Human-aided dispersal and population bottlenecks facilitate parasitism escape in the most invasive mosquito species

Abstract

Human-aided invasion of alien species across the world sometimes leads to economic, health or environmental burdens. During invasion process, species encounter new environments and partially escape some ecological constrains they faced in their native range, while they face new ones. The Asian tiger mosquito Aedes albopictus is one of the most iconic invasive species that was introduced in every inhabited continent over a short period of time due to international trade. It has also been shown to be infected by a prevalent and yet disregarded gregarine entomoparasite Ascogregarina taiwanensis. In this study, we aimed at deciphering the global dynamics of As. taiwanensis infection in natural Ae. albopictus populations and we further explored factors shaping its distribution. We showed that Ae. albopictus populations are highly colonized by several As. taiwanesis genotypes but recently introduced ones are escaping the parasite. We further performed experiments to explain such pattern. First, we hypothesized that founder effects (i.e. population establishment by a small number of individuals) may influence the parasite dynamics. This was confirmed since experimental increase in mosquito density improves the parasite horizontal transmission to larvae. Furthermore, Ae. albopictus larvae do not exhibit density dependent prophylaxis to control the parasite meaning that infection is not mitigated when larval density increases. Secondly, we hypothesized that unparasitized mosquitoes were more prompt to found new populations through active flight dispersal. This was, however, unlikely since parasitized mosquitoes tend to be more active than their unparasitized relatives. Finally, we hypothesized that mosquito passive dispersal (i.e. often mediated by human-aided transportation of dried eggs) affects the parasite infectiveness. Our results support this hypothesis since parasite infection decreases over time when dry eggs are stored. This study highlights the importance of global trade on parasitism escape in one of the most invasive vector species on earth.