Data from: In search of genetic constraints limiting the evolution of egg size: Direct and correlated responses to artificial selection on a prenatal maternal effector

Maternal effects are an important force in nature, but the evolutionary dynamics of the traits that cause them are not well understood. Egg size is known to be a key mediator of prenatal maternal effects with an established genetic basis. Contrary to theoretical expectations for fitness-related traits, there is a large amount of additive genetic variation in egg size observed in natural populations. One possible mechanism for the maintenance of this variation is through genetic constraints caused by a shared genetic basis among traits. Here we created replicated, divergent selection lines for maternal egg investment in Japanese quail (Coturnix japonica) to quantify the role of genetic constraints in the evolution of egg size. We found that egg size responds rapidly to selection, accompanied by a strong response in all egg components. Initially, we observed a correlated response in body size, but this response declined over time, showing that egg size and body size can evolve independently. Furthermore, no correlated response in fecundity (i.e. the number of eggs laid over a 10 day period) was observed. However, the response to selection was asymmetrical, with egg size plateauing after one generation of selection in the high but not the low investment lines. We attribute this pattern to the presence of genetic asymmetries, caused by directional dominance or unequal allele frequencies. Such asymmetries may contribute to the evolutionary stasis in egg size observed in natural populations, despite a positive association between egg size and fitness.

母体效应（maternal effects）在自然界中是一种重要的演化驱动力，但引发该效应的相关性状的演化动力学机制目前仍未得到充分阐释。蛋大小（egg size）是产前母体效应的关键介导因子，且其遗传基础已得到明确界定。与适合度相关性状的理论预期相悖，在自然种群中已观测到蛋大小存在大量的加性遗传变异（additive genetic variation）。维持这类遗传变异的潜在机制之一，可能是性状间共享遗传基础所引发的遗传约束（genetic constraints）。本研究以日本鹌鹑（Coturnix japonica）为研究对象，构建了重复分化选择系以探究母体蛋投入（maternal egg investment），从而量化遗传约束在蛋大小演化过程中的作用。研究结果显示，蛋大小对选择压力响应迅速，且所有蛋组分均伴随显著的响应变化。初始阶段我们观测到体大小出现相关响应，但该响应随时间推移逐渐减弱，表明蛋大小与体大小可独立演化。此外，未观测到繁殖力（fecundity，即10天内的产卵总数）出现相关响应。但对选择压力的响应呈现不对称性：在高投入选择系中，蛋大小仅经过一代选择即达到平台期，而低投入选择系并未出现该现象。我们将该模式归因于定向显性或等位基因频率不均等所引发的遗传不对称性。尽管蛋大小与适合度呈正相关，但这类遗传不对称性或许可以解释自然种群中观测到的蛋大小演化停滞现象。