Abstracts (first author)
Matryoshka hybrid zones: the pinworm Syphacia obvelata in the European house mouse hybrid zone
Hybrid zone studies of host-parasite coevolution have largely taken a host-centric viewpoint: does parasitism affect the outcome of host hybridization by differentially impacting the fitness of host taxa and their hybrid descendants? This viewpoint neglects the possibility that host taxa have distinctive genetic clusters of parasites that may themselves potentially hybridize. Hybridization, bringing together combinations of genes previously untested by natural selection, may affect parasites in similar ways to free-living organisms: revealing a barrier to gene flow, promoting divergence via reinforcement, homogenizing genetic clusters or leading to rapid adaptive diversification via the formation of hybrid parasite species. We use the European house mouse hybrid zone (HMHZ) between Mus m. musculus and M. m. domesticus and their pinworms to explore these issues. First, from a host-centric viewpoint, we tested whether hybrids have greater or lesser load of pinworms than additive expectations. Sampling of 689 mice from 107 localities typed for 1401 diagnostic SNPs across the Bavaria-Bohemia region of the HMHZ we found hybrids have significantly reduced pinworm load, contradicting the idea that pinworms reduce host hybrid fitness through increased load. Second, from a parasite-centric viewpoint, we analyzed the genetic structure of Syphacia obvelata across the same part of the HMHZ by typing 267 female worms from mice at 87 HZ localities and 18 allopatric localities for 10 microsatellite markers and the mtCOX1 gene. We found distinct genetic clusters of S. obvelata meeting at the host HZ, with hybrid pinworm genotypes at the centre. Our results are consistent with independent host-parasite arms races running during isolation of the host taxa: any accumulated co-adaptation will breakdown on hybridisation of host and of parasite. The low parasite loads observed in the HMHZ centre may be due to “immune vigor” of hybrid hosts and/or low fitness of their hybrid parasites.