Meeting Abstract
With its unusual morphology and unexpected habitat (the renal sac of molgulid ascidians), Nephromyces has posed a stubborn phylogenetic puzzle since its first description 140 years ago. Recent data show indisputably that Nephromyces is an apicomplexan. As the only beneficial symbiont thus far found among the otherwise parasitic apicomplexa, Nephromyces presents an exceptional tool to probe the evolution of mutualistic outcomes in symbiotic interactions. In the first stage of a long-term study of Nephromyces-molgulid coevolution, we have begun a comprehensive genomic analysis of Nephromyces. Phylogenetic analysis of apicoplast genomes indicates that Nephromyces is a divergent apicomplexan clade, clearly distinct from, and possibly sister to, “core” apicomplexan clades. 18s rDNA sampling suggests that Cardiosporidium, an apicomplexan parasite of non-molgulid ascidians, is the sister group of Nephromyces. The association of both taxa with ascidians, the presence of symbiotic bacteria (rare among apicomplexa) in both taxa, and the highly derived features of both Nephromyces and molgulids raise the possibility that the Nephromyces-molgulid mutualism evolved from an ancestral apicomplexan-ascidian parasitism. There is surprising genomic diversity in Nephromyces, even among Nephromyces isolated from a single host individual; these data are reinforced by infection mechanisms that indicate the plausibility of multiple Nephromyces infections in individual hosts and by cross-inoculation experiments suggesting that some Nephromyces strains may infect more than one Molgula species. The observed genomic complexity also raises the possibility of functional diversity among different Nephromyces variants, and complex evolutionary and metabolic dynamics of both symbiont-symbiont and symbiont-host interactions in this system.
