Sex-determining mechanisms in animals

Considerations and challenges for sex-aware drug repurposing

Sex differences are essential factors in disease etiology and manifestation in many diseases such as cardiovascular disease, cancer, and neurodegeneration [33]. The biological influence of sex differences (including genomic, epigenetic, hormonal, immunological, and metabolic differences between males and females) and the lack of biomedical studies considering sex differences in their study design has led to several policies. For example, the National Institute of Health's (NIH) sex as a biological variable (SABV) and Sex and Gender Equity in Research (SAGER) policies to motivate researchers to consider sex differences [204]. However, drug repurposing, a promising alternative to traditional drug discovery by identifying novel uses for FDA-approved drugs, lacks sex-aware methods that can improve the identification of drugs that have sex-specific responses [7, 11, 14, 33]. Sex-aware drug repurposing methods either select drug candidates that are more efficacious in one sex or deprioritize drug candidates based on if they are predicted to cause a sex-bias adverse event (SBAE), unintended therapeutic effects that are more likely to occur in one sex. Computational drug repurposing methods are encouraging approaches to develop for sex-aware drug repurposing because they can prioritize sex-specific drug candidates or SBAEs at lower cost and time than traditional drug discovery. Sex-aware methods currently exist for clinical, genomic, and transcriptomic information [1, 7, 155]. They have not expanded to other data types, such as DNA variation, which has been beneficial in other drug repurposing methods that do not consider sex [114]. Additionally, some sex-aware methods suffer from poorer performance because a disproportionate number of male and female samples are available to train computational methods [7]. However, there is development potential for several different categories (i.e., data mining, ligand binding predictions, molecular associations, and networks). Low-dimensional representations of molecular association and network approaches are also especially promising candidates for future sex-aware drug repurposing methodologies because they reduce the multiple hypothesis testing burden and capture sex-specific variation better than the other methods [151, 159]. Here we review how sex influences drug response, the current state of drug repurposing including with respect to sex-bias drug response, and how model organism study design choices influence drug repurposing validation.

Sex differences in the pelvis did not evolve de novo in modern humans

It is commonly assumed that the strong sexual dimorphism of the human pelvis evolved for delivering the relatively large human foetuses. Here we compare pelvic sex differences across modern humans and chimpanzees using a comprehensive geometric morphometric approach. Even though the magnitude of sex differences in pelvis shape was two times larger in humans than in chimpanzees, we found that the pattern is almost identical in the two species. We conclude that this pattern of pelvic sex differences did not evolve de novo in modern humans and must have been present in the common ancestor of humans and chimpanzees, and thus also in the extinct Homo species. We further suggest that this shared pattern was already present in early mammals and propose a hypothesis of facilitated variation as an explanation: the conserved mammalian endocrine system strongly constrains the evolution of the pattern of pelvic differences but enables rapid evolutionary change of the magnitude of sexual dimorphism, which in turn facilitated the rapid increase in hominin brain size.

Sex Ratio of Small Hive Beetles: The Role of Pupation and Adult Longevity

The sex ratio of sexually reproducing animal species tends to be 1:1, which is known as Fisher's principle. However, differential mortality and intraspecific competition during pupation can result in a biased adult sex ratio in insects. The female-biased sex ratio of small hive beetles (SHBs) is known from both laboratory and field studies, but the underlying reasons are not well understood. Here, we used laboratory mass and individual pupation to test if differential mortality between sexes and/or intraspecific interactions can explain this sex ratio. The data show a significant female-biased adult sex ratio in both mass and individual rearing, even when assuming that all dead individuals were males. Our results therefore suggest that neither differential mortality during pupation nor intraspecific interactions are likely to explain the female-biased sex ratio of freshly emerged adult SHBs. We regard it as more likely that either competition during the larval feeding stage or genetic mechanisms are involved. In addition, we compared our data with previously published data on the sex ratio of both freshly emerged and field-collected SHBs to investigate possible gender differences in adult longevity. The data show a significantly greater female bias in the sex ratio upon emergence, compared to field-collected SHBs, suggesting that adult females have a shorter longevity.

Exogenous application of estradiol to eggs unexpectedly induces male development in two turtle species with temperature-dependent sex determination

Steroid hormones affect sex determination in a variety of vertebrates. The feminizing effects of exposure to estradiol and the masculinizing effects of aromatase inhibition during development are well established in a broad range of vertebrate taxa, but paradoxical findings are occasionally reported. Four independent experiments were conducted on two turtle species with temperature-dependent sex determination (Chrysemys picta and Chelydra serpentina) to quantify the effects of egg incubation temperature, estradiol, and an aromatase inhibitor on offspring sex ratios. As expected, the warmer incubation temperatures induced female development and the cooler temperatures produced primarily males. However, application of an aromatase inhibitor had no effect on offspring sex ratios, and exogenous applications of estradiol to eggs produced male offspring across all incubation temperatures. These unexpected results were remarkably consistent across all four experiments and both study species. Elevated concentrations of estradiol could interact with androgen receptors or inhibit aromatase expression, which might result in relatively high testosterone concentrations that lead to testis development. These findings add to a short list of studies that report paradoxical effects of steroid hormones, which addresses the need for a more comprehensive understanding of the role of sex steroids in sexual development.

A case of Intersexuality in the Parthenogenetic Marmorkrebs (Decapoda: Astacida: Cambaridae)

We describe an intersex specimen of the Marmorkrebs, the only obligate parthenogenetic freshwater crayfish with an all-female population. The individual was a fully functional female which possessed male-like first pleopods. Nevertheless, it reproduced successfully and the offspring were normally developed parthenogenetic females, lacking any trace of male traits. The general rarity of aberrant sexual traits in freshwater crayfishes, in particular in Procambarus, is discussed. We suggest that a dysfunction of the sex determining system, which controls the anlagen of the androgenic glands during development, caused the partial male-like phenotype of this Marmorkrebs specimen. The application of this organism for investigations of sex determination and differentiation is recommended.

Effects of sex and age on genotype x environment interaction for beef cattle body weight studied using reaction norm models

The interest in the effect of genotype × environment interaction is increasing because animal breeding programs have become geographically broader. Climate changes in the next decades are also expected to challenge the present breeding goals, increasing the importance of environmental sensitivity. The aim of this work was to analyze genotype × environment interaction effect on cattle BW using the environmental sensitivity predicted by random regression reaction norm models, including sex and age effects as additional dimensions in the study. Genetic parameters were estimated for adjusted BW of Brazilian Nelore cattle at different ages (120, 210, 365, and 450 d), using linear polynomials for random regression analysis. The analyses with sex as a fixed effect (total analyses) were compared with those with sex-separated progenies (male and female progeny analyses, respectively). (Co)variance components were estimated and breeding values calculated EPD. The results showed important differences in reaction norm model genetic parameter estimates according to different age and sex analyses. The results confirmed the presence of an important genotype × environment × sex × age interaction for Nelore cattle BW. The patterns in these results lead to a revision of the importance of sexual and developmental factors on plasticity and adaptation concepts.

A comparative analysis of sex change in Labridae supports the size advantage hypothesis

The size advantage hypothesis (SAH) predicts that the rate of increase in male and female fitness with size (the size advantage) drives the evolution of sequential hermaphroditism or sex change. Despite qualitative agreement between empirical patterns and SAH, only one comparative study tested SAH quantitatively. Here, we perform the first comparative analysis of sex change in Labridae, a group of hermaphroditic and dioecious (non-sex changer) fish with several model sex-changing species. We also estimate, for the first time, rates of evolutionary transitions between sex change and dioecy. Our analyses support SAH and indicate that the evolution of hermaphroditism is correlated to the size advantage. Furthermore, we find that transitions from sex change to dioecy are less likely under stronger size advantage. We cannot determine, however, how the size advantage affects transitions from dioecy to sex change. Finally, contrary to what is generally expected, we find that transitions from dioecy to sex change are more likely than transitions from sex change to dioecy. The similarity of sexual differentiation in hermaphroditic and dioecious labrids might underlie this pattern. We suggest that elucidating the developmental basis of sex change is critical to predict and explain patterns of the evolutionary history of sequential hermaphroditism.

Review. Meiotic drive and sex determination: molecular and cytological mechanisms of sex ratio adjustment in birds

Differences in relative fitness of male and female offspring across ecological and social environments should favour the evolution of sex-determining mechanisms that enable adjustment of brood sex ratio to the context of breeding. Despite the expectation that genetic sex determination should not produce consistent bias in primary sex ratios, extensive and adaptive modifications of offspring sex ratio in relation to social and physiological conditions during reproduction are often documented. Such discordance emphasizes the need for empirical investigation of the proximate mechanisms for modifying primary sex ratios, and suggests epigenetic effects on sex-determining mechanisms as the most likely candidates. Birds, in particular, are thought to have an unusually direct opportunity to modify offspring sex ratio because avian females are heterogametic and because the sex-determining division in avian meiosis occurs prior to ovulation and fertilization. However, despite evidence of strong epigenetic effects on sex determination in pre-ovulatory avian oocytes, the mechanisms behind such effects remain elusive. Our review of molecular and cytological mechanisms of avian meiosis uncovers a multitude of potential targets for selection on biased segregation of sex chromosomes, which may reflect the diversity of mechanisms and levels on which such selection operates in birds. Our findings indicate that pronounced differences between sex chromosomes in size, shape, size of protein bodies, alignment at the meiotic plate, microtubule attachment and epigenetic markings should commonly produce biased segregation of sex chromosomes as the default state, with secondary evolution of compensatory mechanisms necessary to maintain unbiased meiosis. We suggest that it is the epigenetic effects that modify such compensatory mechanisms that enable context-dependent and precise adjustment of primary sex ratio in birds. Furthermore, we highlight the features of avian meiosis that can be influenced by maternal hormones in response to environmental stimuli and may account for the precise and adaptive patterns of offspring sex ratio adjustment observed in some species.

A social basis for the development of primary males in a sex-changing fish

An example of alternative male strategies is seen in diandric protogynous (female first) hermaphrodites, where individuals either mature directly as male (primary males) or first reproduce as female and then change sex to male (secondary males). In some sex-changing fishes, the testes of primary males appear anatomically similar to those of non-sex-changing species, whereas the testes of secondary males have anatomical evidence of their former ovarian function. Here, we provide evidence that in the bluehead wrasse, Thalassoma bifasciatum, these strikingly different male phenotypes arise from differences in the ontogenetic timing of environmental sex determination, timing that can be experimentally altered through changes in the social circumstances. Juveniles differentiated almost exclusively as females when reared in isolation, regardless of whether they were collected from a reef with a high proportion of primary males or from a reef with a low proportion of primary males. In contrast, one individual usually differentiated as a primary male when reared in groups of three. Our results indicate that primary males of the bluehead wrasse are an environmentally sensitive developmental strategy that has probably evolved in response to variation in the reproductive success of primary males in populations of different sizes.

Ultrastructural aspects of gonadal morphogenesis inBufo bufo (Amphibia Anura) 1. sex differentiation

The morphogenesis of gonads in Bufo bufo tadpoles was studied, and ultrastructural differences between sexes were identified. All specimens analyzed initially developed gonads made up of a peripheral fertile layer (cortex) surrounding a small primary cavity. Subsequently a central layer of somatic cells (medulla) developed. Both layers were separated by two uninterrupted basal laminae between which a vestige of the primary cavity persisted. During female differentiation, the peripheral layer continued to be the fertile layer. In males, the central layer blended into the peripheral layer and the basal laminae disappeared. The somatic cells of the central layer came into direct contact with the germ cells; this did not occur in females. Testicular differentiation continued with the migration of germ cells towards the center of the gonad. The somatic elements surrounding the germ cells appeared to play an active role in their transfer to the center of the gonad. The peripheral layer shrank and became sterile. Two basal laminae then re-formed to separate the fertile central layer from the peripheral sterile one. Germ cells have always been thought to perform a passive role in sex differentiation in amphibians. Following the generally accepted "symmetric model", the mechanism of gonad development is symmetrical, with cortical somatic cells determining ovarian differentiation and medullary somatic cells determining testicular differentiation. In contrast, we found that sex differentiation follows an "asymmetric" pattern in which germ cells tend primarily toward a female differentiation and male differentiation depends on a secondary interaction between germ cells and medullary somatic cells.

The effects of aromatase and 5 alpha-reductase inhibitors, antiandrogen, and sex steroids on Bidder's organs development and gonadal differentiation in Bufo bufo tadpoles

Embryos of toads (Bufo bufo) were treated with aromatase (4-OHA) and 5 alpha-reductase (17 beta C) inhibitors, antiandrogen (CPA), estradiol-17 beta, testosterone, and 5 alpha-dihydrotestosterone in order to study the role played by sex steroids in the development and sex differentiation of gonads. Test compounds were administered to tadpoles in water and morphometric and cytometric analyses were carried out on histological sections of the cephalic Bidder's organ (a rudimentary ovary) and of the gonadal region. In Bidder's organs, the number and size of oogonia and oocytes were modified by the treatments. However, the female commitment of the Bidder's organ occurs independently from steroid treatments that lead to an acceleration or slackening of the processes of proliferation and differentiation of oogonia. 4-OHA and androgens caused various degrees of inhibition of ovarian differentiation, with gonads maintaining an undifferentiated condition. Estrogen provoked a shift of the sex ratio towards the female sex, yet slackened gonadal growth. 17 beta C accelerated ovarian differentiation in females while CPA enhanced gonadal differentiation in both sexes by promoting the germ and somatic cell proliferation. We suggest that sex hormones may have a local regulatory role in gonadal differentiation during early developmental stages. Furthermore, the strong tendency of Bidderian germ cells to develop in the oogenetic way regardless of sex genotype and steroid treatments, and the quantitative sex differences found in the control Bidder's organs and gonads, suggest that other factors (such as intracellular mechanisms) may be involved in the initial steps of the process of germ cell differentiation.