Phylogeny, morphology, virulence, ecology, and host range ofOrdospora pajunii(Ordosporidae), a microsporidian symbiont ofDaphniaspp

Symbiosis is increasingly recognized as a dynamic relationship, with the net outcome falling along a continuum from mutualism to parasitism. A key example of this comes from a recently discovered microsporidian symbiont ofDaphnia,the net impact of which was found to vary from negative to positive. We investigated the taxonomic position of this microsporidian and the morphology of infected hosts, as well as the virulence, ecology and host range of the symbiont; we also provide information about its culturing methods. The genetic data indicates that the microsporidian symbiont belongs toOrdospora pajunii, a newly described microsporidian parasite ofDaphnia. We show thatO. pajuniiinfection damages the gut, causing infected epithelial cells to lose microvilli and then rupture. The prevalence of this microsporidian can be high (up to 100% in the lab and 77% of adults in the field). Its overall virulence seems low in most cases, but some genotypes suffer reduced survival and reproduction. Susceptibility and virulence are strongly host-genotype dependent. We found that North AmericanO. pajuniiare able to infect multipleDaphniaspecies, including the European speciesD. longispina, as well as the genusCeriodaphnia. We propose theDaphnia-O. pajuniisymbiosis as a valuable system for studying the mechanisms of context-dependent shifts between mutualism and parasitism, as well as for understanding how symbionts might alter host interactions with resources.ImportanceThe net outcome of symbiosis can shift between parasitism and mutualism, but the mechanisms underlying such changes remain cryptic. A full understanding of what drives shifts of symbiotic relationships will require comprehensive knowledge of the biology and ecology of the symbionts. However, we have few systems in which this is possible, especially for cases where something that is typically a parasite becomes a mutualist. Recently, a microsporidian symbiont ofDaphniawas found to shift from parasitism towards mutualism, but little was known about this symbiont (including its species identity). Here, we describe this endosymbiont ofDaphnia, identifying it as the microsporidiumOrdospora pajunii. We described its phylogeny, morphology, virulence and ecology, enabling future study of this interaction as a model for shifts along the mutualism-parasitism continuum.