Polyploidy is widespread in Microsporidia

Abstract Microsporidia are obligate intracellular eukaryotic parasites with an extremely broad host range. They have both economic and public health importance. Ploidy in microsporidia is variable, with a few species formally identified as diploid and one as polyploid. Given the increase in the number of studies sequencing microsporidian genomes, it is now possible to assess ploidy levels across all currently-explored microsporidian diversity. We estimate ploidy for all microsporidian datasets available on the Sequence Read Archive (SRA) using k-mer-based analyses, and demonstrate that polyploidy is widespread in Microsporidia, and that ploidy change is dynamic in the group. Using genome-wide heterozygosity estimates, we also show that polyploid microsporidian genomes are relatively homozygous, and we discuss the implications of these findings on the timing of polyploidisation events and their origin. Author Summary Microsporidia are single-celled intracellular parasites, distantly related to fungi, that can infect a broad range of hosts, from humans all the way to protozoans. Exploiting the wealth of microsporidian genomic data available, we use k-mer based analyses to assess ploidy status across the group. Understanding a genome’s ploidy is crucial in order to assemble it effectively, and may also be relevant for better understanding a parasite’s behaviour and life cycle. We show that tetraploidy is present in at least six species in Microsporidia, and that these polyploidisation events are likely to have occurred independently. We discuss why these findings may be paradoxical, given that Microsporidia, like other intracellular parasites, have extremely small, reduced genomes.