Data from: Macroevolutionary persistence of heritable endosymbionts: acquisition, retention, and expression of adaptive phenotypes in Spiroplasma

The phylogenetic incongruence between insects and their facultative maternally-transmitted endosymbionts indicates that these infections are generally short-lived evolutionarily. Therefore, long-term persistence of many endosymbionts must depend on their ability to colonize and spread within new host species. At least 17 species of Drosophila are infected with endosymbiotic Spiroplasma that have various phenotypic effects. We transinfected five strains of Spiroplasma from three divergent clades into D. neotestacea to test their capacity to spread in a novel host. A strain that causes male-killing in D. melanogaster (its native host) also does so in D. neotestacea, even though these host species diverged 40-60 mya. A strain native to D. neotestacea (designated sNeo) and the two other strains of the poulsonii clade of Spiroplasma confer resistance to wasp parasitism, suggesting that this trait may be ancestral within this clade of Spiroplasma. Conversely, no strain other than sNeo conferred resistance to the sterilizing effects of nematode parasitism, suggesting that nematode resistance is a recently derived condition. The apparent addition of nematode resistance to a Spiroplasma lineage that already confers resistance to wasp parasitism suggests endosymbionts can increase the repertoire of traits conducive to their spread. The capacity of an endosymbiont to undergo maternal transmission and express adaptive phenotypes in novel hosts, without requiring a period of host-symbiont coevolution, enables the spread of such symbionts immediately after the colonization of a new host. This could be critical for the macroevolutionary persistence of facultative endosymbionts whose sojourn times within individual host species are relatively brief.

昆虫与其兼性母传内共生体（facultative maternally-transmitted endosymbionts）之间的系统发育不一致（phylogenetic incongruence）表明，这类感染在进化进程中通常较为短命。因此，多数内共生体的长期存续，必然依赖其在新宿主类群中成功定殖并扩散的能力。至少有17种果蝇属（Drosophila）昆虫感染了具有多种表型效应的内共生螺原体属（Spiroplasma）物种。我们将来自3个不同演化支的5株螺原体转接种至新种果蝇D. neotestacea，以测试它们在该新宿主中的扩散能力。一株可在其自然宿主黑腹果蝇（D. melanogaster）中诱导杀雄作用（male-killing）的菌株，同样能在D. neotestacea中引发该表型，尽管这两个宿主类群的分化时间已达4000万至6000万年。一株自然宿主为D. neotestacea的菌株（命名为sNeo）以及螺原体Poulsonii演化支的另外两株菌株，均可赋予宿主抵抗寄生蜂寄生（wasp parasitism）的能力，这暗示该抗性性状可能在该螺原体演化支中具有祖先起源。与之相反，除sNeo外的其余菌株均无法赋予宿主抵抗线虫寄生（nematode parasitism）所导致的宿主绝育效应的能力，这表明对线虫的抗性是近期衍生的进化性状。在一株已具备抗寄生蜂寄生能力的螺原体谱系中新增对线虫的抗性，这一现象提示内共生体可扩充有助于自身扩散的性状谱。内共生体无需经历宿主-共生体协同进化（host-symbiont coevolution）阶段，即可在新宿主中实现母传传播并表达适应性表型，这使得这类共生体在定殖新宿主后可立即实现种群扩散。这对于驻留时间相对较短的兼性内共生体的宏观进化（macroevolutionary）存续而言，可能至关重要。