The Drosophila simulans Y chromosome interacts with the autosomes to influence male fitness

Y chromosome-linked variation affects locomotor activity in male Drosophila melanogaster and is robust to differences in thermal environment

Although containing genes important for sex determination, genetic variation within the Y chromosome was traditionally predicted to contribute little to the expression of sexually dimorphic traits. This prediction was shaped by the assumption that the chromosome harbours few protein-coding genes, and that capacity for Y-linked variation to shape adaptation would be hindered by the chromosome's lack of recombination and holandric inheritance. Consequently, most studies exploring the genotypic contributions to sexually dimorphic traits have focused on the autosomes and X chromosome. Yet, several studies have now demonstrated that the Y chromosome harbours variation affecting male fitness, moderating the expression of hundreds of genes across the nuclear genome. Furthermore, emerging results have shown that expression of this Y-linked variation may be sensitive to environmental heterogeneity, leading to the prediction that Y-mediated gene-by-environment interactions will shape the expression of sexually dimorphic phenotypes. We tested this prediction, investigating whether genetic variation across six distinct Y chromosome haplotypes affects the expression of locomotor activity, at each of two temperatures (20 and 28 °C) in male fruit flies (Drosophila melanogaster). Locomotor activity is a sexually dimorphic trait in this species, previously demonstrated to be under intralocus sexual conflict. We demonstrate Y haplotype effects on male locomotor activity, but the rank order and magnitude of these effects were unaltered by differences in temperature. Our study contributes to a growing number of studies demonstrating Y-linked effects moderating expression of traits evolving under sexually antagonistic selection, suggesting a role for the Y chromosome in shaping outcomes of sexual conflict.

Sex-specific aging in animals: Perspective and future directions

Sex differences in aging occur in many animal species, and they include sex differences in lifespan, in the onset and progression of age-associated decline, and in physiological and molecular markers of aging. Sex differences in aging vary greatly across the animal kingdom. For example, there are species with longer-lived females, species where males live longer, and species lacking sex differences in lifespan. The underlying causes of sex differences in aging remain mostly unknown. Currently, we do not understand the molecular drivers of sex differences in aging, or whether they are related to the accepted hallmarks or pillars of aging or linked to other well-characterized processes. In particular, understanding the role of sex-determination mechanisms and sex differences in aging is relatively understudied. Here, we take a comparative, interdisciplinary approach to explore various hypotheses about how sex differences in aging arise. We discuss genomic, morphological, and environmental differences between the sexes and how these relate to sex differences in aging. Finally, we present some suggestions for future research in this area and provide recommendations for promising experimental designs.

Epistatic interactions between sex chromosomes and autosomes can affect the stability of sex determination systems

Sex determination (SD) is an essential and ancient developmental process, but the genetic systems that regulate this process are surprisingly variable. Why SD mechanisms vary so much is a longstanding question in evolutionary biology. SD genes are generally located on sex chromosomes which also carry genes that interact epistatically with autosomes to affect fitness. How this affects the evolutionary stability of SD mechanisms is still unknown. Here, we explore how epistatic interactions between a sexually antagonistic (SA) non‐SD gene, located on either an ancestral or novel sex chromosome, and an autosomal gene affect the conditions under which an evolutionary transition to a new SD system occurs. We find that when the SD gene is linked to an ancestral sex‐chromosomal gene which engages in epistatic interactions, epistasis enhances the stability of the sex chromosomes so that they are retained under conditions where transitions would otherwise occur. This occurs both when weaker fitness effects are associated with the ancestral sex chromosome pair or stronger fitness effects associated with a newly evolved SD gene. However, the probability that novel SD genes spread is unaffected if they arise near genes involved in epistasis. This discrepancy occurs because, on autosomes, SA allele frequencies are typically lower than on sex chromosomes. In our model, increased frequencies of these alleles contribute to a higher frequency of epistasis which may therefore more readily occur on sex chromosomes. Because sex chromosome–autosome interactions are abundant and can take several forms, they may play a large role in maintaining sex chromosomes.

Sex-Ratio Meiotic Drive Shapes the Evolution of the Y Chromosome in Drosophila simulans

The recent emergence and spread of X-linked segregation distorters-called "Paris" system-in the worldwide species Drosophila simulans has elicited the selection of drive-resistant Y chromosomes. Here, we investigate the evolutionary history of 386 Y chromosomes originating from 29 population samples collected over a period of 20 years, showing a wide continuum of phenotypes when tested against the Paris distorters, from high sensitivity to complete resistance (males sire ∼95% to ∼40% female progeny). Analyzing around 13 kb of Y-linked gene sequences in a representative subset of nine Y chromosomes, we identified only three polymorphic sites resulting in three haplotypes. Remarkably, one of the haplotypes is associated with resistance. This haplotype is fixed in all samples from Sub-Saharan Africa, the region of origin of the drivers. Exceptionally, with the spread of the drivers in Egypt and Morocco, we were able to record the replacement of the sensitive lineage by the resistant haplotype in real time, within only a few years. In addition, we performed in situ hybridization, using satellite DNA probes, on a subset of 21 Y chromosomes from six locations. In contrast to the low molecular polymorphism, this revealed extensive structural variation suggestive of rapid evolution, either neutral or adaptive. Moreover, our results show that intragenomic conflicts can drive astonishingly rapid replacement of Y chromosomes and suggest that the emergence of Paris segregation distorters in East Africa occurred less than half a century ago.

Complex interactions between sperm viability and female fertility

Sperm viability is a major male fitness component, with higher sperm viability associated with enhanced sperm competitiveness. While many studies have focussed on sperm viability from the male fitness standpoint, its impact on female fitness is less clear. Here we used a panel of 32 isogenic Drosophila simulans lines to test for genetic variation in sperm viability (percentage of viable cells). We then tested whether sperm viability affected female fitness by mating females to males from low or high sperm viability genotypes. We found significant variation in sperm viability among genotypes, and consistent with this, sperm viability was highly repeatable within genotypes. Additionally, females mated to high sperm viability males laid more eggs in the first seven hours after mating, and produced more offspring in total. However, the early increase in oviposition did not result in more offspring in the 8 hours following mating, suggesting that mating with high sperm-viability genotypes leads to egg wastage for females shortly after copulation. Although mating with high sperm-viability males resulted in higher female fitness in the long term, high quality ejaculates would result in a short-term female fitness penalty, or at least lower realised fitness, potentially generating sexual conflict over optimal sperm viability.