Fitness consequences of outcrossing in a social spider with an inbreeding mating system

Inbreeding mating systems are uncommon because of inbreeding depression. Mating among close relatives can evolve, however, when outcrossing is constrained. Social spiders show obligatory mating among siblings. In combination with a female-biased sex ratio, sib-mating results in small effective populations. In such a system high genetic homozygosity is expected, and drift may cause population divergence. We tested the effect of outcrossing in the social spider Stegodyphus dumicola. Females were mated to sib-males, to a non-nestmate within the population, or to a male from a distant population, and fitness traits of F1s were compared. We found reduced hatching success of broods from between-population crosses, suggesting the presence of population divergence at a large geographical scale that may result in population incompatibility. However, a lack of a difference in offspring performance between inbred and outbred crosses indicates little genetic variation between populations, and could suggest recent colonisation by a common ancestor. This is consistent with population dynamics of frequent colonisations by single sib-mated females of common origin, and extinctions of populations after few generations. While drift or single mutations can lead to population divergence at a relatively short time scale, it is possible that dynamic population processes homogenize these effects at longer time scales.

由于近交衰退（inbreeding depression），近交交配体系并不常见。然而，当异交（outcrossing）受到限制时，近亲交配仍有可能演化。社会蜘蛛类群展现出专性的同胞交配（sib-mating）行为。结合雌性偏倚的性比，同胞交配会导致有效种群（effective population）规模缩小。在这类体系中，种群预期会呈现较高的遗传纯合性（genetic homozygosity），而遗传漂变（drift）可能引发种群分化。本研究以杜氏丝蛛（Stegodyphus dumicola）为对象，探究了异交对其的影响。实验中，雌蛛分别与同胞雄蛛、种群内非同巢雄蛛，以及来自异地种群的雄蛛交配，并对F1代的适合度性状（fitness traits）进行了比较。研究发现，跨种群交配所产卵簇的孵化成功率（hatching success）有所降低，这表明大地理尺度下存在种群分化，且该分化可能引发种群不相容性。然而，近交与异交组合的后代表现并无显著差异，这说明种群间的遗传变异程度较低，也暗示这些种群可能由同一祖先近期扩散定居而来。这与以下种群动态模式相符：具有共同起源的单个同胞交配雌蛛频繁进行定居扩散，且种群在数代后便走向灭绝。尽管遗传漂变或单个突变可在较短时间尺度上引发种群分化，但种群动态过程有可能在更长时间尺度上抵消这类分化效应。