Near chromosome-level genome assembly of the microsporidium Hamiltosporidium tvaerminnensis

Microsporidia are intracellular parasitic fungi whose genomes rank among the smallest of all known eukaryotes. A number of outstanding questions remain concerning the evolution of their large-scale variation in genome architecture, responsible for genome size variation of more than an order of magnitude. This genome report presents a first near-chromosomal assembly of a large-genome microsporidium, Hamiltosporidium tvaerminnensis. Combined Oxford Nanopore, Pacific Biosciences, and Illumina sequencing led to a genome assembly of 17 contigs, 11 of which represent complete chromosomes. Our assembly is 21.64 Mb in length, has an N50 of 1.44 Mb, and consists of 39.56 % interspersed repeats. We introduce a novel approach in microsporidia, PacBio Iso-Seq, as part of a larger annotation pipeline for obtaining high-quality annotations of 3,573 protein-coding genes. Based on direct evidence from the full-length Iso-Seq transcripts, we present evidence for alternative polyadenylation and variation in splicing efficiency, which are potential regulation mechanisms for gene expression in microsporidia. The generated high-quality genome assembly is a necessary resource for comparative genomics that will help elucidate the evolution of genome architecture in response to intracellular parasitism. SignificanceMicrosporidia are a model for genome evolution in response to intracellular parasitism, but we lack high-quality resources from species with large genomes. We present a near complete assembly of a large-genome microsporidium, Hamiltosporidium tvaerminnensis, and obtain high-quality gene annotations through full-length transcripts using Iso-Seq, a novel approach in microsporidia. Our study provides insights into gene regulation and paves the way for comparative genomic analyses aiming to understand the evolution of genome reduction and expansion in these intracellular parasites.