Paternal influences over offspring sex ratio in mammals – tested hypotheses and potential mechanisms

Abstract

The sex ratio of sexually reproducing animal species is usually close to 1:1 and could be influenced by both parents. Deviations from this ratio occur in nature and can have an adaptive role. Although there are several evolutionary hypotheses explaining this bias, there is a significant lack of knowledge of the mechanisms of sex ratio facultative adjustments. While maternal effects are relatively well studied, the paternal effects on the primary sex ratio remain elusive. This review aims to assemble and summarise the knowledge of male control in sex allocation. In particular, we focused on identifying possible mechanisms through which the adjustment could be achieved at pre‐, peri‐ and post‐fertilisation levels. A systematic literature review was performed. We selected 59 articles that provided a potential proximal explanation of observed offspring sex ratio skews for which mammalian males might be responsible. We outlined potential physiological and genetic mechanisms of male‐driven sex allocation operating in male germ cells and zygote stage in mammals. However, most of the proposed hypotheses of how males can bias the offspring sex ratio concern mostly the production of unequal numbers of Y‐ and X‐chromosome‐bearing spermatozoa, and their differences in morphology and physiology. As males possess a wide range of mechanisms to effectively alter the sex ratio of their offspring, they can play a significant role in this phenomenon and should be studied to reduce the gap in our understanding of their role in sex allocation in comparison to the role of females. More empirical studies on different mammalian models are needed to fully characterise the trends of the adjustment. Predicting the sex ratio of the progeny is of great importance, not only in animal husbandry but also for the conservation of endangered species.