Clarifying the Relationships between Microsporidia and Cryptomycota

Some protists with microsporidian‐like cell biological characters, including Mitosporidium, Paramicrosporidium, and Nucleophaga, have SSU rRNA gene sequences that are much less divergent than canonical Microsporidia. We analysed the phylogenetic placement and environmental diversity of microsporidian‐like lineages that group near the base of the fungal radiation and showed that they group in a clade with metchnikovellids and canonical microsporidians to the exclusion of the clade including Rozella, in line with what is currently known of their morphology and cell biology. These results show that the phylogenetic scope of Microsporidia has been greatly underestimated. We propose that much of the lineage diversity previously thought to be cryptomycotan/rozellid is actually microsporidian, offering new insights into the evolution of the highly specialized parasitism of canonical Microsporidia. This insight has important implications for our understanding of opisthokont evolution and ecology, and is important for accurate interpretation of environmental diversity. Our analyses also demonstrate that many opisthosporidian (aphelid+rozellid+microsporidian) SSU V4 OTUs from tropical forest soils group with the short‐branching Microsporidia, consistent with the abundance of their protist and arthropod hosts in soils. This novel diversity of environmental Microsporidia provides a unique opportunity to investigate the evolutionary origins of a highly specialised clade of major animal parasites.