The male handicap: male-biased mortality explains skewed sex ratios in brown trout embryos

Juvenile sex ratios are often assumed to be equal for many species with genetic sex determination, but this has rarely been tested in fish embryos due to their small size and absence of sex-specific markers. We artificially crossed three populations of brown trout and used a recently developed genetic marker for sexing the offspring of both pure and hybrid crosses. Sex ratios (SR = proportion of males) varied widely one month after hatching ranging from 0.15 to 0.90 (mean = 0.39 ± 0.03). Families with high survival tended to produce balanced or male-biased sex ratios, but SR was significantly female-biased when survival was low, suggesting that males sustain higher mortality during development. No difference in SR was found between pure and hybrid families, but the existence of sire × dam interactions suggests that genetic incompatibility may play a role in determining sex ratios. Our findings have implications for animal breeding and conservation because skewed sex ratios will tend to reduce effective population size and bias selection estimates.

对于许多具有遗传性别决定机制的物种而言，幼体性别比例常被假定为均等，但由于鱼类胚胎体型微小且缺乏性别特异性标记，这一假设极少在鱼类胚胎中得到验证。我们人工杂交了三个褐鳟种群，并采用新近开发的遗传标记对纯系杂交与种群间杂交后代的性别进行鉴定。孵化一个月后的幼体性别比例（SR = 雄性占比）差异显著，分布区间为0.15至0.90（平均值为0.39 ± 0.03）。高存活率的家系往往呈现均衡或雄性偏倚的性别比例，但当存活率较低时，性别比例则显著偏向雌性，这表明雄性在发育过程中面临更高的死亡率。纯系家系与杂交家系的性别比例并无显著差异，但父本×母本互作效应的存在表明，遗传不相容性可能在性别比例的调控中发挥作用。本研究结果对动物育种与物种保护具有重要意义，因为偏倚的性别比例往往会降低有效种群规模，并对选择估计产生偏倚影响。