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The present work describes chromosomal polymorphisms in zones of contact between divergent populations of Ctenomys minutus parapatrically distributed in the coastal plain of southern Brazil, and inter-specific hybridization with C. lami a closely related species. A sample of 171 specimens from 32 sample sites distributed along 161 km of the coastal plain was cytogenetically analyzed. Nine polymorphic populations were found: four with specimens with 2n = 46-48 (autosomal arm number (AN) = 76); three only have specimens with 2n = 47 and 48; one population sampled presented specimens with 2n = 43-46 (AN = 74-76) and one population with 2n = 50-52 (AN = 76-80). The remainder populations were fixed for 2n = 42, 46 or 48. The variation is the result of Robertsonian mechanisms of chromosomal evolution and a fusion in tandem rearrangement. The polymorphisms have been considered the result of secondary contact of populations after divergence in allopatry. The geomorphological evolution of the coastal plain provides clues to the possible existence of past geographic barriers acting over populations of Ctenomys, during the Holocene.
The dominant mammals occupying the subterranean niche in South America are rodents of the genus Ctenomys, which form a large group of 56 species with chromosome numbers ranging from 2n = 10 to 70. In southern Brazil, Ctenomys minutus is the species with the widest geographic distribution, inhabiting sandy fields and dunes extending from Jaguaruna beach in the state of Santa Catarina to the town of São José do Norte in the state of Rio Grande do Sul. Eleven karyotypes (2n = 42; 2n = 46a; 2n = 46b; 2n = 47a; 2n = 47b; 2n = 48a; 2n = 48b; 2n = 49a; 2n = 49b; 2n = 50a and 2n = 50b) were described for this species and zones of hybridization are also known. A sample of 51 C. minutus specimens was collected from five sampling sites about 20km apart along the coastal plain of Rio Grande do Sul between the municipalities of Tavares (31 degrees 23'S 51 degrees 09'W) and São José do Norte (31 degrees 52'S 51 degrees 54'W). We were able to extend the known geographic distribution of C. minutus by 90 km, from Tavares southwards to São José do Norte. During our study we found five karyotypes (2n = 46b, 47b, 48b, 49b and 50b), four of which (2n = 47b, 48b, 49b and 50b) have not previously been described for this species.
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A fragment of the mitochondrial cytochrome b gene was studied in 13 species of the South American fossorial rodent Ctenomys using PCR with 'universal' primers and DNA sequencing after cloning. Five different groups of sequences were found, one of which corresponds to the functional mitochondrial gene (mt). The other four groups (A, B, C and D) were believed to be nuclear pseudogenes. Sequences A-C were highly divergent from the mt sequences and included substitutions, deletions and insertions such that they could not possibly have coded a functional protein. They all shared a common insertion between positions 15055 and 15056 suggestive of a common origin, although the A, B and C sequences otherwise differed greatly from each other. The D sequences also could not have been functional on the basis of nucleotide sequence, but the differences with the mt sequences were far more subtle and in a more limited study the D sequences could easily have been classified as a true mtDNA sequence. It is suggested that there were two transfers of the cytochrome b gene from the mitochondrion to the nucleus; the first leading to sequences A-C and the second to the D sequence. Subsequent to transfer, a sequence of duplications within the nucleus appears to have generated the full range of pseudogenes that are observed. This study adds to other recent observations suggesting the frequent transfer of mtDNA sequences to the nucleus and reinforces the necessity of great care in interpreting PCR-generated sequences, particularly those produced with universal primers. There are now data from several species of mammals and birds relating to PCR-generated nuclear copies of cytochrome b, which we review.
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A new Nematode, Gongylonema pithyusensis n. sp., parasitizing the oesophagus mucosa of the dormouse Eliomys quercinus ophiusae Thomas, 1925 in the balearic island of Formentera (Spain) is described. The male of the new species is characterized by the size of spicules and gubernaculum; the female by the morphology of the tail, the posteriour situation of the vulva and the dimensions of the eggs. It is the only Gongylonema species to be known parasitizing Rodents of the family Gliridae.
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A new species of trypanosome, Trypanosoma (Megatrypanum) amilcari, is described in the present paper. This new trypanosome was found in the circulating blood of a rodent species Oryzomys eliurus (Cricetidae). Some biological observations were made.
Ectinorus (E.) uspallatae n. sp. and Delostichus ojedai n. sp. are described from the rodent Abrocomid, Abrocoma uspallata Braun and Mares, 2002, a new small mammal of Argentina.
A new nematode species, Graphidioides subterraneus n. sp., found in the stomach of C. talarum from Argentina is described. The new species more closely resembles G. mazzai Lent & Freitas, 1935, parasite of Galea leucoblephara from Argentina, and G. kravetzi Sutton & Durette-Desset, 1995, parasite of Holochilus brasiliensis from Uruguay. It can be distinguished by shorter spicules, by the shape of the gubernaculum, by shorter uterine branches, and by a different number of ridges of the synlophe all along the body.
Homoplasy among morphological characters has hindered inference of higher level rodent phylogeny for over 100 years. Initial molecular studies, based primarily on single genes, likewise produced little resolution of the deep relationships among rodent families. Two recent molecular studies (Huchon et al., 2002, Mol. Biol. Evol. 19:1053-1065; Adkins et al., 2003, Mol. Phylogenet. Evol. 26:409-420), using larger samples from the nuclear genome, have produced phylogenies that are generally concordant with each other, but many of the deep superfamilial nodes were still lacking substantial statistical support. Data are presented here for a total of approximately 3,600 base pairs from portions of three different nuclear protein-coding genes, CB1, IRBP, and RAG2, from 19 rodents and three outgroups. Separate analyses, with data partitioned according to both genes and codon position, produced conflicting results. Trees obtained from all partitions of CB1 and RAG2 and those obtained from the first- plus second-position sites of IRBP were generally concordant with each other and the trees from the two recent studies, whereas trees obtained from the third-position sites of IRBP were not. Although the IRBP third-position sites represent only 1/9 of the total data set, combined analyses using either parsimony or likelihood resulted in trees in agreement with the IRBP third-position sites and in disagreement with the remaining 8/9 of the sites from this data set and the two recent multigene studies. In contrast, maximum-likelihood analysis using a site-specific rates model did recover a tree that is highly congruent with the trees in the two recent studies. If the IRBP third-position sites are removed from the current data set, then combined likelihood analyses obtain a tree that is highly congruent with those of the two recent studies. This analysis also provides, for the first time in a study of rodent phylogeny, robust statistical support for every bipartition, with just one exception. This tree divides rodents into two major clades. The first contains Myodonta (Muroidea plus Dipodidae) and the only unresolved trichotomy, from which descend Geomyoidea, Pedetidae, and Castoridae. On the other side of the root is a clade containing Sciuroidea plus Gliridae, and Hystricognathi. Some uncertainty remains on the placement of the root. Trees on which the Hystricognathi are the basal sister group to Myodonta, Geomyoidea, Pedetidae, and Castoridae are also found within a Bayesian 95% credible set, as estimated by Metropolis-coupled Markov chain Monte Carlo sampling.
The model of development and evolution of complex morphological structures conceived by Atchley and Hall in 1991 (Biol. Rev. 66:101-157), which establishes that changes at the macroscopic, morphogenetic level can be statistically detected as variation in skeletal units at distinct scales, was applied in combination with the formalism of geometric morphometrics to study variation in mandible shape among populations of the rodent species Thrichomys apereoides. The thin-plate spline technique produced geometric descriptors of shape derived from anatomical landmarks in the mandible, which we used with graphical and inferential approaches to partition the contribution of global and localized components to the observed differentiation in mandible shape. A major pattern of morphological differentiation in T. apereoides is attributable to localized components of shape at smaller geometric scales associated with specific morphogenetic units of the mandible. On the other hand, a clinical trend of variation is associated primarily with localized components of shape at larger geometric scales. Morphogenetic mechanisms assumed to be operating to produce the observed differentiation in the specific units of the mandible include mesenchymal condensation differentiation, muscle hypertrophy, and tooth growth. Perspectives for the application of models of morphological evolution and geometric morphometrics to morphologically based systematic biology are considered.
Many small mammals inhabiting fluctuating and cold environments display enhanced capacity for seasonal changes in nonshivering thermogenesis (NST) and thermoregulatory maximum metabolic rate (MMR). However, it is not known how this plasticity remains in a mammal that rarely experiences extreme thermal fluctuations. In order to answer this question, we determined body mass (m(b)), basal metabolic rate (BMR), NST, MMR, and minimum thermal conductance (C) on a Chilean fossorial caviomorph (Spalacopus cyanus) from a coastal population, acclimated to cold (15 degrees C) and warm (30 degrees C) conditions. NST was measured as the maximum response of metabolic rate (NST(max)) after injection of norepinephrine (NE) in thermoneutrality minus BMR. Maximum metabolic rate was assessed in animals exposed to enhanced heat-loss atmosphere (He-O2) connected with an open-flow respirometer. Body mass and metabolic variables increased significantly after cold acclimation with respect to warm acclimation but to a low extent (BMR, 26%; NST, 10%; and MMR, 12%). However, aerobic scope (MMR/BMR), calculated shivering thermogenesis (ST), and C did not change with acclimation regime. Our data suggest that physiological plasticity of S. cyanus is relatively low, which is in accordance with a fossorial mode of life. Although little is known about MMR and NST in fossorial mammals, S. cyanus has remarkably high NST; low MMR; and surprisingly, a nil capacity of ST when compared with other rodents.