Phylogeography of the subterranean rodent Ctenomys torquatus: an evaluation of the riverine barrier hypothesis

AimSubterranean rodents of the genus Ctenomys are widespread in open habitats; the collared tuco-tuco, C.torquatus, has a wide range crossed by large water-courses. Based on a phylogeographical approach to investigate the riverine barrier hypothesis, we evaluated the strength of a river as a barrier and characterized the effect of large rivers in structuring distinct populations of the collared tuco-tuco. LocationGrasslands of southern Brazil. MethodsSampling included 294 individuals from 33 localities throughout the geographical range of C.torquatus, especially around the main rivers. Phylogeographical patterns were estimated through two mitochondrial DNA sequences (control region and COI). Additionally, 22 microsatellite loci were surveyed to estimate the effect of rivers on population divergence through the F-ST index of genetic differentiation and AMOVAs. Spatial autocorrelation analyses were performed for both molecular markers. Molecular rates of change for mitochondrial DNA were estimated using fossil records and applied to Bayesian demographic analysis to test the relationship with the river geological data. ResultsMolecular-clock analysis estimated the time to most recent common ancestor (TMRCA) to be c. 200kyr and indicated a pattern of recent demographic expansion for C.torquatus. Fu's F-S neutrality test was significant and negative (-13.7). High and significant F-ST and AMOVA among-population comparisons (F-SC) revealed highly differentiated populations, although no AMOVA groupings considering rivers as a segregating factor were significant. Spatial autocorrelation analysis revealed that isolation by distance was detected over approximately 200km. Main conclusionsThe effect of the river was not proportionally larger from the headwaters to the mouth, and no population differentiation was observed after the later events of dispersal over the river or the formation of a water channel. This finding could result from the recent history of the occupation of C.torquatus in that region, combined with a dry Pleistocene climate and lower river levels.