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）是自然界中一类重要的进化驱动力，但引发该效应的性状的进化动力学却尚未得到充分阐明。卵大小（egg size）已被证实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确立。与针对适合度相关性状的理论预期相悖，自然种群中观测到的卵大小存在大量的加性遗传变异（additive genetic variation）。维持这一变异的潜在机制之一，是性状间共享遗传基础所带来的遗传约束（genetic constraints）。本研究针对日本鹌鹑（Coturnix japonica）的母体卵投入构建了重复的分歧选择品系，以量化遗传约束在卵大小进化中的作用。研究发现，卵大小对选择响应迅速，且伴随所有卵组分的显著响应。最初我们观测到了体尺的相关响应，但该响应随时间推移逐渐减弱，表明卵大小与体尺可独立进化。此外，未观测到繁殖力（即10天内产卵数）的相关响应。然而，选择响应存在不对称性：高投入品系在选择一代后卵大小即趋于平稳，而低投入品系则未出现此现象。我们将这一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（genetic asymmetries）。尽管卵大小与适合度呈正相关，但这类遗传不对称性或可解释自然种群中观测到的卵大小进化停滞（evolutionary stasis）现象。