Egg-size plasticity in Apis mellifera: honey bee queens alter egg size in response to both genetic and environmental factors

Social evolution has led to distinct life-history patterns in social insects, but many colony-level and individual traits, such as egg size, are not sufficiently understood. Thus, a series of experiments was performed to study the effects of genotypes, colony size, and colony nutrition on variation in egg size produced by honey bee (Apis mellifera) queens. Queens from different genetic stocks produced significantly different egg sizes under similar environmental conditions, indicating standing genetic variation for egg size that allows for adaptive evolutionary change. Further investigations revealed that eggs produced by queens in large colonies were consistently smaller than eggs produced in small colonies, and queens dynamically adjusted egg size in relation to colony size. Similarly, queens increased egg size in response to food deprivation. These results could not be solely explained by different numbers of eggs produced in the different circumstances but instead seem to reflect an active adjustment of resource allocation by the queen in response to colony conditions. As a result, larger eggs experienced higher subsequent survival than smaller eggs, suggesting that honey bee queens might increase egg size under unfavorable conditions to enhance brood survival, and to minimize costly brood care of eggs that fail to successfully develop, and thus conserve energy at the colony level. The extensive plasticity and genetic variation of egg size in honey bees has important implications for understanding life history evolution in a social context and implies this neglected life history stage in honey bees may have trans-generational effects.

社会演化造就了社会性昆虫各具特色的生活史模式，但诸多群体水平与个体水平的性状（如卵大小）仍未得到充分研究与阐释。为此，本研究开展了一系列实验，以探究基因型、群体规模及群体营养状况对西方蜜蜂（Apis mellifera）蜂王所产卵的大小变异的影响。来自不同遗传品系的蜂王在相似环境条件下所产卵的大小存在显著差异，这表明卵大小存在可支持适应性演化改变的存量遗传变异。进一步研究显示，大群体中蜂王所产的卵始终小于小群体中的卵，且蜂王会根据群体规模动态调整卵的大小；类似地，当遭遇食物匮乏时，蜂王会增大所产卵的尺寸。上述结果无法仅通过不同环境下产卵数量的差异来解释，反而更反映出蜂王会根据群体状况主动调整资源分配策略。研究同时发现，较大的卵后续存活率显著高于较小的卵，这表明西方蜜蜂蜂王可在不利条件下增大卵的尺寸，以提升育幼群体的存活率，同时减少对无法正常发育的卵的高成本育幼照料投入，进而在群体水平节约能量。西方蜜蜂卵大小所具备的广泛表型可塑性与遗传变异，对于理解社会背景下的生活史演化具有重要意义，同时也意味着这一被忽视的生活史阶段可能存在跨代效应。