| Summary: | Microsporidia are eukaryotes with compacted genomes, due to an obligate intracellular lifestyle. Microsporidia may provide valuable models for understanding the minimum requirements for sexual reproduction; however, their sexual status is debated, with most evidence limited to unreliable morphological inferences. Molecular biological investigations into sexual reproduction are sparse: observations of recombination are attributable to mitotic processes and searches of meiosis-related genes are limited in scope. Current proposals of extant sexuality are based on genes similar to fungal high-mobility-group or homeodomain mating-type determinants. Here, molecular genome data were used to infer the sexual status of microsporidia. PCR, cloning and HAPPY mapping were used to investigate variability at the proposed mating-type loci of Antonospora locustae and Paranosema grylli, followed by orthology comparisons with fungal mating-types. No intraspecies or interspecies variability in either locus was found and no orthological relationship to fungal mating-type proteins, refuting suggestions that they determine mating-types. Thus, inferred relationships to the Mucoromycotina are invalid. Alternative microsporidian mating-type determinants may exist. A novel microsporidian infecting commercial cultures of Gryllus bimaculatus was discovered and characterised. Characters of Microsporidium sp. supported the erection of a genus, sister to Trachipleistophora and Vavraia. Within-species variability in RPB1 sequences coincided with an indel used to define Trachipleistophora and Vavraia, thus their generic diagnosis requires review. Host ranges of Vavraia and Microsporidium sp. suggested that insects are natural reservoirs for human infections by Trachipleistophora. Finally, a robust methodology was implemented to identify meiosis-related gene orthologs in eleven microsporidian genome surveys. Observed distributions of meiosis-specific gene orthologs indicate that sex is an ancestral feature of microsporidia, recently lost in Encephalitozoon intestinalis and E. cuniculi. This supports current evolutionary theory that asexual species arise frequently, but are evolutionarily short-lived. A. locustae and Vavraia culicis appear capable of sexual reproduction and are proposed as model species for future investigations into microsporidian sexual processes.
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