Potential role of parasitism in the evolution of mutualism in astigmatid mites: Hemisarcoptes cooremani as a model

Phoresy is a symbiotic interaction that results in dispersal, benefiting th e relocated organism without negatively impacting the phoretic host. It has long been considered that phoresy among astigmatid mites is somehow an int ermediate precursor to the evolution of parasitism within the group. In ast igmatid mites, only the heteromorphic deutonymph (hypopode) participates in phoretic dispersal, and the plesiomorphic hypopode may be the key to under standing the dynamics of the evolution of that parasitism. Hypopodes of Hem isarcoptes cooremani (Acari: Acariformes) and their phoretic beetle host Ch ilocorus cacti (Coleoptera: Coccinellidae) have become the experimental foc us for studies concerned with the potential forces that influence the trans ition of a free-living life style into various coevolved relationships. Pre vious radiolabeling studies applied to H. cooremani and C. cacti determined that hypopodes of H. cooremani acquired resources from adults of C. cacti while in transit, negating the paradigm that the heteromorphy was purely ph oretic. To further probe this relationship, we tested whether materials cou ld be passed from the mites to their hosts. We report here a study using a tritium radiolabel, which indicated that beetles also acquire resources fro m the hypopodes. These results have implications for understanding the comp lex relationship between H. cooremani and C. cacti. We propose that this re lationship should now correctly be defined as mutualistic (not phoretic) an d develop a general model for the potential role of parasitism in the evolu tion of mutualism among the Astigmata.