Temperature and sex related effects of serine protease alleles on larval development in the Glanville fritillary butterfly

The body reserves of adult Lepidoptera are accumulated during larval development. In the Glanville fritillary butterfly, larger body size increases female fecundity, but in males fast larval development and early eclosion, rather than large body size, increase mating success and hence fitness. Larval growth rate is highly heritable, but genetic variation associated with larval development is largely unknown. By comparing the Glanville fritillary population living in the Åland Islands in northern Europe with a population in Nantaizi in China, within the source of the post-glacial range expansion, we identified candidate genes with reduced variation in Åland, potentially affected by selection under cooler climatic conditions than in Nantaizi. We conducted an association study of larval growth traits by genotyping the extremes of phenotypic trait distributions for 23 SNPs in 10 genes. Three genes in clip-domain serine protease family were associated with larval growth rate, development time and pupal weight. Additive effects of two SNPs in the prophenoloxidase-activating proteinase-3 (ProPO3) gene, related to melanization, showed elevated growth rate in high temperature but reduced growth rate in moderate temperature. The allelic effects of the vitellin-degrading protease precursor gene on development time were opposite in the two sexes, one genotype being associated with long development time and heavy larvae in females but short development time in males. Sexually antagonistic selection is here evident in spite of sexual size dimorphism.

成年鳞翅目（Lepidoptera）昆虫的体内储备均在幼虫发育阶段完成积累。在格兰维尔豹蛱蝶（Glanville fritillary）中，成虫体型更大可提升雌性的繁殖力，但对于雄性而言，更快的幼虫发育速率与更早的羽化，而非更大体型，才是提升交配成功率进而提升适合度（fitness）的关键。幼虫生长速率具有较高的遗传力，但目前学界对与幼虫发育相关的遗传变异仍知之甚少。本研究通过对比北欧奥兰群岛（Åland Islands）与中国南台子的格兰维尔豹蛱蝶种群——二者均处于冰后期分布扩张的起源区域——筛选出了奥兰群岛种群中变异程度降低的候选基因，这些基因可能受到了比南台子更为寒冷的气候条件下的自然选择作用。我们针对10个基因中的23个单核苷酸多态性（single nucleotide polymorphism, SNPs）位点进行了基因分型，并基于表型性状分布的极端值开展了幼虫生长性状关联分析。clip结构域丝氨酸蛋白酶（clip-domain serine protease）家族的3个基因与幼虫生长速率、发育时长及蛹重均显著相关。与黑化作用（melanization）相关的酚氧化酶原激活蛋白酶3（ProPO3）基因内的两个单核苷酸多态性位点的加性效应呈现出温度依赖性：在高温环境下可提升幼虫生长速率，而在适中温度环境下则会降低生长速率。卵黄蛋白降解蛋白酶前体（vitellin-degrading protease precursor）基因的等位基因对发育时长的调控效应在雌雄个体中完全相反：某一基因型在雌性个体中与更长的发育时长以及更重的幼虫体重相关，而在雄性个体中则对应更短的发育时长。尽管存在两性体型二态性（sexual size dimorphism），本研究仍观测到了显著的性拮抗选择现象。