Bioscientifica Proceedings

Sex ratios that deviate from 1:1 have been observed in response to a number of stimuli. In this review we will discuss sex ratio biasing, and the evolutionary and molecular mechanisms thought to underlie this phenomena in mammals. The role of embryo quality will be discussed in relation to sex ratio modulation and epigenetic programing of the embryo. Sex ratio skewing has been studied in many species and several factors have been proposed as influencing secondary sex ratios (body condition, maternal dominance, nutrition and developmental asynchrony). In swine, maternal nutrition has repeatedly been shown to influence offspring sex ratios, while maternal dominance and body condition exhibit less consistent evidence supporting their influence. Based on current evidence, we hypothesize that sex ratio biasing is the result of sexual dimorphisms that result in sex specific differences in embryo quality, and these differences lead to sex specific embryonic loss. The mechanisms through which sex specific loss occurs are not fully understood, however sexual dimorphisms in metabolism, gene expression and epigenetic mechanisms during early embryo development suggest that sex ratio modulation might be mediated through these mechanisms. We hypothesize that there are a number of mechanisms for skewing sex ratios in mammals, and that specific mechanisms are elicited in response to specific stimuli.

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