Males can adjust offspring sex ratio in an adaptive fashion through different mechanisms

Density-dependent dispersal reduces conflict over the sex ratio

Haplodiploids-in particular, wasps-are the workhorses of sex-allocation research. This is owing to their unusual system of sex determination, which provides a ready means of sex ratio adjustment. Notably, their sexually asymmetrical mode of genetic inheritance leads mothers and fathers to come into conflict over the sex ratio of their offspring. In the simplest outbreeding scenario, a mother is favoured to employ an even sex ratio while a father prefers that all his mate's offspring are female. An important modulator of evolutionary conflict between mating partners is genetic relatedness, raising the possibility that this sex ratio conflict is reduced in low-dispersal settings with mating occurring between relatives. However, the impact of population viscosity on sex ratio conflict in haplodiploids remains unknown. Here, we develop and analyse a kin-selection model to investigate how the rate of dispersal modulates sex ratio conflict in a haplodiploid, viscous population setting. We find that population viscosity is associated with a reduction in the extent of sex ratio conflict-the effect being very weak under density-independent dispersal and much stronger under density-dependent dispersal.

Parental effects on offspring sex ratio in the Numbat (Myrmecobius fasciatus): does captivity influence paternal sex allocation?

Sex allocation theories predict that under different ecological conditions the production of sons and daughters will affect parental fitness differently. Skewed offspring sex ratios often occur under captive conditions where individuals are exposed to nutritional and social conditions that differ from nature. Here, we analyzed 29 years of offspring sex ratio data from a captive population of an endangered marsupial, the Numbat (Myrmecobius fasciatus). We partitioned variation in offspring sex ratio based on parental origin (captive- vs. wild-bred), parental weight, maternal age, and maternal reproductive history. Our analyses revealed no effect of parental weight or maternal origin on offspring sex ratio-however, there was a significant effect of paternal origin. Data visualization indicated that captive-bred males tended to produce male-biased litters. We discuss the result in relation to recent studies that have shown that male mammals have the capacity to be arbiters of sex allocation and highlight candidate mechanisms, but consider it with caution due to the small sample size from which the result was derived. We performed a population viability analysis (PVA) to explore the potential impact of a sex ratio skew on the sustainability of the captive Numbat population under hypothetical scenarios. Our PVA revealed that supplementation with wild individuals is critical to the persistence of the captive Numbat population and that a biased sex ratio will lead to extinction of the captive colony under certain conditions. Overall, our study demonstrates that covert sex ratio skews can persist undetected in captive populations, which have the potential to become impactful and compromise population sustainability under changed management processes.

Analyzing captive breeding outcomes to inform reintroduction practice: lessons from the pookila (Pseudomys novaehollandiae)

Captive breeding is often used to produce individuals for reintroduction programs in order to reestablish a species in an area where it has become locally extinct. To maximize the likelihood of establishing a self-sustaining population in the wild, an analysis of data from captive breeding programs is commonly undertaken to (1) increase the quantity of individuals and rate at which they can be released, and (2) maintain or improve the genetic and phenotypic quality of individuals. Here we demonstrate how the knowledge gained from these analyses can also be applied to decision-making during the design of subsequent reintroductions to further advance a reintroduction program toward success. We conducted an analysis of data from a captive breeding program for the threatened pookila (Pseudomys novaehollandiae, New Holland mouse) spanning 6 years. We found evidence for relationships between the reproductive output of pookila and behavioral, demographic, experiential, health, and physiological predictors. Based on a biological interpretation of these results, and with reference to a checklist of all known translocation tactics, we recommend 11 specific design elements to maximize the probability of pookila reproduction postrelease (thereby improving the likelihood of reintroduction success). These recommendations should be interpreted as hypotheses to be evaluated and refined in future reintroduction trials for the pookila. The uncertainty around the postrelease survival and reproduction of a species that is common in reintroduction practice warrants the creative use of existing data to inform adaptive management. Indeed, there is a wealth information in well-kept captive breeding records that is currently underused by reintroduction practitioners. The direct integration of knowledge derived from captive breeding (where available) with decision-making for reintroductions, as described here, will help navigate these uncertainties, which would benefit the conservation of both understudied and well-known species around the world.

Sexual antagonism evolves when autosomes influence offspring sex ratio

Sex allocation theory generally assumes maternal control of offspring sex and makes few predictions for populations evolving under paternal control. Using population genetic simulations, we show that maternal and paternal control of the sex ratio lead to different equilibrium sex ratios in structured populations. Sex ratios evolved under paternal control are more female biased. This effect is dependent on the population subdivision; fewer founding individuals leads to both more biased sex ratios and a greater difference between the paternal and maternal equilibria. In addition, sexual antagonism evolves in simulations with both maternally- and paternally-acting loci. Maternally-acting loci continuously accumulate ever more female-biasing effects as male-biasing effects accumulate at paternally-acting loci. The difference in evolved sex-ratio equilibria and the evolution of sexual antagonism can be largely explained by differences in the between-group variance of maternal and paternal effects in the founding generation. These theoretical results apply to any system with biparental autosomal influence over offspring sex, opening up an exciting new line of questioning.

Sex-specific paternal age effects on offspring quality in Drosophila melanogaster

Advanced paternal age has been repeatedly shown to modulate offspring quality via male- and/or female-driven processes, and there are theoretical reasons to expect that some of these effects can be sex-specific. For example, sex allocation theory predicts that, when mated with low-condition males, mothers should invest more in their daughters compared to their sons. This is because male fitness is generally more condition-dependent and more variable than female fitness, which makes it less risky to invest in female offspring. Here, we explore whether paternal age can affect the quality and quantity of offspring in a sex-specific way using Drosophila melanogaster as a model organism. In order to understand the contribution of male-driven processes on paternal age effects, we also measured the seminal vesicle size of young and older males and explored its relationship with reproductive success and offspring quality. Older males had lower competitive reproductive success, as expected, but there was no difference between the offspring sex ratio of young and older males. However, we found that paternal age caused an increase in offspring quality (i.e., offspring weight), and that this increase was more marked in daughters than sons. We discuss different male- and female-driven processes that may explain such sex-specific paternal age effects.

Energy as the cornerstone of environmentally driven sex allocation

In recent years, observations of distinct organisms have linked the quality of the environment experienced by a given individual and the sex it will develop. In most described cases, facing relatively harsh conditions resulted in masculinization, while thriving in favorable conditions promoted the development of an ovary. This was shown indistinctively in some species presenting a genetic sex determination (GSD), which were able to sex-reverse, and in species with an environmental sex determination (ESD) system. However, this pattern strongly depends on evolutionary constrains and is detected only when females need more energy for reproduction. Here, I describe the mechanisms involved in this environmentally driven sex allocation (EDSA), which involves two main energy pathways, lipid and carbohydrate metabolism. These pathways act through various enzymes and are not necessarily independent of the previously known transducers of environmental signals in species with ESD: calcium-redox, epigenetic, and stress regulation pathways. Overall, there is evidence of a link between energy level and the sexual fate of individuals of various species, including reptiles, fish, amphibians, insects, and nematodes. As energy pathways are evolutionarily conserved, this knowledge opens new avenues to advance our understanding of the mechanisms that allow animals to adapt their sex according to the local environment.

Evidence for a male‐biased sex ratio in the offspring of a large herbivore: The role of environmental conditions in the sex ratio variation

Numerous studies have examined whether the primary and/or secondary sex ratio in mammals, including humans, deviates from an equilibrium of 1:1. Although effect size in the sex ratio variation is expected to be low, a large sample size allows the identification of even small deviations from parity. In this study, we investigated whether the sex ratio of roe deer (Capreolus capreolus) offspring at birth approaches parity, using a large data set from roe deer offspring tagged in Baden‐Württemberg (Germany, 1972–2019, N = 12,437). In addition, a systematic re‐analysis of available data on the secondary sex ratios of roe deer was conducted to test whether our finding withstood the accumulation of further data. The null hypothesis that the sex ratio of roe deer (prenatal sex ratio and sex ratio at birth) approaches parity was rejected. Moreover, the secondary sex ratio of roe deer offspring deviated from the male‐biased mean for relatively cold or warm weather conditions during autumn and winter. Our study provides strong evidence for a male‐biased sex ratio in a large herbivore and weak evidence for variations in the secondary sex ratio owing to environmental conditions. The pattern is highly relevant in the context of climate change and its impact on the population dynamics of large herbivores.

Y/X-Chromosome-Bearing Sperm Shows Elevated Ratio in the Left but Not the Right Testes in Healthy Mice

The sex chromosomes play central roles in determining the sex of almost all of the multicellular organisms. It is well known that meiosis in mammalian spermatogenesis produces ~50% Y- and ~50% X-chromosome-bearing sperm, a 1:1 ratio. Here we first reveal that the X-chromosome-encoded miRNAs show lower expression levels in the left testis than in the right testis in healthy mice using bioinformatics modeling of miRNA-sequencing data, suggesting that the Y:X ratio could be unbalanced between the left testis and the right testis. We further reveal that the Y:X ratio is significantly elevated in the left testis but balanced in the right testis using flow cytometry. This study represents the first time the biased Y:X ratio in the left testis but not in the right testis is revealed.

Lifespan decreases with proportion of sons in males but not females of zoo-housed tigers and lemurs

Several studies have shown higher costs of rearing sons than daughters in mammals where males are larger than females. These studies typically focus on females by examining how the offspring sex ratio during a single reproductive event affected mothers' subsequent reproduction or survival probability. Here, we examine relationships between offspring sex ratio during single or multiple reproductive events and several survival metrics in mothers and fathers, using data from zoo-housed tigers (Panthera tigris) and ruffed lemurs (Varecia sp.). Our analyses failed to reveal an overall cost of reproduction or a higher cost of sons to mothers. In male ruffed lemurs, the proportion of sons produced during early life (before 10 years old) was negatively correlated with lifespan later in life. In tigers, males with a higher proportion of sons during their lifetime had shorter lifespans. One likely mechanism is the difference in testosterone levels between males: a high concentration of testosterone can increase the proportion of sons and compromise immune function. Our results suggest studies in wild populations should address the outstanding challenge of understanding consequences of sex allocation for males, and open an opportunity to predict lifespan in an applied conservation context.