Has the inbreeding load for a condition-dependent sexual signalling trait been purged in insular lizard populations?

Sexually selected traits are often condition-dependent and are expected to be affected by genome-wide distributed deleterious mutations and inbreeding. However, sexual selection is a powerful selective force that can counteract inbreeding through purging of deleterious mutations. Inbreeding and purging of the inbreeding load for sexually selected traits has rarely been studied across natural populations with different degrees of inbreeding. Here we investigate inbreeding effects (measured as marker-based heterozygosity) on condition-dependent sexually selected signalling trait and other morphological traits across islet- and mainland populations (n = 15) of an endemic lizard species (Podarcis gaigeae). Our data suggest inbreeding depression on a condition-dependent sexually selected signalling character among mainland subpopulations with low or intermediate levels of inbreeding, but no sign of inbreeding depression among small and isolated islet populations despite their higher overall inbreeding levels. In contrast, there was no such pattern among ten other morphological traits which are primarily naturally selected and presumably not involved in sexual signalling. These results are in line with purging of recessive deleterious alleles, or purging in combination with stochastic fixation of alleles by genetic drift, for a sexual signalling character in the islet environment, which is characterized by low population sizes and strong sexual selection. Higher clutch sizes in islet populations also raise interesting questions regarding the possibility of antagonistic pleiotropy. Purging and other non-exclusive explanations of our results are discussed.

性选择性状（sexually selected traits）通常具有条件依赖性，且会受到全基因组分布的有害突变与近交的影响。然而，性选择是一种强有力的选择压力，可通过清除有害突变以抵消近交的不利影响。针对性选择性状的近交及其近交负荷清除，在不同近交程度的自然种群中鲜有研究。
本研究以一种特有蜥蜴（Podarcis gaigeae）的15个小岛与大陆种群（n=15）为研究对象，探究了基于标记的杂合度（marker-based heterozygosity）所衡量的近交效应，对条件依赖性性选择信号性状以及其他形态性状的影响。
研究数据显示，在近交程度较低或中等的大陆亚种群中，条件依赖性性选择信号性状存在近交衰退（inbreeding depression）现象；但在整体近交程度更高的小型孤立小岛种群中，却未发现近交衰退的迹象。与之形成对比的是，其余10种主要受自然选择、且推测不参与性信号传递的形态性状，并未呈现出此类模式。
上述结果与小岛环境中（该环境以种群规模小、性选择强度大为特征）性信号性状的隐性有害等位基因清除现象相符，或是与清除结合遗传漂变导致的等位基因随机固定现象相符。小岛种群的窝卵数更高，这也引出了关于拮抗多效性（antagonistic pleiotropy）可能性的有趣问题。本研究对有害突变清除及其他可共同解释本研究结果的非排他性假说进行了讨论。