Pelodera: cosmopolitan phoretic saprotrophs and neglected models for origins of nematode parasitism.

Pelodera (A. Schneider, 1866) is a Clade V nematode genus of the Rhabditidae family and close relative of Caenorhabditis elegans. The genus comprises 26 species poised between free-living, commensal, and parasitic lifestyles, as well as species with underappreciated roles in soil nutrient cycling. Pelodera strongyloides, the type species, is a cause of follicular larva migrans among humans and other mammals but can be confused with hookworm folliculitis. With minimal genetic resources available, the capacity to identify and diagnose infectious species and strains of Pelodera are limited. Thus, the ecology of Pelodera species as both parasites and commensalists is likely underreported and possibly mischaracterised. Similarly, decomposition biome studies report Pelodera and other Rhabditidae as dominant taxa during decay but require greater genetic resources to classify species. To renew appreciation for these neglected model organisms, this review collates available literature to detail Pelodera associations across mammalian and invertebrate hosts and discusses traits that drive host association such as dauer formation, waving (nictation), phoresy, and tissue invasion. The potential for Pelodera as a satellite model to C. elegans is also discussed since Pelodera are readily culturable while having inducible parasitic forms. Research into Pelodera will not only improve understanding of their ecology and contribution to decomposition but may prove invaluable in identification of behaviours underpinning parasitism and parasite origins. Utilising Pelodera facultative parasites may be essential to triangulate and resolve differences between C. elegans and obligate nematode parasites with similar niches such as hookworms, lungworms, Strongyloides, and gastrointestinal nematodes of livestock.