We investigated the relationship between satellite copy number and chromosomal evolution in tuco-tucos (genus Ctenomys), a karyotypically diverse clade of rodents. To explore phylogenetic relationships among 23 species and 5 undescribed forms, we sequenced the complete mitochondrial cytochrome b genes of 27 specimens and incorporated 27 previously published sequences. We then used quantitative dot-blot techniques to assess changes in the copy number of the major Ctenomys satellite DNA (satDNA), named RPCS. Our analysis of the relationship between variation in copy number of RPCS and chromosomal changes employed a maximum-likelihood approach to infer the copy number of the satellite RPCS in the ancestors of each clade. We found that amplifications and deletions of RPCS were associated with extensive chromosomal rearrangements even among closely related species. In contrast, RPCS copy number stability was observed within clades characterized by chromosomal stability. This example reinforces the suspected role of amplification, deletion, and intragenomic movement of satDNA in promoting extensive chromosomal evolution.

Recurrent Amplifications and Deletions of Satellite DNA Accompanied Chromosomal Diversification in South American Tuco-tucos (Genus Ctenomys, Rodentia: Octodontidae): A Phylogenetic Approach

Two species of tuco-tucos, Ctenomys talarum and C. australis, overlap geographically in south-eastern Buenos Aires Province, Argentina. C. talarum (100-180 g) occupies habitats with clayey and harder soils, while the larger C. australis (300-600 g) occupies sandy and quite friable soils. This study compares the digging apparatus and digging performance of both species, and also provides additional ecological data to describe the adaptations of both species. When confronted with sandy, friable soils, both tuco-tucos exclusively use their forelimbs (‘scratch digging’ mode) to break the soil. When confronted with harder and clayey soils, C. australis used its forelimbs and, less frequently, its incisors, but no appreciable effectiveness was observed in breaking the soil. On the other hand, C. talarum behaved as a ‘scratch’ and ‘chisel-tooth digger’ species, using both forelimbs and incisors to dig tunnels in extremely hard soils. C. talarum showed relatively larger forelimb extensors than C. australis. Moreover, I hypothesize that the structure of the ulna in C. talarum may increase the effective forces of the digital and carpal flexors, thus accounting for the enhanced digging ability in this species. Outward forces exerted by the distal elements (claws-digits-carpals) when forelimbs impact the soil appear to be a critical factor in loosening soil fragments. In addition, the increased upper incisor procumbency of C. talarum correlated with an effective chisel-tooth digging behaviour. Finally, I briefly examine phylogenetic and biogeographical factors that may account for some C. australis morphological and physiological traits. It is suggested that the digging ability of C. australis could be a by-product of its larger size, a likely adaptation to evade predation.

Functional morphology, comparative behaviour, and adaptation in two sympatric subterranean rodents genus Ctenomys (Caviomorpha: Octodontidae)

We present the most comprehensive study to date of species groups in Ctenomys (tuco-tucos), a species-rich genus of Neotropical rodents. To explore phylogenetic relationships among 38 species and 12 undescribed forms we sequenced the complete mitochondrial cytochrome-b genes of 34 specimens and incorporated 50 previously published sequences. Parsimony, likelihood, and Bayesian phylogenetic analyses were performed using additional hystricognath rodents as outgroup taxa. The basal dichotomy of Ctenomys splits C. sociabilis from the remaining tuco-tucos, within which 8 main species groups were identified: boliviensis, frater, mendocinus, opimus, magellanicus, talarum, torquatus, and tucumanus. Whereas most of these groups refer to previous clades proposed on the basis of chromosomes or morphology, the torquatus and magellanicus species groups are novel taxonomic hypotheses. However, relationships among species groups are poorly resolved. Furthmore, the positions of C. leucodon, C. maulinus, and C. t...

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Species groups and the evolutionary diversification of tuco-tucos, genus Ctenomys (Rodentia: Ctenomyidae)

Molecular systematics of the South American caviomorph rodents: relationships among species and genera in the family Octodontidae.

The chromosomes of subterranean rodents of the South American genus Clenomys are highly variable with diploid numbers ranging from 10 to 70 The Phylogenetic relationships of this group have been analysed cladistically using G-banded karyorypes as have the chromosomal rearrangements involved in its karyotypic differentiation One group, called the 'Corrientes group', has very variable chromosomes but low allozymic and morphological differentiation among its members This group has been analysed with respect to chromosomal speciation Using a member of another subfamily ( Octodonlomys gliroides ) as an outgroup, the results indicate that karyotypes with low diploid and fundamental numbers are plesiomorphic The range of diploid numbers studied here is between 22 and 70, while the fundamental numbers are between 40 and 86 It was found that the main chromosomal rearrangement that transforms karyotypes towards higher diploid and fundamental numbers is the acquisition of new chromosomal material via unknown mechanisms, followed by pericentric inversions that generate new chromosomal arms, centric fusions and centric fissions In spite of their low differentiation regarding allozymic and morphological features, it was found that the karyomorphs of the Corrientes group have enough chromosomal differentiation to consider them as distinct species Beside the range of diploid and fundamental numbers of this group (42–70 and 80–84 respectively), their pairwise chromosomal differences are high The most closely related of them differ in one nonhomologous arm, one Robertsonian change and a whole chromosome duplication The most differentiated taxa differ in 20 arms with lack of homology, 12 Robertsonian changes (one with monobrachial homology), six pericentric inversions and the above mentioned probable arm duplication For these reasons, it is probable that some kind of chromosomal speciation has occurred in the Corrientes group

Phylogenetic analysis of G-banded karyotypes among the South American subterranean rodents of the genus Ctenomys (Caviomorpha: Octodontidae), with special reference to chromosomal evolution and speciation

Cytogenetic analysis was performed in six nominal taxa of the genus Ctenomys with a diploid number of 2n=47/48. The studied species were: C. australis (2n=48, FN=76); C. mendocinus (2n=47/48, FN=68/75/76); C. porteousi (2n=47/48, FN=71/72/73); C. azarae (2n=47, FN=71); C. sp. (‘chasiquensis’) (2n=47/48); and C. talarum (2n=48, FN=80). The first three species shared the whole complement, C. talarum shares with them 19 arms from a total of 43 (44%). In all species analyzed constitutive heterochromatin was detected in most short arms, and in several centromeres. Polymorphisms for several pairs involving the heterochromatic short arms together with a complex polymorphism of pair A1 were found in C. azarae, C. sp., C. mendocinus and C. porteousi. Intraindividual variation found in one specimen of C. porteousi involving heterochromatic arms is discussed. These results lead us to propose the inclusion of all species except C. talarum, within a complex called the mendocinus-group. A new case of conservatism for chromosomal number in the genus Ctenomys is found in this group.

Chromosomal polymorphism and small karyotypic differentiation in a group of Ctenomys species from Central Argentina (Rodentia: Octodontidae)

Les caryotypes de 4 especes de Ctenomys: C. bergi, C. fodax, C. knightii, C. pundti, ainsi que celui d'une espece probablement nouvelle de Patagonie, sont decrits. Des corrections sont faites aux caryotypes publies de C. latro et C. occultus. Les caryotypes de C. opimus luteolus, C. tucumanus et C. sociabilis sont confirmes par du materiel nouveau ou de nouvelles localites. Des variants caryotypiques sont aussi mis en evidence chez C. rionegrensis et C. talarum. Les caryotypes en bandes C de 20 especes et 26 populations sont decrits et illustres

M. A. Barros

Papers

Chromosomal polymorphism and small karyotypic differentiation in a group of Ctenomys species from Central Argentina (Rodentia: Octodontidae)

New karyotypes and C-banding patterns of the subterranean rodents of the genus Ctenomys (Caviomorpha, Octodontidae) from Argentina