Data from: Heterozygosity predicts clutch and egg size but not plasticity in a house sparrow population with no evidence of inbreeding

We investigated the link between heterozygosity and the reaction norm attributes of reproductive performance in female house sparrows (Passer domesticus). We collected data on clutch size, egg size, hatching success, and nestling survival in 2 816 nesting attempts made by 791 marked individuals over a 16 year period. Pedigree analysis revealed no evidence of inbreeding. Neither parent-offspring regression nor an animal model revealed significant heritability in clutch or egg size. We selected 42 females that laid at least seven clutches at our study site and used a survey of 21 autosomal microsatellite loci to estimate heterozygosity for each female. We controlled for phenotypic plasticity and found that both clutch and egg size showed significant positive correlations with heterozygosity. We found no evidence that heterozygosity influenced the slope of individual reaction norms. Further analysis suggested that clutch size was affected by heterozygosity across the genome, but egg size had more complex relationships, with evidence favoring the influence of multiple loci. Given the apparent lack of inbreeding and large population size, our results suggest associative overdominance as the likely mechanism for the impact of heterozygosity, but also created a puzzle about the process producing associations between neutral markers and the genes affecting clutch size or egg size. One possible explanation is a long term residual effect of the historical bottleneck that occurred when house sparrows were introduced into North America. The existence of HFCs in a population with considerable phenotypic plasticity and little inbreeding implies that the effects of heterozygosity may be more significant than previously thought.

本研究探讨了雌性家麻雀（Passer domesticus）的杂合性（heterozygosity）与繁殖表现的反应规范（reaction norm）特征之间的关联。我们在16年的研究周期内，对791只环志个体的2816次筑巢尝试收集了窝卵数、卵大小、孵化成功率以及雏鸟存活率相关数据。系谱分析（pedigree analysis）未发现近交（inbreeding）的迹象。亲子回归（parent-offspring regression）与动物模型（animal model）均未揭示窝卵数或卵大小存在显著遗传力（heritability）。我们筛选出42只在本研究区域至少产7窝卵的雌性个体，并通过对21个常染色体微卫星位点（autosomal microsatellite loci）的分析，估算了每只雌性个体的杂合性水平。我们在分析中控制了表型可塑性（phenotypic plasticity）的影响，结果发现窝卵数与卵大小均与杂合性呈现显著正相关。未发现杂合性会影响个体反应规范斜率的证据。进一步分析显示，窝卵数受全基因组杂合性的影响，而卵大小的关联更为复杂，有证据表明其受多个位点的调控。鉴于研究群体几乎不存在近交且种群规模较大，我们的研究结果表明关联超显性（associative overdominance）可能是杂合性产生影响的潜在机制，但同时也提出了一个谜题：中性标记与影响窝卵数或卵大小的基因之间的关联是如何形成的？一种可能的解释是家麻雀被引入北美时经历的历史种群瓶颈（bottleneck）所留下的长期残余效应。在一个具有显著表型可塑性且近交程度极低的种群中发现杂合性-适合度关联（Heterozygosity-fitness correlations, HFCs），这意味着杂合性的影响可能比此前所认为的更为显著。