Data from: No evidence that genetic compatibility drives extra-pair behavior in female blue-footed boobies

The function of female birds' extra-pair (EP) behavior has remained an unresolved question in ornithology and behavioral ecology for > 30 yr. The genetic compatibility hypothesis (GCH) proposes that females benefit by acquiring biological sires that yield more heterozygous, and therefore fitter, offspring than their social mates. We used ten polymorphic microsatellite loci to test GCH predictions and its assumption that fitness increases with heterozygosity in blue-footed boobies Sula nebouxii, a long-lived tropical seabird. Our predictions were not supported. Heterozygosity was uncorrelated with quality indicators (fledging probability, fledgling or adult body size or mass, adult ornamentation, mean breeding success). Females were no more likely to have EP behavior or chicks when their social mates were less heterozygous or compatible, nor were EP males more heterozygous or compatible than the males they cuckolded. Finally, EP chicks were no more heterozygous than within-pair chicks overall or in half-sib comparisons, nor were within-pair chicks from all-within-pair nests more heterozygous that those with EP nest-mates. There are both methodological and biological explanations for these consistently negative results. Inadequate sample size is possible but unlikely, since our samples were comparable or larger than those of similar studies with significant findings. Lack of identity disequilibrium (within-individual heterozygosity correlation) among our marker loci could be responsible, and suggests either insufficient marker coverage or lack of inbreeding, bottleneck, and/or admixture. An independent social pedigree revealed infrequent inbreeding, suggesting that pressure on females to select sires that maximize offspring heterozygosity may be genuinely lax. Alternatively, it is possible that the GCH is only upheld when selection on young is strongest; this would not be detected in our sample, which was taken during an extremely productive year. Whatever their cause, our results expand the taxonomic breadth of avian EP behavior analyses and should be considered in future evaluations of the GCH.

三十余年来，鸟类雌性个体的配对外（extra-pair, EP）行为的演化功能始终是鸟类学与行为生态学领域未被解决的核心问题之一。遗传相容性假说（genetic compatibility hypothesis, GCH）提出，雌性个体通过选择社会性配偶之外的雄性作为生父，可产生相较于与社会性配偶所育后代具有更高杂合度、因而适合度更强的子代，从而获得演化收益。我们以十对多态性微卫星位点（microsatellite loci）为分子标记，对蓝脚鲣鸟（Sula nebouxii）——一种长寿命热带海鸟——的遗传相容性假说预测及其“适合度随杂合度升高而提升”的核心假设进行了检验。我们的研究结果并未支持该假说。杂合度与各项适合度指标（雏鸟出飞概率、雏鸟或成鸟的体型与体重、成鸟装饰性羽饰、平均繁殖成功率）均无相关性。当社会性配偶的杂合度较低或相容性较差时，雌性个体并未更大概率发生配对外行为或产下配对外子代；同时，配对外雄性的杂合度与相容性也并未显著高于其所戴绿帽的原配雄性。最终，无论是整体比较还是半同胞子代比较，配对外子代的杂合度均未显著高于配对内子代；此外，全配对内巢的配对内子代的杂合度，也未显著高于存在配对外同巢子代的巢内的配对内子代。本研究得到的一系列阴性结果可从方法学与生物学两个层面进行解释。样本量不足存在可能性但概率较低，因为我们的样本规模与已有获得显著性结果的同类研究相当甚至更大。我们的标记位点间缺乏同一性不平衡（个体内杂合度相关性），这或许是原因之一，这一结果提示要么是标记覆盖度不足，要么是研究种群未发生近交、瓶颈效应或遗传混合。一项独立的社会谱系分析显示该种群近交事件极为罕见，这表明雌性个体选择可最大化子代杂合度的生父所承受的演化压力可能确实较弱。另一种可能性是，仅当对幼体的选择压力最强时，遗传相容性假说才成立；而我们的样本采集自繁殖极为成功的年份，因此无法检测到这一效应。无论本研究阴性结果的成因如何，其结果都拓展了鸟类配对外行为分析的分类学范围，未来在对遗传相容性假说进行评估时应予以考虑。