Evidence from beta-tubulin phylogeny that microsporidia evolved from within the fungi

Microsporidia are obligate intracellular parasites that were thought to be an ancient eukaryotic lineage based on molecular phylogenies using ribosoma l RNA and translation elongation factors. However, this ancient origin of m icrosporidia has been contested recently, as several other molecular phylog enies suggest that microsporidia are closely related to fungi. Most of the protein trees that place microsporidia with fungi are not well sampled, how ever, and it is impossible to resolve whether microsporidia evolved from a fungus or from a protistan relative of fungi. We have sequenced beta-tubuli ns from 3 microsporidia, 4 chytrid fungi, and 12 zygomycete fungi, expandin g the representation of beta-tubulin to include all four fungal divisions a nd a wide diversity of microsporidia. In phylogenetic trees including these new sequences, the overall topology of the fungal beta-tubulins generally matched the expected relationships among the four fungal divisions, althoug h the zygomycetes were polyphyletic in some analyses. The microsporidia con sistently fell within this fungal diversification, and not as a sister grou p to fungi. Overall, beta-tubulin phylogeny suggests that microsporidia evo lved from a fungus sometime after the divergence of chytrids. We also found that chytrid alpha- and beta-tubulins are much less divergent than are tub ulins from other fungi or microsporidia. In trees in which the only fungal representatives were the chytrids, microsporidia still branched with fungi (i.e., with chytrids), suggesting that the affiliation between microsporidi an and fungal tubulins is not an artifact of long-branch attraction.