Eccrinales (Trichomycetes) are not fungi, but a clade of protists at the early divergence of animals and fungi

The morphologically diverse orders Eccrinales and Amoebidiales have been considered members of the fungal class Trichomycetes (Zygomycota) for the last 50 years. These organisms either inhabit the gut or are ectocommensals on the exoskeleton of a wide range of arthropods-Crustacea, Insecta, and Diplopoda-in varied habitats. The taxonomy of both orders is based on a few micromorphological characters. One species, Amoebidium parasiticum, has been axenically cultured and this has permitted several biochemical and phylogenetic analyses. As a consequence, the order Amoebidiales has been removed from the Trichomycetes and placed in the class Mesomycetozoea. An affinity between Eccrinales and Amoebidiales was first suggested when the class Trichomycetes was erected by Duboscq et al. [Arch. Zool. Exp. Gen. 86 (1948) 29]. Subsequently, molecular markers have been developed to study the relationship of these orders to other groups. Ribosomal gene (18S and 28S) sequence analyses generated by this study do not support a close association of these orders to the Trichomycetes or to other fungi. Rather, Eccrinales share a common ancestry with the Amoebidiales and belong to the protist class Mesomycetozoea, placed at the animal-fungi boundary.