Multiple sex chromosome drivers in a mammal with three sex chromosomes

Yes, polygenic sex determination is a thing!

The process of sexual development in animals is modulated by a variety of mechanisms. Some species respond to environmental cues, while, in others, sex determination is thought to be controlled by a single 'master regulator' gene. However, many animals respond to a combination of environmental cues (e.g., temperature) and genetic factors (e.g., sex chromosomes). Even among species in which genetic factors predominate, there is a continuum between monofactorial and polygenic systems. The perception that polygenic systems are rare may result from experiments that lack the statistical power to detect multiple loci. Intellectual biases against the existence of polygenic sex determination (PSD) may further arise from misconceptions about the regulation of developmental processes and a misreading of theoretical results on the stability of polygenic systems of sex determination.

The role of conflict in the formation and maintenance of variant sex chromosome systems in mammals

The XX/XY sex chromosome system is deeply conserved in therian mammals, as is the role of Sry in testis determination, giving the impression of stasis relative to other taxa. However, the long tradition of cytogenetic studies in mammals documents sex chromosome karyotypes that break this norm in myriad ways, ranging from fusions between sex chromosomes and autosomes to Y chromosome loss. Evolutionary conflict, in the form of sexual antagonism or meiotic drive, is the primary predicted driver of sex chromosome transformation and turnover. Yet conflict-based hypotheses are less considered in mammals, perhaps because of the perceived stability of the sex chromosome system. To address this gap, we catalog and characterize all described sex chromosome variants in mammals, test for family-specific rates of accumulation, and consider the role of conflict between the sexes or within the genome in the evolution of these systems. We identify 152 species with sex chromosomes that differ from the ancestral state and find evidence for different rates of ancestral to derived transitions among families. Sex chromosome-autosome fusions account for 79% of all variants whereas documented sex chromosome fissions are limited to three species. We propose that meiotic drive and drive suppression provide viable explanations for the evolution of many of these variant systems, particularly those involving autosomal fusions. We highlight taxa particularly worthy of further study and provide experimental predictions for testing the role of conflict and its alternatives in generating observed sex chromosome diversity.

A rapid approach for sex assignment by RAD-seq using a reference genome

Sex identification is a common objective in molecular ecology. While many vertebrates display sexual dimorphism, determining the sex can be challenging in certain situations, such as species lacking clear sex-related phenotypic characteristics or in studies using non-invasive methods. In these cases, DNA analyses serve as valuable tools not only for sex determination but also for validating sex assignment based on phenotypic traits. In this study, we developed a bioinformatic framework for sex assignment using genomic data obtained through GBS, and having an available closely related genome assembled at the chromosome level. Our method consists of two ad hoc indexes that rely on the different properties of the mammalian heteromorphic sex chromosomes. For this purpose, we mapped RAD-seq loci to a reference genome and then obtained missingness and coverage depth values for the autosomes and X and Y chromosomes of each individual. Our methodology successfully determined the sex of 165 fur seals that had been phenotypically sexed in a previous study and 40 sea lions sampled in a non-invasive way. Additionally, we evaluated the accuracy of each index in sequences with varying average coverage depths, with Index Y proving greater reliability and robustness in assigning sex to individuals with low-depth coverage. We believe that the approach presented here can be extended to any animal taxa with known heteromorphic XY/ZW sex chromosome systems and that it can tolerate various qualities of GBS sequencing data.

Hormone profiles of 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 has 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 more likely driven by genetic factors rather than steroid hormone concentrations.

New perspectives on the causes and consequences of male meiotic drive

Gametogenesis is vulnerable to selfish genetic elements that bias their transmission to the next generation by cheating meiosis. These so-called meiotic drivers are widespread in plants, animals, and fungi and can impact genome evolution. Here, we summarize recent progress on the causes and consequences of meiotic drive in males, where selfish elements attack vulnerabilities in spermatogenesis. Advances in genomics provide new insights into the organization and dynamics of driving chromosomes in natural populations. Common themes, including small RNAs, gene duplications, and heterochromatin, emerged from these studies. Interdisciplinary approaches combining evolutionary genomics with molecular and cell biology are beginning to unravel the mysteries of drive and suppression mechanisms. These approaches also provide insights into fundamental processes in spermatogenesis and chromatin regulation.

Did the creeping vole sex chromosomes evolve through a cascade of adaptive responses to a selfish x chromosome?

The creeping vole Microtus oregoni exhibits remarkably transformed sex chromosome biology, with complete chromosome drive/drag, X-Y fusions, sex reversed X complements, biased X inactivation, and X chromosome degradation. Beginning with a selfish X chromosome, I propose a series of adaptations leading to this system, each compensating for deleterious consequences of the preceding adaptation: (1) YY embryonic inviability favored evolution of a selfish feminizing X chromosome; (2) the consequent Y chromosome transmission disadvantage favored X-Y fusion ("XP "); (3) Xist-based silencing of Y-derived XP genes favored a second X-Y fusion ("XM "); (4) X chromosome dosage-related costs in XP XM males favored the evolution of XM loss during spermatogenesis; (5) X chromosomal dosage-related costs in XM 0 females favored the evolution of XM drive during oogenesis; and (6) degradation of the non-recombining XP favored the evolution of biased X chromosome inactivation. I discuss recurrent rodent sex chromosome transformation, and selfish genes as a constructive force in evolution.

Genotypic sex shapes maternal care in the African pygmy mouse, Mus minutoides

Sexually dimorphic behaviours, such as parental care, have long been thought to be mainly driven by gonadal hormones. In the past two decades, a few studies have challenged this view, highlighting the direct influence of the sex chromosome complement (XX versus XY or ZZ versus ZW). The African pygmy mouse, Mus minutoides, is a wild mouse species with naturally occurring XY sex reversal induced by a third, feminizing X* chromosome, leading to three female genotypes: XX, XX* and X*Y. Here, we show that sex reversal in X*Y females shapes a divergent maternal care strategy (maternal aggression, pup retrieval and nesting behaviours) from both XX and XX* females. Although neuroanatomical investigations were inconclusive, we show that the dopaminergic system in the anteroventral periventricular nucleus of the hypothalamus is worth investigating further as it may support differences in pup retrieval behaviour between females. Combining behaviours and neurobiology in a rodent subject to natural selection, we evaluate potential candidates for the neural basis of maternal behaviours and strengthen the underestimated role of the sex chromosomes in shaping sex differences in brain and behaviours. All things considered, we further highlight the emergence of a third sexual phenotype, challenging the binary view of phenotypic sexes.

Do sex-linked male meiotic drivers contribute to intrinsic hybrid incompatibilities? Recent empirical studies from flies and rodents

Intrinsic hybrid incompatibility is one of the important isolating barriers between species. In organisms with sex chromosomes, intrinsic hybrid incompatibility often follows two rules: Haldane's rule and large-X effects. One explanation for these two rules is that sex chromosomes are hotspots for meiotic drivers that can cause intrinsic hybrid incompatibility between geographically isolated populations. Although this hypothesis seems plausible and several empirical data are consistent with it, we are still unsure whether such mechanisms occur in nature, particularly with respect to speciation with gene flow. Here, we review empirical studies that have investigated the roles of meiotic drive in sex-chromosome evolution and speciation and propose future studies necessary for testing this hypothesis.

Bypassing Mendel's First Law: Transmission Ratio Distortion in Mammals

Mendel's law of segregation states that the two alleles at a diploid locus should be transmitted equally to the progeny. A genetic segregation distortion, also referred to as transmission ratio distortion (TRD), is a statistically significant deviation from this rule. TRD has been observed in several mammal species and may be due to different biological mechanisms occurring at diverse time points ranging from gamete formation to lethality at post-natal stages. In this review, we describe examples of TRD and their possible mechanisms in mammals based on current knowledge. We first focus on the differences between TRD in male and female gametogenesis in the house mouse, in which some of the most well studied TRD systems have been characterized. We then describe known TRD in other mammals, with a special focus on the farmed species and in the peculiar common shrew species. Finally, we discuss TRD in human diseases. Thus far, to our knowledge, this is the first time that such description is proposed. This review will help better comprehend the processes involved in TRD. A better understanding of these molecular mechanisms will imply a better comprehension of their impact on fertility and on genome evolution. In turn, this should allow for better genetic counseling and lead to better care for human families.