"Parasite turnover zone" at secondary contact: A new pattern in host-parasite population genetics.

We describe here a new pattern of population genetic structure in a host-parasite system that can arise after secondary contact of previously isolated populations. Due to different generation times, and therefore different tempos of molecular evolution, the host and parasite populations reach different degrees of genetic differentiation during their separation (e.g., in refugia). Consequently, upon secondary contact, the host populations are able to re-establish a single panmictic population across the area of contact, while the parasite populations stop their dispersal at the secondary contact zone and create a narrow hybrid zone. From the host's perspective, the parasite's hybrid zone functions on a microevolutionary scale as a "parasite turnover zone": while the hosts are passing from area A to area B, their parasites turn genetically from the area A genotypes to the area B genotypes. We demonstrate this novel pattern with a model composed of Apodemus mice and Polyplax lice by comparing maternally inherited markers (complete mitochondrial genomes, and complete genomes of the vertically transmitted symbiont Legionella polyplacis) with single nucleotide polymorphisms derived from louse genomic data. We discuss the circumstances that may lead to this pattern and possible reasons why it has been overlooked in studies of host-parasite population genetics.