Biology subjects

Saunders, P. A.

Publications and source records attributed to Saunders, P. A..

6 recordsLinked to original sources

bioRxiv2026

Thermally driven sex reversal reveals divergent sex determination dynamics in wild viviparous reptile populations

Climate change can threaten population viability by disrupting sex ratios in species whose sex is influenced by temperature. While species with sex chromosomes were historically considered immune, in some species, temperatures can override genetic sex determination via sex reversal, leaving them vulnerable to climate-driven sex ratio shifts. The Tasmanian spotted snow skink (Carinacincus ocellatus), a viviparous reptile with an XX/XY system, provides a compelling case study. While laboratory studies demonstrated that extreme thermal conditions induce female-to-male sex reversal (XX males), its occurrence in the wild remains unexplored, limiting our understanding of actual climate impacts. Integrating 23 years of phenotypic and genetic sexing data across two climatically distinct populations, we provide the first evidence of sex reversal in a wild viviparous reptile. XX reversal occurred in both populations, affecting up to 23.5% of XX births in the warmer population, and was associated with colder minimum daily temperatures. Despite high birth rates in some years, sex-reversed adults were rare. We also identified putative XY females, suggesting bidirectional sex reversal and reinforcing the extreme plasticity of reptilian sex determination. Ultimately, sex reversal could act as an evolutionary trap, potentially compromising population viability as climate instability increases.

evolutionary biology↗

bioRxiv2024

Influence of gonadal and chromosomal sex on the brain transcriptome in a mouse species with natural sex reversal

Sex chromosomes are expected to play a role in shaping the transcriptional architecture of sexual dimorphism, through the direct expression of sex-linked genes, by regulating autosomal genes, or in interactions with hormones. Yet, their degree of involvement remains elusive partly because chromosomal sex (XX/XY or ZZ/ZW) and gonadal sex (ovaries or testes) are usually inextricably intertwined. They are however dissociated in the African pygmy mouse, Mus minutoides, in which a feminizing X (X*) has evolved resulting in three female genotypes (XX, XX* and X*Y) and one male genotype (XY). Despite hormonal levels similar to the other females, X*Y females show distinctive phenotypes with greater fertility, divergent maternal care strategies and the masculinization of some traits (e.g. enhanced aggressiveness). By comparing the brain transcriptome of the four sexual genotypes, we show, here, that differential gene expression is mainly linked to gonadal sex (male vs. female) but also and significantly, to chromosomal sex, with expression patterns matching the singularity of X*Y female traits. Genes with such patterns are over-represented on sex and sex-linked chromosomes, and some are strong candidates to explain X*Y-specific behavioral and reproductive traits. We also report the preferential inactivation of the X* chromosome in XX* females, which could explain their trait similarities with XX females. Overall, we show that sex chromosomes have profoundly impacted the brain transcriptome in ways that reflect their new transmission modes and new resulting conflicts. This opens exciting prospects on the evolution of sex differences in relation to the dynamic of sex chromosome evolution.

evolutionary biology↗

bioRxiv2023

Using a handful of transcriptomes to detect sex-linked markers in a lizard with homomorphic sex chromosomes

To understand the biology of a species it is often crucial to be able to differentiate males and females. Many species lack distinguishable sexually dimorphic traits, but in those that possess sex chromosomes, molecular sexing offers a good alternative. Designing molecular sexing assays is typically achieved through the comparison of male and female genomic sequences, often from reduced-representation sequencing. However, in many non-model species sex chromosomes are poorly differentiated, and identifying sex-limited sequences and developing PCR-based sexing assays is challenging without additional genomic resources. Here we highlight a simple procedure for detection of sex-linked markers based on transcriptomes that circumvents limitations of other approaches. We apply it to the spotted snow skink Carinascincus ocellatus, a lizard with homomorphic XY chromosomes that also experiences environmentally-induced sex reversal. With transcriptomes from 3 males and 3 females alone, we identify thousands of putative Y-linked sequences. We confirm linkage through alignment of assembled transcripts to a distantly related genome, and readily design PCR primers to sex C. ocellatus and related species. In addition to providing an important molecular sexing tool for these species, this approach also facilitated valuable comparisons of sex determining systems on a large taxonomic scale.

genetics↗

bioRxiv2022

Separating the effects of sex hormones and sex chromosomes on behavior in the African pygmy mouse Mus minutoides, a species with XY female sex reversal.

In mammals, most sex differences in phenotype are controlled by gonadal hormones, but recent work on transgenic mice have shown that sex chromosomes can have a direct influence on sex-specific behaviors. In this study, we take advantage of the naturally occurring sex reversal in a mouse species, Mus minutoides, to investigate for the first time the relationship between sex chromosomes, hormones and behaviors in a wild species. In this model, a feminizing variant of the X chromosome, named X*, produces three types of females with different sex chromosome complements (XX, XX*, and X*Y), associated with alternative behavioral phenotypes, while all males are XY. We thus compared the levels of three major circulating steroid hormones (testosterone, corticosterone and estradiol) in the four sex genotypes to disentangle the influence of sex chromosomes and sex hormones on behavior. First, we did not find any difference in testosterone levels in the three female genotypes, although X*Y females are notoriously more aggressive. Second, in agreement with their lower anxiety-related behaviors, X*Y females and XY males display lower baseline corticosterone concentration than XX and XX* females. Instead of a direct hormonal influence, this result rather suggests that sex chromosomes may have an impact on the baseline corticosterone level, which in turn may influence behaviors. Third, estradiol concentrations do not explain the enhanced reproductive performance and maternal care behavior of the X*Y females compared to the XX and XX* females. Overall, this study highlights that most of the behaviors varying along with sex chromosome complement of this species are driven by genetic factors rather than steroid hormone concentrations.

evolutionary biology↗

bioRxiv2021

Multiple Sex Chromosome Drivers in a Mammal with Three Sex Chromosomes

A few mammals have unusual sex determining systems whereby fertile XY females live alongside XX females and XY males. These systems are regarded as evolutionary paradoxes because of the production of sex-reversed individuals and non-viable embryos, but they nevertheless seem stable over evolutionary time. Several hypotheses have been proposed to account for their stability, including models involving sex chromosome drive (i.e., biased transmission of sex chromosomes to the next generation). Here we corroborate this hypothesis in Mus minutoides, a close relative of the house mouse in which the presence of XY females is due to the evolution of a third sex chromosome: a feminizing X. Through extensive molecular sexing of pups at weaning, we reveal the existence of a remarkable male sex chromosome drive system in this species, whereby direction and strength of drive is conditional upon the genotype of males partners: males transmit their Y to almost 80% of their offspring when mating with XX females, and only 36% when mating with XY females. Using mathematical modelling, we explore the joint evolution of these unusual sex-determining and drive systems, revealing that different sequences of events could have led to the evolution of this bizarre system, and that the "conditional" nature of sex chromosome drive stabilizes the feminizing X, and even precludes a return to a standard XX/XY system.

evolutionary biology↗

bioRxiv2018

Impact of deleterious mutations, sexually antagonistic selection and mode of recombination suppression on transitions between male and female heterogamety.

Deleterious mutations accumulating on non-recombining Y chromosomes can drive XY to XY turnovers, but are thought to prevent XY to ZW turnovers, because the latter require fixation of the ancestral Y. Using individual-based simulations, we explored whether and how a dominant W allele can spread in a young XY system that gradually accumulates deleterious mutations. We also investigated how sexually antagonistic (SA) polymorphism on the ancestral sex chromosomes, and the mechanism controlling X-Y recombination suppression affect these transitions. In contrast with XY to XY turnovers, XY to ZW turnovers cannot be favored by Y chromosome mutation load. If the arrest of X-Y recombination depends on genotypic sex, transitions are strongly hindered by deleterious mutations, and totally suppressed by very small SA cost, because deleterious mutations and female-detrimental SA alleles would have to fix with the Y. If, however, the arrest of X-Y recombination depends on phenotypic sex, X and Y recombine in XY ZW females, allowing for the purge of Y-linked deleterious mutations and loss of the SA polymorphism, causing XY to ZW turnovers to occur at a neutral rate. We generalize our results to other types of turnovers (e.g., triggered by non-dominant sex-determining mutations) and discuss their empirical relevance.

evolutionary biology↗