Microsporidia and Their Closest Relatives: Modern Ideas about the Evolution of Unique Group of Extreme Parasites

The review considers modern ideas about the evolution and phylogeny of the obligate intracellular eukaryotic parasites (microsporidia). In the process of adaptation to intracellular parasitism, the genome and functional apparatus of these organisms underwent a significant reduction and modification. The systematic position of this group remained unclear for a long time, and it was unknown for certain how the evolution of microsporidia occurred, when and how their unique adaptations to a parasitic lifestyle occurred. A true breakthrough in the study of the evolution of microsporidia was made only in the last few years. The groups occupying a position on the tree of life closest to microsporidia were discovered and described: Cryptomycota (Rozellida) and Aphelida. Sequencing of the genomes of representatives of these groups (that also belong to intracellular parasites) and the first genomic data on representatives of basal branches of microsporidia (obtained in 2013–2020) allowed to take a fresh look at the evolution of microsporidia and their closest relatives, to detect intermediate forms along the pathway of adaptation to intracellular parasitism.