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Open Access Publications from the University of California

The genome of the xerotolerant mold Wallemia sebi reveals adaptations to osmotic stress and suggests cryptic sexual reproduction


Wallemia (Wallemiales, Wallemiomycetes) is a genus of xerophilic Fungi of uncertain phylogenetic position within Basidiomycota. Most commonly found as food contaminants, species of Wallemia have alsobeen isolated from hypersaline environments. The ability to tolerate environments with reduced wateractivity is rare in Basidiomycota. We sequenced the genome of W. sebi in order to understand its adaptations for surviving in osmotically challenging environments, and we performed phylogenomic andultrastructural analyses to address its systematic placement and reproductive biology. W. sebi has a compact genome (9.8 Mb), with few repeats and the largest fraction of genes with functional domains compared with other Basidiomycota. We applied several approaches to searching for osmotic stress-relatedproteins. In silico analyses identied 93 putative osmotic stress proteins; homology searches showedthe HOG (High Osmolarity Glycerol) pathway to be mostly conserved. Despite the seemingly reducedgenome, several gene family expansions and a high number of transporters (549) were found that alsoprovide clues to the ability of W. sebito colonize harsh environments. Phylogenetic analyses of a 71-protein dataset support the position of Wallemia as the earliest diverging lineage of Agaricomycotina, whichis conrmed by septal pore ultrastructure that shows the septal pore apparatus as a variant of the Tremella-type. Mating type gene homologs were idented although we found no evidence of meiosis duringconidiogenesis, suggesting there may be aspects of the life cycle of W. sebi that remain cryptic

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