A split sex ratio in solitary and social nests of a facultatively social bee

A classic prediction of kin selection theory is that a mixed population of social and solitary nests of haplodiploid insects should exhibit a split sex ratio among offspring: female biased in social nests, male biased in solitary nests. Here we provide the first evidence of a solitary-social split sex ratio, using the sweat bee Megalopta genalis (Halictidae). Data from 2502 offspring collected from naturally occurring nests across six years spanning the range of the M. genalis reproductive season show that despite significant yearly and seasonal variation, the offspring sex ratio of social nests is consistently more female biased than in solitary nests. This suggests that split sex ratios may facilitate the evolutionary origins of cooperation based on reproductive altruism via kin selection.

亲缘选择理论（kin selection theory）的经典预测表明，单倍二倍体昆虫（haplodiploid insects）的社会型巢穴与独居型巢穴混合种群，其子代应呈现性比分化现象：社会型巢穴内子代偏雌性，独居型巢穴内子代偏雄性。本研究以汗蜂Megalopta genalis（隧蜂科Halictidae）为研究对象，首次提供了独居-社会型性比分化的实证证据。在覆盖M. genalis繁殖季全时段的6年研究周期内，我们采集了来自自然巢穴的2502头子代样本。数据显示：尽管子代性比存在显著的年度与季节波动，但社会型巢穴的子代性比始终较独居型巢穴更偏向雌性。这一结果表明，性比分化或可通过亲缘选择助力基于生殖利他主义的合作行为的进化起源。