Accepted_test

Microsporidia are a widespread and species-rich group of obligate intracellular parasites of animals and some protists. They combine highly specialized and primitive features in their organization. The biosynthetic pathways and molecular machineries are seriously reduced in these parasites, and the genomes are small and compactly organized. Recent metagenomic studies demonstrated a huge "hidden" diversity of early microsporidia and related groups of organisms, while only a limited number of described representatives is known. The isolation and thorough study of new organisms from this group is of special interest. One of the main challenges is to reconstruct the early steps of evolutionary history of this remarkable group of parasites, which was strongly influenced by reductive evolution. Early microsporidia were isolated from a broad range of hosts: from protists (gregarines from marine annelids, free-living amoebae) to Metazoa (tardigrades, insects). The sequenced genomes were rather small and compact. In some lineages we observed the complete or partial reduction of mitochondrial genome, while in others it was not affected by tendency to degeneration. Phylogenomic analysis demonstrated the widespread occurrence of co-infections with two or more hyperparasitic microsporidian species in one superhost. Phylogenomic and comparative genomic analyses suggested that early evolution of microsporidia involved a series of independent losses and acquisitions of genes and molecular machineries in different lineages, probably as a result of adaptations to the various hosts.