Data from: Dynamics of the continent-wide spread of a Drosophila defensive symbiont

Facultative symbionts can represent important sources of adaptation for their insect hosts and thus have the potential for rapid spread. Drosophila neotestacea harbors a heritable symbiont, Spiroplasma, that confers protection against parasitic nematodes. We previously found a cline in Spiroplasma prevalence across central Canada, ending abruptly at the Rocky Mountains. Resampling these populations nine years later revealed that Spiroplasma had increased substantially across the region, resembling a Fisherian wave of advance. Associations between Spiroplasma infection and host mitochondrial DNA indicate that the increase was due to local increase of Spiroplasma-infected flies. Finally, we detected Spiroplasma west of the Rocky Mountains for the first time and showed that defense against nematodes occurs in flies with a western genetic background. Because nematode infection is common throughout D. neotestacea’s range, we expect Spiroplasma to spread to the Pacific coast.

兼性共生菌（facultative symbionts）可作为昆虫宿主重要的适应性来源，因而具备快速扩散的潜力。新测果蝇（Drosophila neotestacea）宿主有一种可遗传共生菌——螺原体（Spiroplasma），该菌能够为宿主提供抵御寄生线虫的保护能力。我们此前在加拿大中部地区发现了螺原体感染率的梯度分布，该分布在落基山脉处骤然终止。时隔九年对这些种群进行重新采样后发现，该区域内螺原体的感染率已大幅提升，其扩散模式类似费希尔前进波（Fisherian wave of advance）。螺原体感染与宿主线粒体DNA之间的关联表明，此次感染率提升源于本地受螺原体感染的果蝇种群的增长。最后，我们首次在落基山脉以西地区检测到螺原体，并证实具有西部遗传背景的果蝇同样可借助该共生菌抵御线虫。由于线虫感染在新测果蝇（Drosophila neotestacea）的整个分布范围内均较为普遍，我们预计螺原体将扩散至太平洋沿岸。