Sex chromosomes, sex ratios and sex gaps in longevity in plants

Variation in adult sex ratios in tetrapods is linked to sex chromosomes through mortality differences between males and females

Sex chromosomes can determine male and female phenotypes, and the resulting sex differences may have significant impacts on ecology and life history. One manifestation of this link is that ZW/ZZ sex-determination systems are associated with more male-skewed adult sex ratio (ASR, proportion of males in the adult population) than XX/XY systems across tetrapods (amphibians, reptiles, birds, and mammals). Here, we investigate four demographic processes: male and female offspring production, sex differences in juvenile and adult mortalities and in timing of maturation that can contribute to ASR variation between XX/XY and ZW/ZZ systems, using phylogenetic analyses of a large dataset collected from tetrapod species in the wild. We show that sex differences in adult mortality reliably predict ASR that is also more male-biased in XX/XY species than in ZW/ZZ species. Sex differences in juvenile mortality and in maturation time also contribute to ASR skews, but do not differ consistently between XX/XY and ZW/ZZ systems. Phylogenetic path analyses confirm an influence of sex-determination system on ASR through sex-biased adult mortalities. Together these results infer that sex chromosomes can impact, via demographic pathways, frequency-dependent selection emerging from the relative number of males and females. We call for follow-up studies to uncover the potentially complex web of associations between sex determination, population dynamics, and social behavior.

Mother's Curse effects on lifespan and aging

The Mother's Curse hypothesis posits that mothers curse their sons with harmful mitochondria, because maternal mitochondrial inheritance makes selection blind to mitochondrial mutations that harm only males. As a result, mitochondrial function may be evolutionarily optimized for females. This is an attractive explanation for ubiquitous sex differences in lifespan and aging, given the prevalence of maternal mitochondrial inheritance and the established relationship between mitochondria and aging. This review outlines patterns expected under the hypothesis, and traits most likely to be affected, chiefly those that are sexually dimorphic and energy intensive. A survey of the literature shows that evidence for Mother's Curse is limited to a few taxonomic groups, with the strongest support coming from experimental crosses in Drosophila. Much of the evidence comes from studies of fertility, which is expected to be particularly vulnerable to male-harming mitochondrial mutations, but studies of lifespan and aging also show evidence of Mother's Curse effects. Despite some very compelling studies supporting the hypothesis, the evidence is quite patchy overall, with contradictory results even found for the same traits in the same taxa. Reasons for this scarcity of evidence are discussed, including nuclear compensation, factors opposing male-specific mutation load, effects of interspecific hybridization, context dependency and demographic effects. Mother's Curse effects may indeed contribute to sex differences, but the complexity of other contributing factors make Mother's Curse a poor general predictor of sex-specific lifespan and aging.

Sexual antagonism in sequential hermaphrodites

Females and males may have distinct phenotypic optima, but share essentially the same complement of genes, potentially leading to trade-offs between attaining high fitness through female versus male reproductive success. Such sexual antagonism may be particularly acute in hermaphrodites, where both reproductive strategies are housed within a single individual. While previous models have focused on simultaneous hermaphroditism, we lack theory for how sexual antagonism may play out under sequential hermaphroditism, which has the additional complexities of age-structure. Here, we develop a formal theory of sexual antagonism in sequential hermaphrodites. First, we construct a general theoretical overview of the problem, then consider different types of sexually antagonistic and life-history trade-offs, under different modes of genetic inheritance (autosomal or cytoplasmic), and different forms of sequential hermaphroditism (protogynous, protoandrous or bidirectional). Finally, we provide a concrete illustration of these general patterns by developing a two-stage two-sex model, which yields conditions for both invasion of sexually antagonistic alleles and maintenance of sexually antagonistic polymorphisms.

Y chromosome toxicity does not contribute to sex-specific differences in longevity

While sex chromosomes carry sex-determining genes, they also often differ from autosomes in size and composition, consisting mainly of silenced heterochromatic repetitive DNA. Even though Y chromosomes show structural heteromorphism, the functional significance of such differences remains elusive. Correlative studies suggest that the amount of Y chromosome heterochromatin might be responsible for several male-specific traits, including sex-specific differences in longevity observed across a wide spectrum of species, including humans. However, experimental models to test this hypothesis have been lacking. Here we use the Drosophila melanogaster Y chromosome to investigate the relevance of sex chromosome heterochromatin in somatic organs in vivo. Using CRISPR-Cas9, we generated a library of Y chromosomes with variable levels of heterochromatin. We show that these different Y chromosomes can disrupt gene silencing in trans, on other chromosomes, by sequestering core components of the heterochromatin machinery. This effect is positively correlated to the level of Y heterochromatin. However, we also find that the ability of the Y chromosome to affect genome-wide heterochromatin does not generate physiological sex differences, including sexual dimorphism in longevity. Instead, we discovered that it is the phenotypic sex, female or male, that controls sex-specific differences in lifespan, rather than the presence of a Y chromosome. Altogether, our findings dismiss the 'toxic Y' hypothesis that postulates that the Y chromosome leads to reduced lifespan in XY individuals.

How much does the unguarded X contribute to sex differences in life span?

Females and males often have markedly different mortality rates and life spans, but it is unclear why these forms of sexual dimorphism evolve. The unguarded X hypothesis contends that dimorphic life spans arise from sex differences in X or Z chromosome copy number (i.e., one copy in the "heterogametic" sex; two copies in the "homogametic" sex), which leads to a disproportionate expression of deleterious mutations by the heterogametic sex (e.g., mammalian males; avian females). Although data on adult sex ratios and sex-specific longevity are consistent with predictions of the unguarded X hypothesis, direct experimental evidence remains scant, and alternative explanations are difficult to rule out. Using a simple population genetic model, we show that the unguarded X effect on sex differential mortality is a function of several reasonably well-studied evolutionary parameters, including the proportion of the genome that is sex linked, the genomic deleterious mutation rate, the mean dominance of deleterious mutations, the relative rates of mutation and strengths of selection in each sex, and the average effect of mutations on survival and longevity relative to their effects on fitness. We review published estimates of these parameters, parameterize our model with them, and show that unguarded X effects are too small to explain observed sex differences in life span across species. For example, sex differences in mean life span are known to often exceed 20% (e.g., in mammals), whereas our parameterized models predict unguarded X effects of a few percent (e.g., 1-3% in Drosophila and mammals). Indeed, these predicted unguarded X effects fall below statistical thresholds of detectability in most experiments, potentially explaining why direct tests of the hypothesis have generated little support for it. Our results suggest that evolution of sexually dimorphic life spans is predominantly attributable to other mechanisms, potentially including "toxic Y" effects and sexual dimorphism for optimal investment in survival versus reproduction.

Sex determination and sex chromosome evolution in land plants

Linnaeus's very first opus, written when he was 22 years old, dealt with the analogy that exists between plants and animals in how they 'propagate their species', and a revised version with a plate depicting the union of male and female Mercurialis annua plants became a foundational text on the sexuality of plants. The question how systems with separate males and females have evolved in sedentary organisms that appear ancestrally bisexual has fascinated biologists ever since. The phenomenon, termed dioecy, has important consequences for plant reproductive success and is of commercial interest since it affects seed quality and fruit production. This theme issue presents a series of articles that synthesize and challenge the current understanding of how plants achieve dioecy. The articles deal with a broad set of taxa, including Coccinia, Ginkgo, Mercurialis, Populus, Rumex and Silene, as well as overarching topics, such as the field's terminology, analogies with animal sex determination systems, evolutionary pathways to dioecy, dosage compensation, and the longevity of the two sexes. In this introduction, we focus on four topics, each addressed by several articles from different angles and with different conclusions. Our highlighting of unclear or controversial issues may help future studies to build on the current understanding and to ask new questions that will expand our knowledge of plant sexual systems. This article is part of the theme issue 'Sex determination and sex chromosome evolution in land plants'.