Data from: Asymmetrical sexual isolation but no postmating isolation between the closely related species Drosophila suboccidentalis and Drosophila occidentalis

Background: During the speciation process several types of isolating barriers can arise that limit gene flow between diverging populations. Studying recently isolated species can inform our understanding of how and when these barriers arise, and which barriers may be most important to limiting gene flow. Here we focus on Drosophila suboccidentalis and D. occidentalis, which are closely related mushroom-feeding species that inhabit western North America and are not known to overlap in geographic range. We investigate patterns of reproductive isolation between these species, including premating, postmating prezygotic, and postzygotic barriers to gene flow. Results: Using flies that originate from a single population of each species, we find that the strength of premating sexual isolation between these species is asymmetric: while D. occidentalis females mate with D. suboccidentalis males at a reduced but moderate rate, D. suboccidentalis females discriminate strongly against mating with D. occidentalis males. Female hybrids will mate at high rates with males of either species, indicating that this discrimination has a recessive genetic basis. Hybrid males are accepted by females of both species. We do not find evidence for postmating prezygotic or postzygotic isolating barriers, as females use the sperm of heterospecific males and both male and female hybrids are fully fertile. Conclusions: Premating isolation is substantial but incomplete, and appears to be the primary form of reproductive isolation between these species. If these species do hybridize, the lack of postzygotic barriers may allow for gene flow between them. Given that these species are recently diverged and are not known to be sympatric, the level of premating isolation is relatively strong given the lack of intrinsic postzygotic isolation. Further work is necessary to characterize the geographic and genetic variation in reproductive isolating barriers, as well as to determine the factors that drive reproductive isolation and the consequences that isolating barriers as well as geographic isolation have had on patterns of gene flow between these species.

背景：在物种形成过程中，可产生多种生殖隔离障碍（isolating barriers），以限制分化种群间的基因流。对新近分化物种的研究，有助于我们理解这些隔离障碍如何产生、何时产生，以及何种障碍对限制基因流最为关键。本研究聚焦于西方果蝇（Drosophila suboccidentalis）和北美果蝇（D. occidentalis）：二者为亲缘关系紧密的食菌果蝇类群，栖息于北美西部，目前尚无二者地理分布重叠的记录。结果：本研究采用源自各物种种群的单群果蝇进行实验，发现二者间的交配前性隔离（premating sexual isolation）强度存在不对称性：北美果蝇雌虫与西方果蝇雄虫的交配率虽有所降低，但仍处于中等水平；而西方果蝇雌虫会强烈排斥与北美果蝇雄虫交配。杂种雌虫可与两种果蝇的雄虫均以较高频率交配，表明这种交配偏好具有隐性遗传基础。两种果蝇的雌虫均可接受杂种雄虫。未检测到交配后合子前隔离（postmating prezygotic）或合子后隔离（postzygotic）存在的相关证据：雌虫可利用异种雄虫的精子，且雌雄杂种个体均完全可育。结论：交配前隔离虽显著但并不完全，且是这两个物种间最主要的生殖隔离形式。若二者发生杂交，由于缺乏合子后隔离障碍，可能会实现种群间的基因流。鉴于这两个物种新近分化且目前尚无同域分布记录，在缺乏内在合子后隔离的情况下，其交配前隔离强度相对较高。未来仍需开展进一步研究，以阐明生殖隔离障碍的地理与遗传变异特征，明确驱动生殖隔离形成的因素，以及生殖隔离与地理隔离对二者间基因流模式的影响。