Using molecular markers to assess species distribution, contact zones, and hybridization in Oklahoma pocket gophers (geomys).

Abstract

Speciation is the evolutionary process which leads to the formation of new and distinct species. Understanding the mechanisms involved in the speciation process is imperative for understanding evolutionary biology and species diversity. A promising approach to understanding the mechanisms involved in speciation is the study of hybrid zones where genetic exchange between distinct species can produce mixed or recombinant genotypes. Analysis of hybrid zones can provide insight into the processes that isolate groups from each other. Cryptic species are those that are difficult to identify based on morphological characteristics, but are reproductively or genetically distinct. A less restrictive, but similar term, is species complex. A species complex contains two or more closely related species with similar morphologies that have species boundaries that are difficult to define. Both of these terms can be applied to the genus Geomys (pocket gophers). Because of the morphological similarities between species in this genus, molecular analyses often are necessary for species identification. Currently, two species of Geomys are known to occur in Oklahoma. Geomys bursarius (plains pocket gopher) occurs in western Oklahoma, whereas G. breviceps (Baird's pocket gopher) occurs in eastern Oklahoma. There has been limited molecular research done on Oklahoma Geomys, which has led to a lack of understanding of species boundaries throughout the state. Hybridization between G. bursarius and G. breviceps has been reported to occur in Oklahoma where the two species come into contact, which can further complicate species identification. Additionally, the identification of pocket gophers in the Oklahoma panhandle is in question. The panhandle populations are presumed to belong to G. bursarius, but it has been hypothesized that a third species, G. jugossicularis (Hall's pocket gopher), may reside there as well. The goals of my research were to utilize molecular markers to 1) determine if G. jugossicularis occurs in the Oklahoma panhandle, 2) evaluate the boundary line between G. bursarius and G. breviceps in central Oklahoma, and 3) determine the location of contact zones in the state and assess whether hybrid individuals occur within these zones. The complete mitochondrial cytochrome-b gene was used to aid in species identification. Microsatellite markers were used to identify unique genetic clusters in Oklahoma, address the possibility of hybridization in Oklahoma, as well as evaluate genetic diversity within and between species. Geomys jugossicularis was identified in the Oklahoma panhandle (Beaver and Cimarron Counties) as well as in the Texas panhandle in Dallam and Hartley Counties. Four admixed individuals between G. jugossicularis and G. bursarius were identified in Beaver and Cimarron Counties. The boundary line between these two species most likely meanders in and out of the Oklahoma and Texas panhandles, although additional research is needed to determine the extent and location of the contact zone between these two species in this region. The boundary line between G. bursarius and G. breviceps was found in central Oklahoma and is similar to that proposed based on morphological data. It is likely that multiple contact zones exist along the boundary. Contact zones were evaluated in Tulsa and Cleveland Counties and it was determined that contact zones may be wider than originally suggested. Four admixed individuals were identified in Seminole, Logan, and Marshall Counties. Based on findings from this project and previous research, the tension zone model is likely the best fit model to describe the maintenance of hybrid zones in central Oklahoma. The overall goals of this research were to gain a better understanding of species boundaries, including the locations of contact zones; hybridization; and genetic diversity within and among Oklahoma pocket gophers. My research allowed an opportunity to advance the growing knowledge of cryptic species and the occurrence of hybridization between species. Most importantly, my project has contributed to a broader understanding of Geomys in Oklahoma, which will allow for future research opportunities. Future research will focus on characterizing individuals using the whole genome approach as well as studying hybrid zones in more detail. With use of the whole genome approach, future research will be able to determine which regions of the genome are under selection in each hybrid zone. By determining common characteristics among zones, there will be a better understanding of the mechanisms necessary for speciation.