Overview of the results, their exploitation and dissemination
Comparative genomic analyses have identified species-specific genes and genes under diversifying selection between closely related, host-specific species of the O. unilateralis complex, and we found that these were enriched in pathogenesis functions, and mostly in heat-labile enterotoxins. These results were reported in a manuscript, which is under revision for the journal Molecular Ecology. Population genomic analyses revealed population subdivision showing that the sibling species of the O. unilateralis complex were highly divergent with very low shared polymorphism. We found population subdivisions within the recognized sibling species, which could be explained by the presence of further cryptic species or phylogeographic patterns. The data of this part is still being analysed with a planned manuscript in 6 months.
Work performed from the beginning of the project
A total of 59 strains of three species of the O. unilateralis complex, specific to three different ant species, were collected across Thailand. They were sequenced with Illumina HiSeq3000 at high coverage. Initially, the raw reads of these samples were mapped back to the unique genome reference of O. polyrhachis-furcata (Wichadakul et al. 2015) for detection of single nucleotide polymorphisms (SNPs). This first set of analyses showed that the different species shared so little polymorphism that the scan of differentiation between species along the genome could not reveal any outlier regions that would allow us to detect genes under divergent selection. Therefore, we undertook first a study of comparative genomics in order to detect species-specific genes and genes with high rates of non-synonymous substitutions among species, likely revealing divergent selection. By comparing the genomes of the three closely related species from Thailand and a more distant taxon from the USA associated to distinct ant host species, we found that species-specific genes were enriched in pathogenesis functions, and mostly in heat-labile enterotoxins. Furthermore these genes were over-represented among those with significant footprints of positive selection; other categories of genes suspected to be important for virulence and pathogenicity in entomopathogenic fungi (e.g. chitinases, lipases, proteases, core genes of secondary metabolites) were much less represented, although a few candidates were found to evolve under positive selection. An analysis including orthologs from a broader phylogenetic context in entomopathogenic fungi showed that positive selection on enterotoxins was specific to the O. unilateralis complex. Together with previous studies showing overexpression of an enterotoxin during behavior manipulation of diseased ants, our findings support the view that heat-labile enterotoxins are important effectors involved in host adaptation and coevolution in Ophiocordyceps entomopathogenic fungi.
The de novo and improved assemblies of different species were then used as references to call SNPs in their respective species. SNPs for each species were used to infer intra-specific population structures and will be used in the next six months to detect selective sweeps.
