No abstract

Delimiting species is very conflicting in the case of very young taxa that are in the process of diversification, and even more difficult if the species inhabit a heterogeneous environment. In this case, even population delimitation is controversial. The South American genus of subterranean rodents Ctenomys is highly speciose, with 62 species that appeared in the lapse of 3 Myr. Within the genus, the 'perrensi' group, formed by three named species and a group of forms of unknown taxonomic status, inhabits the Iberá wetland, in northern Argentina. Almost every locality shows a particular chromosomal complement. To understand the relationships and the evolutionary process among species and populations, we examined mitochondrial DNA sequences and microsatellite genotypes. We found an isolation-by-distance pattern with evidence of cluster-like behaviour of the system. The mitochondrial DNA network revealed two different groups, separated by one of the main rivers of the region. Clustering methods delimited 12 different populations and five metapopulation lineages that seem to be evolving independently. We found evidence of ancient migration among localities at the centre of the distribution but no signals of current migration among the 12 delimited clusters. Some of the genetic clusters found included localities with different chromosomal numbers, which points to the existence of gene flow despite chromosomal variation. The evolutionary future of these five lineages is controlled by the dynamics of their habitat: if stable, they may become distinct species; otherwise, they may collapse into a hybrid swarm, forming a single evolving metapopulation.

The chromosomal speciation hypothesis suggests that irregularities in synapsis, recombination, and segregation in heterozygotes for chromosome rearrangements may restrict gene flow between karyotypically distinct populations and promote speciation. Ctenomys talarum is a South American subterranean rodent inhabiting the coastal regions of Argentina, whose populations polymorphic for Robertsonian and tandem translocations seem to have a very restricted gene flow. To test if chromosomal differences are involved in isolation among its populations, we examined chromosome pairing, recombination, and meiotic silencing of unsynapsed chromatin in male meiosis of simple and complex translocation heterozygotes using immunolocalization of the MLH1 marking mature recombination nodules and phosphorylated histone γH2A.X marking unrepaired double-strand breaks. We observed small asynaptic areas labeled by γH2A.X in pericentromeric regions of the chromosomes involved in the trivalents and quadrivalents. We also observed a decrease of recombination frequency and a distalization of the crossover distribution in the heterozygotes and metacentric homozygotes compared to acrocentric homozygotes. We suggest that the asynapsis of the pericentromeric regions are unlikely to induce germ cell death and decrease fertility of the heterozygotes; however, suppressed recombination in pericentromeric areas of the multivalents may reduce gene flow between chromosomally different populations of the Talas tuco-tuco.

No abstract