Authors

R S Low¹; A P Jackson¹;  ¹ University of Liverpool

Discussion

Blastocystis hominis is an intestinal pathogen of humans associated with Irritable Bowel Syndrome, with the smallest known Stramenopile genome. To understand the evolutionary causes for its size and to examine potential adaptations for pathogenicity, we conducted a comparative analysis of Stramenopile genomes.
In the absence of effective out-groups for comparison, we produced a genome for Proteromonas lacertae, the closest known relative of Blastocystis, and a transcriptome for Cafeteria roenbergensis, a free-living Stramenopile, which allowed us to establish character states in the ancestor of Blastocystis spp.
Our analyses show that B. hominis has lost all flagellum components otherwise conserved in Stramenopiles including dyneins and kinesins. Besides motility, we observed no other losses of major cell function. Rather, we show reduced diversity of genes associated with adhesion (EGF domains), protein interactions (WD domains) and gene regulation (Ankyrin domains).
Overall, evolution of the B. hominis genome has evolved a streamlining of genomic complexity. We analysed the phylodiversity of conserved gene families of diverse functions and found that B. hominis routinely retains fewer ancestral gene lineages than other Stramenopiles. We suggest that this gene loss, combined with an expansion of IG domain-based cell-surface proteins, reflects a history of increasing dependency on the gut mucosa.