Molecular Relationships Among Octodontidae (Mammalia: Rodentia: Caviomorpha)

We examined ten species of octodontid rodents and representatives of three outgroups in a complete 13 × 13 DNA-hybridization matrix. The results were indexed as differences in median melting-point depressions (T,_ms), symmetrized, subjected to phylogenetic analysis using FITCH, bootstrapped, and exhaustively taxon-jackknifed. Within Octodontidae, four clades were recovered with 100% bootstrap and complete jackknife support: Tympanoctomys barrerae with Octomys mimax, Octodontomys gliroides alone, Octodon spp., and Aconaemys spp. with Spalacopus cyanus; the latter two clades were closer to each other than either was to Octodontomys or Octomys-Tympanoctomys, but were slightly nearer to and united with Octodontomys, with 89% bootstrap support. However, relationships among the three Aconaemys species and Spalacopus were not completely resolved by our experiments. Ctenomys coyhaiquensis, Abrocoma bennetti, and Lagostomus maximus represented successive outgroups to Octodontidae, while one-way comparisons with Cavia porcellus and Microcavia australis suggested that these caviids are almost as distant from octodontoids as is the chinchilloid Lagostomus. When the data were suitably corrected for percentage hybridization and saturation, division of the distances by the rate previously determined for most amniotes (0.48%/myr) suggested that the basal divergence among the caviomorph rodents examined occurred about 59 myrbp, and that Octodontidae originated 40 myrbp and diversified into extant lineages beginning 14 myr ago. Calibration against the date of the earliest known caviomorph (late Eocene or about 37.5 myrbp) gave a rate of 0.75%/myr, which would suggest later dates for subsequent caviomorph cladogenesis. It is notable that, based respectively on the slower or faster rates, the tetraploid Tympanoctomys barrerae must have diverged from its sister-taxon Octomys mimax 10 or 6.5 myr ago.     

Juvenile vocalizations ofCtenomys talarum (Rodentia: Octodontidae)

We analysed the physical structure and functional interpretation of juvenile vocalizations ofCtenomys talarum Thomas, 1898. Two sounds, one true vocalization (care-elicitation call) and one mechanical sound (nursing sound) were recorded during the nestling period. Care-elicitation calls were emitted by isolated pups and caused the mother to respond by immediately approaching the pups to take care of them. This maternal response to the care-elicitation calls ofC. talarum pups, together with the production of these vocalizations during the first weeks after birth, when pups depend exclusively on their mother to obtain food and maintain their body temperature, give preliminary support for the recent theory that care-elicitation cries are honest advertisements of offspring need.     

Phylogenetic position ofAbalosia and the evolution of the extant Octodontinae (Rodentia, Caviomorpha, Octodontidae)

South American octodontid rodents of the subfamily Octodontinae currently show low species richness but great morphological and chromosomal diversity. This diversity is interpreted alternatively as the remnant of a wider past radiation or as the result of saltational evolution. These hypotheses are discussed in relation to a phylogenetic analysis of the Late Pliocene octodontineAbalosia castellanosi. My results suggest thatAbalosia, together withTympanoctomys andOctomys, is part of a clade of desert specialist propalinal octodontids, which would have differentiated east of the Andes in the emergent semi-deserts of western Argentina. The presence ofAbalosia in the coastal region of central Argentina during the Upper Marplatan Age (Late Pliocene) suggests a pulse of expansion of such arid environments, probably coeval with the global climatic deterioration detected around the transition Gauss-Matuyama magnetic ages. The phylogenetic position ofA. castellanosi suggests that extinction events affected the clade of the octodontine desert specialists. Accordingly, regardless of how rough or gradual the differentiation of the octodontine’s diversity has been, the living representatives seem to be remnants of a wider radiation.     

Sequence evolution of the major satellite DNA of the genus Ctenomys (Octodontidae, Rodentia)

Sequence variability of RPCS (repetitive PuvII Ctenomys sequence), the major satellite DNA of octodontid Ctenomys rodents, was analysed in species belonging to three groups of species representing the two patterns of karyotypic evolution in the genus: stable and dynamic karyotypes among closely related species. The studied species represent the overall range of RPCS copy number (2000--6.6x10(6) copies per haploid genome) in the genus. RPCS sequence was characterised by PCR amplification of the genomic consensus sequence and cloned monomers. Our results suggest that RPCS genomic consensus sequence variability correlates with RPCS copy number stability and karyotypic stastis, but not with high or low RPCS copy number values. In contrast, the RPCS gcs shows a mutational profile that is similar across all analysed species. Our data suggest that an RPCS ancestral library of variants was maintained through the cladogenesis of the genus. There is also evidence pointing to the simultaneous contribution of processes of concerted evolution that resulted in a reduced representation of some ancestral variants and their partial replacement for new ones. In addition, analysis of distribution of the variability along the monomer suggests that subsequences of the RPCS are subject to some degree of constraint, probably driven by the recent replicative activity of RPCS in species with high copy number.     

Breeding biology of the fossorial rodent Ctenomys talarum (Rodentia: Octodontidae)

The paper describes the breeding biology of the South American herbivorous fossorial rodent Ctenomys talarum (tuco-tuco), covering the following aspects: length of breeding season, prenatal mortality, mean litter size and its correlates, sex ratio and its density-dependence, and late sexual maturity of both sexes. These parameters are compared and contrasted with those of other conspecifics and other subterranean rodents.     

Dental evolution in Neophanomys (Rodentia, Octodontidae) from the late Miocene of central Argentina

The evolutionary pattern of the molar morphology of the small caviomorph (Octodontidae) Neophanomys from the late Miocene Cerro Azul Formation of central Argentina is analyzed. Two new species (chronomorphs) are recognized, which constitute an anagenetically evolving lineage with a gradual and directional pattern of increasing molar hypsodonty. Dental changes related to increasing hypsodonty are comparable to those of the octodontid lineage Chasichimys also recovered from the Cerro Azul Formation. However, Neophanomys shows comparatively less variation in gross morphology and there are no evidences that this lineage achieved euhypsodonty. In contrast, important changes in enamel microstructure (schmelzmuster) are observed among different populations of Neophanomys, supporting the hypothesis that these changes can occur at least partially independently from modifications in dental gross morphology. The patterns of dental evolution detected in the Neophanomys and Chasichimys-Xenodontomys lineages and the unequivocal polarity of the changes involved, related to increasing hypsodonty, reinforce the hypothesis that chronological differences exist among late Miocene outcroppings of Cerro Azul Formation in central Argentina.     

Testing Magnetic Orientation in a Solitary Subterranean Rodent Ctenomys talarum (Rodentia: Octodontidae)

To test for the hypothesis that Ctenomys talarum can use the earth's magnetic field for spatial orientation, we carried out field and laboratory experiments to analyse if C. talarum burrows present any geomagnetic orientation in their natural habitat, if C. talarum show any spontaneous directional preference when starting to excavate their burrows and if this subterranean rodent is capable to use the earth's magnetic field to orient towards a goal in a complex maze. No correlation between the burrowing direction and the earth's magnetic field was found. We could not find any evidence for any spontaneous directional preference when starting to excavate the burrows in C. talarum. The change of the horizontal vector of the geomagnetic field did not affect the ability of this rodent to orient towards a goal in an artificial labyrinth. Explanations for these results and other possible mechanisms of orientation that could be used by C. talarum are discussed.     

The population genetic structure of Ctenomys porteousi (Rodentia: Octodontidae)

Several aspects of the population biology and the population genetic structure of Ctenomys porteousi were studied. Chromosomal and allozyme polymorphisms in two local populations separated by 10 km were used to infer genetic structure. Heterochromatic addition/deletion rearrangements for six autosomal pairs were employed as genetic markers. Allozyme variants were analysed by the study of thirty-one presumptive loci. Thirteen loci (43%) were polymorphic, average observed heterozygosity per locus was 10%, and mean number of alleles per locus was 1.57. Chromosomal and allozyme frequencies were homogeneous across populations. The estimated F_st values were very low, 0.009 and 0.022 for chromosomal and allozyme data respectively. Gene flow was indirectly estimated by the method of Wright, Nm values (mean number of migrants per generation per deme) was very high (27.2 and 11.3 for chromosomal and allozyme data respectively). The F_is values suggest a significant defect of heterozygotes that could be explained by the Wahlund effect. The ecological data indicate the occurrence of sharp changes in density and in distribution pattern in short time. The genetic and ecological data suggest that the population structure of C. porteousi differs from that found in other species of Ctenomys affected by chromosomal mechanisms of speciation.     

New Ctenomys karyotypes (Rodentia,Octodontidae) from north-eastern Argentina and from Paraguay confirm the extreme chromosomal multiformity of the genus

Bone-marrow karyotypes of 68 specimens of the subterranean octodontid rodent genus Ctenomys from 16 different populations of north east Argentina and one from Paraguay have been studied. A surprising variety of chromosome numbers was found, ranging from 2n=42 to 2n=70. Some of the karyomorphs are clearly assigned to named species by topotypy: C. conoveris 2n=50, FN=56; C. argentinus, 2n=44, FN=54; C. perrensi, 2n=50, FN=84; C. dorbignyi, 2n=70, FN=84; C. roigi, 2n=48, FN=80; C. yolandae, 2n=50, FN=78. Four populations of Corrientes Province similar in morphology to C. perrensi were found to be polymorphic and polytypic; they maintain the same FN=84, but diploid numbers increase from 2n=54 to 2n=58 from SW to the NE, thus suggesting Robertsonian rearrangements. In the middle of this cline, a stable karyomorph of 2n=62, FN=84 was found in two different populations, suggesting to belong to an undescribed species. Another karyomorph of 2n=42, FN=76 found in Curuzú Laurel, Corrientes, may also prove to represent another undescribed species. One karyomorph of 2n=52, FN=74, and another of 2n=56, FN=78 from Paraná and Ubajaý (Entre Ríos Province, Argentina) respectively are close to C. rionegrensis. The relationships among these karyomorphs is considered in light of data on sperm morphology. The hypothesis is advanced that karyotypic rearrangements among the FN=84 group may be the result of Robertsonian repatterning from a 2n=70 original widespread form. Fixation of chromosomal variants is correlated with patchy distribution and small size of unstable demes, and may or may not have resulted in reproductive isolation.     

Comparative cytogenetics of spiny rats of the genus Proechimys (Rodentia, Echimyidae) from the Amazon region

We made a comparative analysis of the cytogenetics of spiny rat species of the genus Proechimys collected from several sites of the Madeira River basin (Amazonas State, Brazil) and Jari River valley (Pará State, Brazil). Individuals were assigned to three groups based on diploid and fundamental numbers: 2n = 28, FN = 46 (P. cuvieri and P. gr. longicaudatus); 2n = 38, FN = 52 (Proechimys gr. guyannensis), and 2n = 40, FN = 54 (P. gardneri). The nucleolar organizer region (NOR) was interstitial on the long arm of one submetacentric pair, as seen in all species of Proechimys analyzed thus far. However, its position in the karyotype was variable. A duplication of the NOR in one of the homologues was detected in P. gr. longicaudatus from the Aripuan? basin along the mid Madeira. The C-band pattern varied between species and, together with the NOR, allowed the identification of two evolutionary units in P. gr. longicaudatus in the region of the mid Madeira River (cytotypes A and B). The morphology and banding of the sex chromosomes were species specific. A range extension is suggested for the geographic distribution of P. gardneri and P. gr. longicaudatus. Moreover, we suggest that species of Proechimys with 2n = 38 chromosomes are restricted to east of the Negro River and north of the Amazon River. We also revised the published chromosome data available for Proechimys.     

Seasonal changes in body composition of Ctenomys talarum (Rodentia: Octodontidae): an herbivore subterranean rodent

Ctenomys talarum is a subterranean herbivorous rodent whose burrow systems exhibit particular characteristics, distinct from other subterranean environments. We studied seasonal variation in body composition of C. talarum in relation to energetic requirements. Body lipid content seasonally changed in C. talarum, related to reproductive cycle and thermorregulatory mechanisms. A decrease in protein body content was found only in spring. Ash content of females was lowest when most of them are in post partum estro. Observed variations in water body content could be associated with plant water content and/or metabolic regulation. Our results show the occurrence of seasonal variations in body composition in C. talarum, which could be related to the high cost of reproduction and the subterranean life style of this species.     

A new host of Trypanosoma cruzi from Jujuy, Argentina: Octodontomys gliroides (Gervais & D'Orbigny, 1844) (Rodentia, Octodontidae).

To identify wild hosts of Trypanosoma cruzi, surveys were conducted in the subandean valleys of Jujuy Province, Argentina, between June 1986 and March 1987. Seventy two mammals from 13 different species were examined by xenodiagnosis. Fifty two of them were mostly rodents trapped at the localities of Maimará, León and Tilcara, and the remainder had been kept in captivity at the Estación Biológica Experimental, in Jujuy. Trypanosoma cruzi infection was detected only in 2 Octodontomys gliroides (2 pos./8 exam. 25%) from all 72 examined mammals. Isolates were called Octodontomys Argentina 1 and 2 (OA1 and OA2). Both infected animals were caught at the archaelogical ruin of Pucará, at Tilcara. Repeated searches for triatomines in the ruin itself and in neighbour houses rendered negative results. Groups of mice inoculated with either OA1 or OA2 isolates became infected between 7 (OA1) to 12 days (OA2) postinoculation PI. Parasitemia peaks were observed between day 12th-14th PI. Scarce amastigote nests were found in myocardium and skeletal muscle. Mortality was observed only for mice inoculated with OA1. Isoenzyme patterns of OA1 and OA2 were identical to one found in dogs and slightly different from that of human parasites in Argentina. Bones from Octodontomys sp., were recently found in a cave, dated 10200-8600 BC, in Pumamarca, near Tilcara, Jujuy. There are evidences that O. gliroides cohabited with man in ancient times and was associated to the domestic cycle of T. cruzi transmission, playing a role like that of domestic caves in Bolivia.