Human impact in naturally patched small populations: genetic structure and conservation of the burrowing rodent, tuco-tuco (Ctenomys lami).

Isolated or semi-isolated small populations are commonly found among species, due to a naturally patchy occupancy of suitable habitats or also as a result of habitat alterations. These populations are subject to an increased risk of local extinction because they are more vulnerable to demographic, genetic, and environmental stochasticity. Considering that natural areas have been becoming progressively more fragmented and smaller, understanding the genetic structure and evolutionary dynamics of small populations is critical. Ctenomys lami has 26 karyotypes distributed in a small area (936 km(2)) continually modified by human actions. We assessed the genetic geographical structure of this species, examining 178 specimens sampled on a fine scale, using information from chromosomal variability, mitochondrial DNA control region and cytochrome c oxidase subunit I sequences, and 14 microsatellite loci. The observed isolation-by-distance pattern and a clinal genetic variation suggest a stepping-stone population model. The results did not indicate genetic structuring associated with distinct karyotypes. However, mitochondrial and nuclear molecular markers demonstrated the existence of 2 demes, which are not completely isolated but are probably reinforced by a geographical barrier. The vulnerability of C. lami is greater than previously supposed, and our data support the designation of one Evolutionary Significant Unit and one Management Unit, and also the inclusion of this species' conservation status as vulnerable.