Construction of a reference genome for Starmerella batistae and annotation of Starmerella species reveal an unexpected evolutionary relationship with Schizosaccharomyces pombe and suggest an alternative enzymatic route for sophorolipid production

The Starmerella clade is known for displaying osmotolerant and acidophilic traits from their association with bees. Several species in this genus can produce sophorolipids, which are commercially produced as biosurfactants. Here, we isolated a yeast contaminant from the laboratory environment, identified as Starmerella batistae, able to thrive at low pH and relative high temperatures. We sequenced and conducted a de novo genome assembly in three chromosomes and a mitochondrial genome for S. batistae (ca. 9.3 Mb). Based on this reference genome we functionally annotated 29 Starmerella species, using the publicly available sequences. Phylogenetic analysis across different yeast clades revealed a close relationship between Starmerella and Schizosaccharomyces yeasts. Fifteen genes were uniquely shared between Sz. pombe and S. batistae, of which twelve were involved in cell morphology, reflecting the fact that S. batistae cells are elongated rather than round. We found that all the Starmerella sophorolipid-producing strains shared a close common ancestor. One-to-one orthologs of S. bombicola sophorolipid pathway were only found in S. kuoi (full pathway, but inverted), and in S. powellii and S. floricola (partial pathway). These findings support the notion that alternative pathways for the production of sophorolipids have evolved in different Starmerella lineages.