Transcriptome analysis of the honey bee fungal pathogen, Ascosphaera apis: implications for host pathogenesis

We present a comprehensive transcriptome analysis of the fungus Ascosphaera apis, aneconomically important pathogen of the Western honey bee (Apis mellifera) that causeschalkbrood disease. Our goals were to further annotate the A.

apis reference genome and toidentify genes that are candidates for being differentially expressed during host infectionversus axenic culture.

Results: We compared A. apis transcriptome sequence from mycelia grown on liquid or solid mediawith that dissected from host-infected tissue.

454 pyrosequencing provided 252 Mb offiltered sequence reads from both culture types that were assembled into 10,087 contigs.Transcript contigs, protein sequences from multiple fungal species, and ab initio genepredictions were included as evidence sources in the Maker gene prediction pipeline,resulting in 6,992 consensus gene models. A phylogeny based on 12 of these protein-coding loci further supported the taxonomic placement of Ascosphaera as sister to the coreOnygenales.

Several common protein domains were less abundant in A. apis compared withrelated ascomycete genomes, particularly cytochrome p450 and protein kinase domains.

Anovel gene family was identified that has expanded in some ascomycete lineages, but notothers. We manually annotated genes with homologs in other fungal genomes that haveknown relevance to fungal virulence and life history.

Functional categories of interestincluded genes involved in mating-type specification, intracellular signal transduction, andstress response. Computational and manual annotations have been made publicly available onthe Bee Pests and Pathogens website.

Conclusions: This comprehensive transcriptome analysis substantially enhances our understanding of theA.

apis genome and its expression during infection of honey bee larvae. It also providesresources for future molecular studies of chalkbrood disease and ultimately improved diseasemanagement.

Published on: 2012-06-29

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