels, and decrease motor ability. While the main effects of paraquat are well-characterized, less is known about the naturally occurring variation in paraquat susceptibility. The purpose of this study was to map the genes contributing to low-dose paraquat susceptibility in Drosophila melanogaster, and to determine if susceptibility differs between the sexes. One hundred of the Drosophila Genetic Reference Panel (DGRP) lines were scored for susceptibility via climbing ability and used in a genome-wide association study (GWAS). Variation in seventeen genes in females and thirty-five genes in males associated with paraquat susceptibility. Only two candidate genes overlapped between the sexes despite a significant positive correlation between male and female susceptibilities. Many associated polymorphisms had significant interactions with sex, with most having conditionally neutral effects. Conditional neutrality between the sexes probably stems from sex-biased expression which may result from partial resolution of sexual conflict. Candidate genes were verified with RNAi knockdowns, gene expression analyses, and DA quantification. Several of these genes are novel associations with paraquat susceptibility. This research highlights the importance of assessing both sexes when studying toxicant susceptibility.

Androgenic regulation of sexually dimorphic expression of RNA binding motif protein 48 in the developing mouse cortex and hippocampus

To further reveal the molecular mechanism underlying sexual differentiation of the mouse cerebral cortex and hippocampus, we reanalyzed our previous microarray study with Gene Ontology (GO) term enrichment and found that the GO term "RNA binding" was over-represented among the 89 sexually dimorphic candidate genes. Thus, we selected 16 autosomal genes annotated to the term RNA binding and profiled their mRNA expression in the developing male and female mouse cortex/hippocampus. During the first three weeks after birth, sex differences in mRNA levels of Khdrbs2, Nanos2, Rbm48, and Tdrd3 were observed in the mouse cortex/hippocampus. Of these genes, only the female-biased expression of Rbm48 in neonates was abolished by prenatal exposure to testosterone propionate (TP), while postnatal treatment of TP three weeks after birth increased Rbm48 and Tdrd3 mRNA levels in both sexes. Regardless of sex, the postnatal cortex/hippocampus also showed a marked increase in the content of androgen receptor (Ar) and estrogen receptor β (Esr2), but a decrease in estrogen receptor α (Esr1) and aromatase (Cyp19a1), which might confer the different responses of Rbm48 to prenatal and postnatal TP. Our results suggest that androgen-regulated, sexually dimorphic Rbm48 expression might present a novel molecular mechanism by which perinatal androgens control development of sexual dimorphism in cortical and hippocampal structure and function.

Sex differences in avoidance behavior after perceiving potential risk in mice

Background

Sex has been considered as a potential factor regulating individual behaviors in different contexts. Recently, findings on sex differences in the neuroendocrine circuit have expanded due to exact measurements and control of neuronal activity, while findings on sex differences in behavioral phenotypes are limited. One efficient way to determine the miscellaneous aspects of a sexually different behavior is to segment it into a set of simpler responses induced by discrete scenes.

Methods

In the present study, we conducted a battery of behavioral tests within a variety of unique risky scenes, to determine where and how sex differences arise in responses under those scenes.

Results

A significant sex difference was observed in the avoidance responses measured in the two-way active and the passive avoidance tests. The phenotype observed was higher mobility in male mice and reduced mobility in female mice, and required associative learning between an escapable risk and its predictive cue. This was limited in other scenes where escapable risk or predictive cue or both were missing.

Conclusions

Taken together, the present study found that the primary sex difference occurs in mobility in the avoidance response after perceiving escapable risks.

Electronic supplementary material

The online version of this article (doi:10.1186/s12993-017-0126-3) contains supplementary material, which is available to authorized users.

Hypothalamic control of the male neonatal testosterone surge

Sex differences in brain neuroanatomy and neurophysiology underpin considerable physiological and behavioural differences between females and males. Sexual differentiation of the brain is regulated by testosterone secreted by the testes predominantly during embryogenesis in humans and the neonatal period in rodents. Despite huge advances in understanding how testosterone, and its metabolite oestradiol, sexually differentiate the brain, little is known about the mechanism that actually generates the male-specific neonatal testosterone surge. This review examines the evidence for the role of the hypothalamus, and particularly the gonadotropin-releasing hormone (GnRH) neurons, in generating the neonatal testosterone surge in rodents and primates. Kisspeptin-GPR54 signalling is well established as a potent and critical regulator of GnRH neuron activity during puberty and adulthood, and we argue here for an equally important role at birth in driving the male-specific neonatal testosterone surge in rodents. The presence of a male-specific population of preoptic area kisspeptin neurons that appear transiently in the perinatal period provide one possible source of kisspeptin drive to neonatal GnRH neurons in the mouse.

Sex differences and rapid estrogen signaling: A look at songbird audition

The actions of estrogens have been associated with brain differentiation and sexual dimorphism in a wide range of vertebrates. Here we consider the actions of brain-derived 'neuroestrogens' in the forebrain and the accompanying differences and similarities observed between males and females in a variety of species. We summarize recent evidence showing that baseline and fluctuating levels of neuroestrogens within the auditory forebrain of male and female zebra finches are largely similar, and that neuroestrogens enhance auditory representations in both sexes. With a comparative perspective we review evidence that non-genomic mechanisms of neuroestrogen actions are sexually differentiated, and we propose a working model for nonclassical estrogen signaling via the MAPK intracellular signaling cascade in the songbird auditory forebrain that is informed by the way sex differences may be compensated. This view may lead to a more comprehensive understanding of how sex influences estradiol-dependent modulation of sensorimotor representations.

Sexual differentiation of the brain requires perinatal kisspeptin-GnRH neuron signaling

Sex differences in brain function underlie robust differences between males and females in both normal and disease states. Although alternative mechanisms exist, sexual differentiation of the male mammalian brain is initiated predominantly by testosterone secreted by the testes during the perinatal period. Despite considerable advances in understanding how testosterone and its metabolite estradiol sexually differentiate the brain, little is known about the mechanism that generates the male-specific perinatal testosterone surge. In mice, we show that a male-specific activation of GnRH neurons occurs 0-2 h following birth and that this correlates with the male-specific surge of testosterone occurring up to 5 h after birth. The necessity of GnRH signaling for the sexually differentiating effects of the perinatal testosterone surge was demonstrated by the persistence of female-like brain characteristics in adult male, GnRH receptor knock-out mice. Kisspeptin neurons have recently been identified to be potent, direct activators of GnRH neurons. We demonstrate that a population of kisspeptin neurons appears in the preoptic area of only the male between E19 and P1. The importance of kisspeptin inputs to GnRH neurons for the process of sexual differentiation was demonstrated by the lack of a normal neonatal testosterone surge, and disordered brain sexual differentiation of male mice in which the kisspeptin receptor was deleted selectively from GnRH neurons. These observations demonstrate the necessity of perinatal GnRH signaling for driving brain sexual differentiation and indicate that kisspeptin inputs to GnRH neurons are essential for this process to occur.

Representing sex in the brain, one module at a time

Sexually dimorphic behaviors, qualitative or quantitative differences in behaviors between the sexes, result from the activity of a sexually differentiated nervous system. Sensory cues and sex hormones control the entire repertoire of sexually dimorphic behaviors, including those commonly thought to be charged with emotion such as courtship and aggression. Such overarching control mechanisms regulate distinct genes and neurons that in turn specify the display of these behaviors in a modular manner. How such modular control is transformed into cohesive internal states that correspond to sexually dimorphic behavior is poorly understood. We summarize current understanding of the neural circuit control of sexually dimorphic behaviors from several perspectives, including how neural circuits in general, and sexually dimorphic neurons in particular, can generate sexually dimorphic behaviors, and how molecular mechanisms and evolutionary constraints shape these behaviors. We propose that emergent themes such as the modular genetic and neural control of dimorphic behavior are broadly applicable to the neural control of other behaviors.

Sex-specific processing of social cues in the medial amygdala

Animal–animal recognition within, and across species, is essential for predator avoidance and social interactions. Despite its essential role in orchestrating responses to animal cues, basic principles of information processing by the vomeronasal system are still unknown. The medial amygdala (MeA) occupies a central position in the vomeronasal pathway, upstream of hypothalamic centers dedicated to defensive and social responses. We have characterized sensory responses in the mouse MeA and uncovered emergent properties that shed new light onto the transformation of vomeronasal information into sex- and species-specific responses. In particular, we show that the MeA displays a degree of stimulus selectivity and a striking sexually dimorphic sensory representation that are not observed in the upstream relay of the accessory olfactory bulb (AOB). Furthermore, our results demonstrate that the development of sexually dimorphic circuits in the MeA requires steroid signaling near the time of puberty to organize the functional representation of sensory stimuli.

DOI:http://dx.doi.org/10.7554/eLife.02743.001

Many animals emit and detect chemicals known as pheromones to communicate with other members of their own species. Animals also rely on chemical signals from other species to warn them, for example, that a predator is nearby. Many of these chemical signals—which are present in sweat, tears, urine, and saliva—are detected by a structure called the vomeronasal organ, which is located at the base of the nasal cavity.

When this organ detects a particular chemical signal, it broadcasts this information to a network of brain regions that generates an appropriate behavioral response. Two structures within this network, the accessory olfactory bulb and the medial amygdala, play an important role in modifying this signal before it reaches its final destination—a region of the brain called the hypothalamus. Activation of the hypothalamus by the signal triggers changes in the animal's behavior. Although the anatomical details of this pathway have been widely studied, it is not clear how information is actually transmitted along it.

Now, Bergan et al. have provided insights into this process by recording signals in the brains of anesthetized mice exposed to specific stimuli. Whereas neurons in the accessory olfactory bulb responded similarly in male and female mice, those in the medial amygdala showed a preference for female urine in male mice, and a preference for male urine in the case of females. This is the first direct demonstration of differences in sensory processing in the brains of male and female mammals.

These differences are thought to result from the actions of sex hormones, particularly estrogen, on brain circuits during development. Consistent with this, neurons in the medial amygdala of male mice with reduced levels of estrogen showed a reduced preference for female urine compared to control males. Similarly, female mice that had been previously exposed to high levels of estrogen as pups showed a reduced preference for male urine compared to controls.

In addition to increasing understanding of how chemical signals—including pheromones—influence the responses of rodents to other animals, the work of Bergan et al. has provided clues to the neural mechanisms that underlie sex-specific differences in behaviors.

DOI:http://dx.doi.org/10.7554/eLife.02743.002

Sex-specific processing of social cues in the medial amygdala

Animal-animal recognition within, and across species, is essential for predator avoidance and social interactions. Despite its essential role in orchestrating responses to animal cues, basic principles of information processing by the vomeronasal system are still unknown. The medial amygdala (MeA) occupies a central position in the vomeronasal pathway, upstream of hypothalamic centers dedicated to defensive and social responses. We have characterized sensory responses in the mouse MeA and uncovered emergent properties that shed new light onto the transformation of vomeronasal information into sex- and species-specific responses. In particular, we show that the MeA displays a degree of stimulus selectivity and a striking sexually dimorphic sensory representation that are not observed in the upstream relay of the accessory olfactory bulb (AOB). Furthermore, our results demonstrate that the development of sexually dimorphic circuits in the MeA requires steroid signaling near the time of puberty to organize the functional representation of sensory stimuli.DOI: http://dx.doi.org/10.7554/eLife.02743.001.

Sex determination strategies in 2012: towards a common regulatory model?

Sex determination is a complicated process involving large-scale modifications in gene expression affecting virtually every tissue in the body. Although the evolutionary origin of sex remains controversial, there is little doubt that it has developed as a process of optimizing metabolic control, as well as developmental and reproductive functions within a given setting of limited resources and environmental pressure. Evidence from various model organisms supports the view that sex determination may occur as a result of direct environmental induction or genetic regulation. The first process has been well documented in reptiles and fish, while the second is the classic case for avian species and mammals. Both of the latter have developed a variety of sex-specific/sex-related genes, which ultimately form a complete chromosome pair (sex chromosomes/gonosomes). Interestingly, combinations of environmental and genetic mechanisms have been described among different classes of animals, thus rendering the possibility of a unidirectional continuous evolutionary process from the one type of mechanism to the other unlikely. On the other hand, common elements appear throughout the animal kingdom, with regard to a) conserved key genes and b) a central role of sex steroid control as a prerequisite for ultimately normal sex differentiation. Studies in invertebrates also indicate a role of epigenetic chromatin modification, particularly with regard to alternative splicing options. This review summarizes current evidence from research in this hot field and signifies the need for further study of both normal hormonal regulators of sexual phenotype and patterns of environmental disruption.