Population genetics of Thomomydoecus minor: Ectoparasitic chewing lice on Thomomys bottae pocket gophers of the Rio Grande Valley in New Mexico

Thomomydoecus minor is an ectoparasitic chewing louse that lives in intimate association with the pocket gopher Thomomys bottae. Chewing lice are wingless, obligate, host specific parasites that spend their entire life cycle on the fur of their host. Pocket gophers are fossorial and asocial, cut off for most of their lives even from members of their own species. Thus, the life histories of both chewing lice and pocket gophers have been predicted to limit transmission of lice from host to host, thus limiting gene flow among louse infrapopulations found on different hosts and increasing the effect of louse population bottlenecks that occur when lice colonize new host individuals. The geographic location of special interest in this study was a section of the Rio Grande Valley in New Mexico called the San Acacia constriction. This is a zone of secondary contact where two subspecies of pocket gophers meet and hybridize to a limited extent. Restricted hybridization between these hosts was predicted to influence genetic structure of the corresponding louse populations. In the present study, genomic DNA was isolated from 118 chewing lice collected in 2011 from 3 localities surrounding the San Acacia constriction and from 39 samples of lice collected in 1992 from one of the same localities. A portion of the mitochondrial cytochrome-c oxidase subunit I (COI) gene was sequenced and used to construct a phylogenetic tree, which indicated two distinct haplotypes, with one of these occurring north of the host hybrid zone and the other occurring south of it. These two haplotypes likely diverged 78,000-200,000 years ago. Haplotype distribution coincides with the geographic break in suitable pocket gopher habitat imposed by the San Acacia constriction, indicating that either geography or limits on host hybridization in this region cause an impediment to gene flow between northern and southern chewing louse populations. Eight novel microsatellite loci developed for this study revealed greater levels of genetic variation than were available in previous studies of chewing louse populations, which relied on allozymes. Like the mtDNA data, microsatellite data supported a distinct separation between northern and southern louse populations coincident with geography. Furthermore, distinct infrapopulations on different host individuals were detected in microsatellite genetic distance measures and AMOVA analyses, thus supporting previous predictions of louse population subdivision resulting from a life history whereby host pocket gophers serve as isolated islands of habitat for chewing lice with horizontal transmission of lice between unrelated hosts being relatively rare. Contrary to previous predictions, louse populations appeared to be in HardyWeinberg Equilibrium and showed little or no evidence of population bottlenecks or inbreeding. Despite a life history that has been thought to impose frequent bottlenecks on chewing louse populations, relatively stable genetic diversity was maintained over a 19.5year, 175-generation time span between collection dates at the same sampling locality. POPULATION GENETICS OF THOMOMYDOECUS MINOR; ECTOPARASITIC CHEWING LICE ON THOMOMYS BOTTAE POCKET GOPHERS OF THE RIO GRANDE VALLEY IN NEW MEXICO