Chromosome plasticity in Ctenomys (Rodentia Octodontidae): chromosome 1 evolution and heterochromatin variation

Investigating the evolutionary dynamics of diploid number variation in Ctenomys (Ctenomyidae, Rodentia)

Contrary to predictions from classical hybrid sterility models of chromosomal speciation, some organisms display high rates of karyotype variation. Ctenomys are the current mammals with the greatest interspecific and intraspecific chromosomal variation. A large number of species have been studied cytogenetically. The diploid numbers of chromosomes range from 2n = 10 to 2n = 70. Here, we analyzed karyotype evolution in Ctenomys using comparative phylogenetic methods. We found a strong phylogenetic signal with chromosome number. This refutes the chromosomal megaevolution model, which proposes the independent accumulation of multiple chromosomal rearrangements in each closely related species. We found that Brownian motion (BM) described the observed characteristic changes more thoroughly than the Ornstein-Uhlenbeck and Early-Burst models. This suggests that the evolution of chromosome numbers occurs by a random walk along phylogenetic clades. However, our data indicate that the BM model alone does not fully characterize the chromosomal evolution of Ctenomys.

Integrative lineage delimitation in rodents of the Ctenomys Corrientes group

The tuco-tucos rodents (genus Ctenomys) of the Corrientes group comprise several populations that inhabit the vast area under the influence of the Iberá wetland. Lineage delimitation within the recently diverged Corrientes group is a challenging task as morphological differentiation is not conspicuous between populations. However, delimitation is crucial for evolutionary studies and conservation issues. In this study, we performed a phylogenetic analysis including cytochrome b (cyt-b) sequences from taxa that had never been studied in a comprehensive context. We integrated previously published chromosomal studies, mitochondrial phylogenies and simple sequence repeat (SSR) variability analyses, and applied a delimitation criterion over the basis of chromosomal incompatibilities and genetic exclusivity. Under this integrative approach seven independently evolving lineages were delimited in the Corrientes group: Ctenomys roigi, which conserves its former definition, Ctenomys dorbignyi and Ctenomys perrensi complex which were redefined, Sarandicito which includes the population of Paraje Sarandicito and probably a group of nearby poorly studied populations, and Iberá i, ii and iii distributed at both sides of the Iberá wetland. We discuss future perspectives to evaluate the proposed lineages and conservation issues concerning these tuco-tucos.

Chromosomal polymorphisms in African vlei rats, Otomys irroratus (Muridae: Otomyini), detected by banding techniques and chromosome painting: inversions, centromeric shifts and diploid number variation

Pericentric inversions are important for evolutionary biology because of their potential role in speciation. They may result in reproductive isolation due to illegitimate pairing of homologues at meiosis which leads to the production of aneuploid gametes (containing deletions or duplications of chromosomal segments), and consequently mediate chromosomal divergence. In this study, we describe the prevalence of pericentric inversions in the African vlei rat, Otomys irroratus (OIR). The species is characterized by intraspecific chromosomal variation (2n = 23-32) across its distribution in southern Africa. Here, we analyzed 55 individuals collected from 7 localities in South Africa by G- and C-banding and chromosome painting with flow sorts of Myotomys unisulcatus. Of the 55 specimens that were analyzed, 47% contained inversions or centromeric shifts on 4 autosomes (OIR1, 4, 6 and 10) which were present singly in specimens (i.e. none of the specimens contained all 4 inversions concurrently). These inversions were found in both homozygous and heterozygous state over a wide geographic range suggesting that they are floating polymorphisms. Given the potential role of inversions in post-mating isolation (through production of aneuploid gametes), the prevalence of inversions as floating polymorphisms in the vlei rats suggests that they are probably retained in the population through suppression of recombination in the inverted regions of the chromosomes.

Karyotypic and molecular polymorphisms in Ctenomys torquatus (Rodentia: Ctenomyidae): taxonomic considerations

The rodent genus Ctenomys (tuco-tucos) comprises more than 60 described species, and shows extraordinary inter- and intraspecific karyotypic variation. The most widely distributed species of Ctenomys in Brazil is C. torquatus. Although several cytogenetic studies have been done, the karyotypic variability of this species is still poorly known. In this paper we report two new diploid numbers for C. torquatus: 2n = 40 and 2n = 42, both showing AN = 72. The new distribution limits of C. torquatus here reported include localities in the southern, central and western parts of Rio Grande do Sul (RS) State in southern Brazil. The phylogenetic relationship between C. torquatus from Alegrete, RS, and Ctenomys sp. from Corrientes, Argentina, is described by means of mtDNA cytochrome b analysis. Although both entities share similar karyotypes and sperm morphology, these two species are not phylogenetically close.