Data from: Divergence before the host shift? Prezygotic reproductive isolation among three varieties of a specialist fly on a single host plant

Although divergence via host-plant shifting is a common theme in the speciation of some phytophagous insects, it is not clear whether host shifts are typically initiators of speciation or if they instead contribute to divergence events already in progress. While host shifts appear to be generally associated with speciation events for flies in the genus Strauzia, three sympatric varieties of the sunflower fly [Strauzia longipennis (Wiedemann)] co-occur on the same host plant in the Midwestern United States and may have evolved reproductive barriers without a host shift. The strength of two prezygotic reproductive barriers was compared among the three S. longipennis varieties: one barrier that is often associated with divergent ecological selection (allochronic isolation), and another that is more likely to be independent of ecological selection (pre-copulatory sexual isolation). The presence and relative strength of each barrier between fly varieties were evaluated using microsatellites, no choice mating experiments, studies of allochronic isolation, and field collection data. Evidence for both allochronic isolation and pre-copulatory sexual isolation was detected between the three varieties of S. longipennis. The measure of isolation calculated for each barrier between the three varieties was lower than measures calculated between different species of Strauzia found on different hosts, suggesting that subsequent host shifts may increase the degree of reproductive isolation. For Strauzia and other specialist insects, some reproductive isolation may evolve prior to, and indeed may facilitate, host shifts.

尽管通过寄主植物转换实现分化是部分植食性昆虫物种形成过程中的常见模式，但目前尚不清楚寄主转换通常是物种形成的启动因素，还是仅对已在进行中的分化事件起到推动作用。虽然对于Strauzia属（Strauzia）蝇类而言，寄主转换似乎普遍与其物种形成事件相关联，但在美国中西部地区，向日葵蝇[Strauzia longipennis (Wiedemann)]的三个同域变种共享同一寄主植物，且可能在未发生寄主转换的情况下演化出了生殖隔离屏障。研究比较了这三个S. longipennis变种之间两类合子前生殖隔离屏障的强度：一类为通常与分化性生态选择相关的异时隔离（allochronic isolation），另一类则为更可能独立于生态选择的交配前性隔离（pre-copulatory sexual isolation）。研究人员通过微卫星标记（microsatellites）、无选择交配实验、异时隔离研究以及野外采集数据，评估了各蝇类变种间生殖隔离屏障的存在与否及其相对强度。研究在这三个S. longipennis变种之间同时检测到了异时隔离与交配前性隔离的证据。两类屏障在变种间测得的隔离强度，均低于不同寄主上的不同Strauzia属物种间的隔离强度测算值，这表明后续的寄主转换可能会进一步提升生殖隔离的程度。对于Strauzia属昆虫及其他专食性昆虫而言，部分生殖隔离可能会在寄主转换之前演化形成，且实际上可能会推动寄主转换事件的发生。