Postmating Reproductive isolation between strains of <i>Drosophila willistoni</i>

Speciation can occur through the presence of reproductive isolation barriers that impede mating, restrict cross-fertilization, or render inviable/sterile hybrid progeny. The <i>D. willistoni</i> subgroup is ideally suited for studies of speciation, with examples of both allopatry and sympatry, a range of isolation barriers, and the availability of one species complete genome sequence to facilitate genetic studies of divergence. <i>D. w. willistoni</i> has the largest geographic distribution among members of the <i>Drosophila willistoni</i> subgroup, spanning from Argentina to the southern United States, including the Caribbean islands. A subspecies of <i>D. w. willistoni, D. w. quechua</i>, is geographically separated by the Andes mountain range and has evolved unidirectional sterility, in that only male offspring of <i>D. w. quechua</i> females × <i>D. w. willistoni</i> males are sterile. Whether <i>D. w. willistoni</i> flies residing east of the Andes belong to one or more <i>D. willistoni</i> subspecies remains unresolved. Here we perform fecundity assays and show that F1 hybrid males produced from crosses between different strains found in Central America, North America, and northern Caribbean islands are reproductively isolated from South American and southern Caribbean island strains as a result of unidirectional hybrid male sterility. Our results show the existence of a reproductive isolation barrier between the northern and southern strains and suggest a subdivision of the previously identified <i>D. willistoni willistoni</i> species into 2 new subspecies.物种形成可通过生殖隔离屏障的介导得以实现，此类屏障可阻碍交配、限制异体受精，或使杂交后代丧失存活能力或出现不育表型。威利斯果蝇物种群（Drosophila willistoni subgroup）是研究物种形成的理想模型类群，其兼具异域分布与同域分布类群，涵盖多种生殖隔离屏障，且已有一个物种的全基因组序列可用于物种分化的遗传学研究。威利斯果蝇威利斯亚种（Drosophila willistoni willistoni）是该物种群中地理分布范围最广的成员，其分布范围从阿根廷延伸至美国南部，涵盖加勒比群岛。威利斯果蝇克丘亚亚种（Drosophila willistoni quechua）作为其亚种，因安第斯山脉的地理隔离而演化出单向不育特性：仅当以克丘亚亚种雌性个体与威利斯亚种雄性个体杂交时，其子代雄性会表现出不育表型。目前，安第斯山脉以东的威利斯果蝇种群隶属于单一种还是多个威利斯果蝇亚种，这一问题尚未得到解决。本研究通过繁殖力检测实验证实，对中美洲、北美洲及加勒比海北部群岛的不同菌株进行杂交，所获得的F1杂交雄性个体，与南美洲及加勒比海南部群岛菌株之间存在生殖隔离，该隔离由单向杂交雄性不育介导。研究结果表明，北部菌株与南部菌株之间存在生殖隔离屏障，提示可将此前界定的威利斯果蝇威利斯亚种细分为两个新的亚种。