Data from: Small spermatophore size and reduced female fitness in an isolated butterfly population

The Glanville fritillary butterfly (Melitaea cinxia) has a small population (Ne ~100) on the small island of Pikku Tytärsaari (PT) in the Gulf of Finland. The population has remained completely isolated for ~100 generations, which has resulted in greatly reduced genetic variation and high genetic load (low fitness). In particular, females lay small egg clutches with low egg-hatching rate in comparison with a large reference population in the Åland Islands (ÅL). Here, we analyze to what extent egg clutch size and egg-hatching rate are influenced by male population and spermatophore size. Spermatophore size increases with male body size, is smaller after the first mating, and is smaller in the small PT population. In the ÅL population but not in the PT population, egg-hatching rate increases with spermatophore size. Egg-hatching rate of PT females is higher when mated with ÅL males than when mated with PT males (heterosis), but there is no such effect on clutch size. Clutch size of ÅL females is however reduced when mated with PT males. These results indicate that both male and female traits contribute to reduced reproductive fitness in the small isolated population.

格朗维尔贝母蝶（Glanville fritillary butterfly, *Melitaea cinxia*）在芬兰湾的皮库·蒂塔尔萨里小岛（Pikku Tytärsaari，简称PT）上的种群有效规模（Ne）约为100。该种群已完全隔离约100个世代，由此导致遗传变异大幅缩减以及遗传负荷升高（适合度低下）。相较于奥兰群岛（Åland Islands，简称ÅL）的大型参考种群，该种群雌蝶所产卵块更小，且卵孵化率更低。本研究旨在分析雄蝶种群特征与精包（spermatophore）大小对卵块大小及卵孵化率的影响程度。精包大小随雄蝶体型增大而增加，首次交配后精包尺寸会减小，且在PT小型种群中精包普遍更小。在奥兰群岛种群中，卵孵化率随精包尺寸提升而升高，但该规律并未在PT种群中出现。PT雌蝶与奥兰群岛雄蝶交配时的卵孵化率，显著高于与PT本土雄蝶交配的情况（该现象属于杂种优势（heterosis）），但该效应并未对卵块大小产生影响。然而，奥兰群岛雌蝶与PT雄蝶交配时，其卵块规模会出现下降。上述研究结果表明，雌雄两性的性状共同作用，导致小型隔离种群的繁殖适合度降低。