A microsporidia possesses robust transcriptional response to heat shock impacting diverse cellular processes despite lack of HSF

The majority of fungal species prefer the 12°–30° C range and relatively few species tolerate temperatures higher than 35° C . Our understanding of the mechanisms underpinning the ability of some species to grow at higher temperatures is incomplete. Nosema ceranae is an obligate intracellular fungal parasite that infects honey bees and can cause individual mortality and contribute to colony collapse. Despite a reduced genome, this species is strikingly thermotolerant, growing optimally at the colony temperature of 35° C. In characterizing the HSR in N. ceranae, we found that this and other microsporidian species have lost the transcriptional regulator HSF and possess a reduced set of putative core HSF1-dependent HSR target genes. Despite these losses, N. ceranae demonstrates robust upregulation of the remaining HSR target genes after heat shock. In addition, thermal stress leads to alterations in genes involved in various metabolic pathways, ribosome biogenesis and translation, and DNA repair. These results provide important insight into the stress responses of microsporidia. Such new understanding will allow new comparisons with other pathogenic fungi and potentially enable discovery of novel treatment strategies for microsporidia infections affecting food production and human health.