Trachymyrmex ant and Leucocoprinus fungus genotypes

Over the past few decades, large-scale phylogenetic analyses of fungus-gardening ants and their symbiotic fungi have depicted strong concordance among major clades of ants and their symbiotic fungi, yet within clades, fungus sharing is widespread among unrelated ant lineages. Sharing has been explained using a diffuse coevolution model within major clades. Understanding horizontal exchange within clades has been limited by conventional genetic markers that lack both interspecific and geographic variation. To examine whether reports of horizontal exchange were indeed due to symbiont sharing or the result of employing relatively uninformative molecular markers, samples of Trachymyrmex arizonensis and Trachymyrmex pomonae and their fungi were collected from native populations in Arizona and genotyped using conventional marker genes and genome-wide single nucleotide polymorphisms (SNPs). Conventional markers of the fungal symbionts generally exhibited cophylogenetic patterns that were consistent with some symbiont sharing, but most fungal clades had low support. SNP analysis, in contrast, indicated that each ant species exhibited fidelity to its own fungal subclade with only one instance of a colony growing a fungus that was otherwise associated with a different ant species. This evidence supports a pattern of codivergence between Trachymyrmex species and their fungi, and thus a diffuse coevolutionary model may not accurately predict symbiont exchange. These results suggest that fungal sharing across host species in these symbioses may be less extensive than previously thought.

近数十年来，针对菌培蚂蚁（fungus-gardening ants）与其共生真菌的大规模系统发育分析（phylogenetic analyses）显示，蚂蚁的主要演化支（clade）与其共生真菌之间存在高度的演化一致性；但在同一演化支内部，非亲缘蚂蚁类群间却广泛存在真菌共享现象。针对演化支内部的真菌共享现象，此前学界多以弥散协同进化（diffuse coevolution）模型进行解释。然而，传统遗传标记（genetic markers）同时缺乏种间与地理分布层面的遗传变异，这极大限制了学界对演化支内部水平共生体交换事件的认知。为验证此前报道的水平共生体交换事件，究竟是真实存在的共生体共享现象，还是因使用信息量相对不足的分子标记（molecular markers）所导致的假象，研究团队从美国亚利桑那州的原生种群中采集了亚利桑那曲须切叶蚁（Trachymyrmex arizonensis）、波莫纳曲须切叶蚁（Trachymyrmex pomonae）及其共生真菌的样本，并分别通过传统标记基因与全基因组单核苷酸多态性（single nucleotide polymorphisms, SNPs）对样本完成基因分型。对真菌共生体的传统标记分析整体呈现出与部分共生体共享现象相符的共系统发育（cophylogenetic）模式，但绝大多数真菌演化支的拓扑支持度较低。与之相对，全基因组单核苷酸多态性分析结果显示，每种蚂蚁均严格忠于其自身的真菌亚演化支，仅观测到1例蚁群培育的真菌与其他蚂蚁物种存在关联的案例。该证据支持曲须切叶蚁属（Trachymyrmex）物种与其共生真菌之间存在协同分化（codivergence）的演化模式，因此弥散协同进化模型或许无法准确预测共生体的交换事件。本研究结果表明，这类共生关系中跨宿主物种的真菌共享现象，或许比此前学界的认知要更为有限。