Why the -omic future of Apicomplexa should include gregarines.

Gregarines, a polyphyletic group of apicomplexan parasites infecting mostly non-vertebrates hosts, remains poorly known at taxonomic, phylogenetic and genomic levels. However, it represents an essential group for understanding evolutionary history and adaptive capacities of apicomplexan parasites to the remarkable diversity of their hosts. Because they have a mostly extracellular lifestyle, gregarines have developed other cellular developmental forms and host-parasite interactions, compared with their much better studied apicomplexan cousins, intracellular parasites of vertebrates (Hemosporidia, Coccidia, Cryptosporidia). This review highlights the promises offered by the molecular exploration of gregarines, that have been until now left on the side of the road of the comparative -omic exploration of apicomplexan parasites. Elucidating molecular bases for both their ultrastructural, functional and behavioural similarities and differences, compared with those of the typical apicomplexan models, is expected to provide entirely novel clues on the adaptive capacities developed by Apicomplexa over evolution. A challenge remains to identify which gregarines should be explored in priority, as recent metadata from open and host-associated environments have confirmed how underestimated is our current view on true gregarine biodiversity. It is now time to turn to gregarines to widen the currently highly skewed view we have of adaptive mechanisms developed by Apicomplexa.