Understanding phylogenetic relationships among species in the Nosema/Vairimorpha clade: What does genetic similarity say about host switching in the microsporidia?

Microsporidian species with high genetic identity based on rDNA phylogenies have been isolated from different insect species and even higher level host taxa across the globe. Complicating the picture, for genera such as the lepidopteran Nosema and Vairimorpha, isolates from different hosts may be morphologically identical while some species complexes include isolates that differ significantly in morphology, tissue tropism, host range and pathogenicity. Morphological characters such as octospore formation in the Vairimorpha, once thought to be apomorphies, cannot be used to separate these two genera or possibly even species. Difficulties in distinguishing isolates in turn complicates research on microsporidia suspected of host-switching to new and economically or ecologically important hosts. We used a combination of rRNA, HSP70, and RPB1 genes to evaluate and clarify the relationships among a large collection of species and isolates in the Nosema/Vairimorpha clade. Our data will be useful for classifying new isolates, describing species and evaluating the potential for microsporidia to invade new hosts.