On the occurrence of the neogregarine Apicystis bombi (Apicomplexa) in South America: an unassembled puzzle

Worldwide declines in critical hymenopteran pollinators, such as bumble bees and honey bees, in recent decades have sparked a surge in research aimed at identifying the factors behind these declines. Among the suspected contributors, infectious diseases have garnered significant attention. In this context, we conduct a comprehensive review of the potential impact of naturalized Palearctic exotic bumble bee species, specifically Bombus terrestris and Bombus ruderatus, on the occurrence of Apicystis bombi in southern South America. Apicystis bombi is a unique apicomplexan protist that belongs to the neogregarines, which as a group exhibit relatively high virulence towards a range of insects. Our review encompasses the available information on its morphology, molecular characteristics, pathology, host range, and geographic distribution. Additionally, we explore three plausible hypotheses, each not necessarily exclusive of the others, in an effort to shed light on the occurrence of A. bombi in South America. These hypotheses include the two classically accepted, but less supported in view of current data (entry via B. terrestris, entry via B. ruderatus), and one postulated for the first time here with arguably better support (pre-existing presence before the introduction of B. terrestris and B. ruderatus, plus the possibility of multiple Apicystis species coexisting). We aim for this review to stimulate interest in this relatively obscure parasitic microbe that affects crucial insect pollinators.