POPULATION GENETICS OF CREEK CHUB (SEMOTILUS ATROMACULATUS) IN A POSTGLACIAL, AGRICULTURAL LANDSCAPE

The population genetics of species occupying formerly glaciated regions are not only impacted by glacial retreat but also agricultural land use that is typical of such regions. Areas which have experienced glaciation often display a lowered amount of genetic variability and minimal population structure, and these effects become more predominant with increasing distance from a potential refugial population. Meanwhile, agricultural land use over the recent past has also been demonstrated to disrupt population structure distribution through disturbance regimes. The purpose of this study was to assess potential post-glacial and agricultural effects on populations of creek chub (Semotilus atromaculatus) in two agricultural watersheds that differ in the glacial history. The Saint Joseph River (SJR) watershed, Indiana and Michigan, USA was entirely glaciated during the last glacial maxima, while the Little Miami River (LMR) watershed in Ohio, USA, is situated on the boundary of the glacier. The degree of agricultural land use also varies between and within the two watersheds. Using eight microsatellite loci, 312 individuals were genotyped from 13 sites in SJR and 2,318 individuals from 29 sites in LMR. Measures of genetic differentiation showed that there was strong differentiation between watersheds. Analyses within watersheds recovered additional but weaker differentiation that was mostly associated with the geography of sub-watersheds and isolation by distance. Proximity to the glacial boundary appeared to play a minimal role in genetic differentiation and genetic variation. Differentiation among localities was not directly associated with the glacial boundary within LMR, and localities in this watershed had lower allelic richness and heterozygosity than those in the fully glaciated SJR. After accounting for the positive correlation of stream distance in LMR using partial Mantel test, both glacial history and agricultural land use were positively correlated with genetic differentiation. However, these predictor variables were also strongly correlated with one another which prevented disentangling the two potential effects. Within SJR, no 10 relationship of genetic differentiation with agricultural land use was recovered. My study shows that there is not a simple relationship between glacial history, contemporary land use, and genetic differentiation in creek chub. Rather, it appears that the patterns of genetic variation observed may be more closely linked to the dispersal behavior of creek chub within and among watersheds, and the history of effective population size within watersheds.