Data from: Infectious adaptation: potential host range of a defensive endosymbiont in Drosophila

Maternally transmitted symbionts persist over macroevolutionary time scales by undergoing occasional lateral transfer to new host species. To invade a new species, a symbiont must survive and reproduce in the new host, undergo maternal transmission, and confer a selective benefit sufficient to overcome losses due to imperfect maternal transmission. Drosophila neotestacea is naturally infected with a strain of Spiroplasma that restores fertility to nematode-parasitized females, which are otherwise sterilized by parasitism. We experimentally transferred Spiroplasma from D. neotestacea to four other species of mycophagous Drosophila that vary in their ability to resist and/or tolerate nematode parasitism. In all four species, Spiroplasma achieved within-host densities and experienced rates of maternal transmission similar to that in D. neotestacea. Spiroplasma restored fertility to nematode-parasitized females in one of these novel host species. Based on estimates of maternal transmission fidelity and the expected benefit of Spiroplasma infection in the wild, we conclude that Spiroplasma has the potential to spread and become abundant within D. putrida, which is broadly sympatric with D. neotestacea and in which females are rendered completely sterile by nematode parasitism. Thus, a major adaptation within D. putrida could arise via lateral transmission of a heritable symbiont from D. neotestacea.

母系传播共生体（maternally transmitted symbionts）通过偶尔发生的水平转移（lateral transfer）至新宿主物种，得以在宏进化时间尺度（macroevolutionary time scales）上持续存续。若要成功入侵新宿主，共生体需在新宿主体内存活并繁殖、完成母系传播（maternal transmission），且赋予足够强的选择优势（selective benefit）以抵消不完全母系传播（imperfect maternal transmission）造成的适合度损耗。新测果蝇（Drosophila neotestacea）天然感染有一株螺原体（Spiroplasma），该菌株可恢复被线虫（nematode）寄生的雌虫的育性——该类雌虫若无该共生体则会因寄生而不育。我们通过实验将螺原体从新测果蝇转移至另外4种食真菌果蝇（mycophagous Drosophila），这些宿主在抵抗和/或耐受线虫寄生的能力上存在差异。在这4个宿主物种中，螺原体的宿主体内密度（within-host densities）与母系传播速率均与新测果蝇中的水平相当。在其中一种新宿主中，螺原体成功恢复了被线虫寄生雌虫的育性。基于母系传播保真度（maternal transmission fidelity）的估算结果，以及野外环境中螺原体感染所带来的预期选择收益，我们推断螺原体具备在腐食果蝇（Drosophila putrida）种群中扩散并达到高丰度的潜力。该果蝇与新测果蝇广泛同域分布（sympatric），且其雌虫会因线虫寄生而完全不育。因此，腐食果蝇的一项关键适应性性状可通过从新测果蝇水平转移可遗传共生体（heritable symbiont）的方式演化而来。