Demographic pathways linking variation in adult sex ratio to genetic sex determination systems in tetrapods

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Ivett Pipoly, Center for Natural Science, MTA-PE Evolutioary Ecology Research Group, Veszprém, Hungary, Veronika Bókony, Eötvös Loránd Research Network, Lendület Evolutionary Ecology Research Group, Budapest, Hungary, Jean-Michel Gaillard and Jean-Francois Lemaitre, Laboratoire de Biométrie et Biologie Évolutive, Université Lyon, Lyon, France, Tamás Székely, Milner Centre for Evolution, University of Bath, Bath, United Kingdom, András Liker, Behavioural Ecology Research Group, Center for Natural Science, University of Pannonia, Veszprém, Hungary

Background/Question/Methods
Adult sex ratio (ASR, proportion of males in the adult population) is a fundamental trait of populations with effects on behaviour, ecology and population dynamics. ASR is often unbalanced in the wild, as populations in many species exhibit male-biased or female-biased sex ratios. Previous research showed that direction of ASR bias is associated with the type of sex-chromosome system in tetrapods, as ASR is consistently more male-biased in species with ZW sex-chromosome systems than species with XY systems. However, the mechanisms leading to this association are unclear. To understand this link between sex-chromosome system and ASR bias, we investigated whether four demographic metrics, i.e. birth sex ratio, sex differences in juvenile and adult mortality, and sex differences in maturation time, differ between XY and ZW systems and, at the same time, also contribute to interspecific variation in ASR. We used a unique dataset of 456 species encompassing most groups of wild tetrapods (52 amphibians, 93 reptiles, 187 birds and 124 mammals), and phylogenetically controlled statistics (phylogenetic generalized least squares models and path analyses).
Results/Conclusions
Our results with an extended dataset corroborated that ASR is more male-biased in species with ZW than with XY sex chromosomes. We also found that adult mortality is more male-biased in XY than in ZW systems. ASR is increasingly female-biased with increasingly male-biased mortality in both juvenile and adult stages, and also with increasingly male-biased maturation times. Result of phylogenetic path analysis supported a model in which the sex bias in adult mortality links ASR and the type of genetic sex determination: sex bias in adult mortality was the only demographic metric that was significantly predicted by sex-chromosome type and, at the same time, was also associated with ASR. We will discuss the implications of these findings for future studies in ecology and evolutionary biology.