Genetic similarity between species of Akodon (Rodentia, Cricetidae)

Genetic similarity between species of rodents of the Akodon genus (A. dolores, A. molinae, and A. azarae) has been estimated by analysis of electrophoretic zymograms corresponding to 23 loci. Nei's coefficient between A. dolores and A. molinae was within the range usually found in conspecific populations. This evidence plus the successful production of "hybrids" (Merani et al., J. Exp. Zool., 206:343-346, '78) suggests that A. dolores and A. molinae may represent geographic races of the same species.     

A new genus and species of lungworm (Nemata: Metastrongyloidea) from Akodon mollis Thomas, 1894 (Rodentia: Cricetidae) in Peru

Akodonema luzsarmientae n.g., n.sp. (Nemata: Metastrongyloidea) is described from the pulmonary arteries and heart from several individuals of "soft grass mouse," Akodon mollis (Rodentia: Cricetidae), collected in the region of Ancash, Peru. The new genus and species is distinguished by a reduction of the dorsal ray to 2 small widely separated papillae.     

Meiotic behavior of gonosomically variant females of Akodon azarae (Rodentia, Cricetidae)

The meiotic behavior of sex chromosomes has been investigated in variant females of Akodon azarae, both in pachytene oocytes and metaphase I. In somatic cells, these females have a heteromorphic sex pair, in which the minor chromosome has been previously interpreted as a major deletion of the long arm of the X chromosome (dX). After microspreading for synaptonemal complex analysis, pachytene oocytes show two axes of very different lengths (100:17.1), which correspond to the sex chromosomes X and dX. True synapsis is abnormally restricted (43.3%) between these sex chromosomes; on the other hand, self-synapsis of both the X and dX chromosomes is frequent (60%). Single, nonsynapsed axes or axial segments are thickened. Strong chromatin condensation occurs around nonsynapsed axes or axial segments, giving many of these sex pairs an appearance similar to an XY body ("sex vesicle"). The minor gonosome axis differs from that of the Y chromosome of male meiosis, as the former is shorter (relative to the X) and has a different synaptic behavior. In 17 metaphases I from XdX variant females, only heteromorphic, end-to-end joined sex pairs were observed. These variant females differ from the variant females of the wood lemming Myopus schisticolor in several respects, but a similar mechanism seems to be prevalent in other species of the genus Akodon. Self-synapsis of unequal gonosomes in oocytes is assumed as an escape from functional deterioration, following the hypothesis put forward by others.     

Temporal variation of allele frequencies in populations of Akodon dolores (Rodentia,Cricetidae)

Summary Six population samples of the South American cricetid rodent Akodon dolores were collected at the same site at six-month intervals over a three year period. Changes in density were detected. Seven out of 18 loci analyzed by means of starch gel electrophoresis were polymorphic. Only two of these loci (Est-4 and G6pdh) showed statistically significant variation in allele frequencies following a seasonal pattern. There was no correlation between allele frequencies and population density. When animals were grouped into two classes according to body weight, a clear difference in allele distribution at the Est-4 and G6pdh loci was observed between individuals 39 g or less and those heavier than 39 g. As the first group comprises predominantly younger animals, the data indicate that changes in the age-structure of population, rather than density variations, are responsible for the cyclic pattern of allele frequencies fluctuations.     

DNA of Akodon (Rodentia,Cricetidae). I. Biophysical characterization

Buoyant density in CsCl, melting temperature, and G + C base content of the DNA from four species of Akodon (Rodentia, Cricetidae) were determined. The buoyant density values of 1.699–1.701 g/cm³ were in accordance with the data reported for other cricetids. No satellite bands were seen in neutral CsCl. The T _m values determined in 1 × SSC ranged from 86.2 to 87.0 C, which corresponds to G + C contents of 41.2–43.2%. There was good agreement in DNA base composition of the four species, although values were slightly higher in A. obscurus, suggesting a certain degree of interspecies variability.This study was supported by grants from Comisión de Investigaciones Científicas de la Provincia de Buenos Aires, Consejo Nacional de Investigaciones Científicas y Técnicas, and Organización de Estados Americanos.     

A new angiostrongylid (Nematoda) species from the pulmonary arteries of Akodon azarae (Rodentia: Cricetidae) in Argentina

Angiostrongylus morerai n. sp. (Nematoda: Angiostrongylidae) is described from the pulmonary arteries of Azara's grass mouse Akodon azarae (Rodentia: Cricetidae) in Argentina. It is distinguished from its congeners principally by the morphology of the dorsal ray, which is as long, or longer, than the externodorsals and has 2 long branches; the spicule lengths are also greater (400-465 microm). This is the first record of a metastrongyloid from sigmodontine rodents in Argentina.     

A new species of Syphacia (Nematoda: Oxyuridae) from Akodon azarae (Rodentia: Cricetidae) in Argentina

Eight species of Syphacia (Nematoda: Oxyuridae) have been reported from South America in rodents of the Sigmodontinae, only 1 of which has been recorded in Argentina. Syphacia (Syphacia) carlitosi n. sp. is described from the ceca of Akodon azarae bibianae and Akodon azarae hunteri (Sigmodontinae: Akodontini) captured in 3 provinces in the northeast region of Argentina. The new species is differentiated principally by the shape of the cephalic plate; distribution of submedian papillae and amphids; presence, extent, and shape of cervical alae in females; absence of lateral alae; absence of deirids; spicular and gubernaculum length; shape and structure of accessory hook of gubernaculum; and distance of mamelons, excretory pore, and vulva from the anterior extremity. This is the second record of Syphacia parasitizing rodents of the tribe Akodontini.     

The sex-determining zinc finger sequences in XY females of Akodon azarae (Rodentia, Cricetidae)

Wild populations of Akodon azarae comprise females with a karyotype indistinguishable from that of males. These individuals were formerly assumed to be Xx, the x being an X chromosome with a deletion of most of its long arm. By using a DNA probe derived from the testis-determining region of the human Y chromosome (comprising a candidate gene for the testis-determining factor, Y-linked zinc finger [ZFY]), we demonstrate that A. azarae gonosomally variant females are XY and not Xx. The ZFY sequences in A. azarae are amplified and located in two different families of EcoRI fragments derived from Y-chromosome DNA. No rearrangement or change in the state of methylation of ZFY or ZFX (X-linked zinc finger) sequences were found in XY females. We propose that sex reversal in A. azarae may be mediated by a gene or genes other than ZFX or ZFY.     

Phylogenetic relationships among species of the genus Oligoryzomys (Rodentia,Cricetidae) from Central and South America

González‐Ittig, R. E., Salazar‐Bravo, J., Barquez, R. M. & Gardenal, C. N. (2010). Phylogenetic relationships among species of the genus Oligoryzomys (Rodentia, Cricetidae) from Central and South America. —Zoologica Scripta, 39, 511–526. The genus Oligoryzomys includes several species very similar in external morphology, which has resulted in a confusing specific taxonomy. Accurate species identification is particularly important because several species of Oligoryzomys act as natural hosts of hantaviruses affecting humans. Here, we assign specific status to individuals from a wide geographical area of Argentina and Chile using sequences of the mtDNA control region. We also compare cytochrome b sequences of 14 species recognized from Central and South America to infer the phylogenetic relationships within the genus. In addition, the results were analysed using available data on chromosome numbers, and the host–parasite relationships reported for the genus Hantavirus. We confirm the geographical distribution of Oligoryzomys longicaudatus (Argentina, Chile), Oligoryzomys nigripes (Argentina, Paraguay, Brazil), Oligoryzomys chacoensis (Argentina, Bolivia, Paraguay), Oligoryzomys fornesi (Argentina, Paraguay), Oligoryzomys destructor (Argentina, Bolivia) and Oligoryzomys microtis (Bolivia, Brazil). Oligoryzomys longicaudatus is strongly related to the Oligoryzomys flavescens complex, which comprises four clades; O. nigripes is closely related to Oligoryzomys stramineus, and Oligoryzomys vegetus, to Oligoryzomys fulvescens from Central America. Oligoryzomys chacoensis, O. destructor, O. fornesi, O. longicaudatus, O. microtis, O. nigripes, O. stramineus, Oligoryzomys moojeni, Oligoryzomys rupestris, O. fulvescens and O. vegetus are confirmed as valid species, whereas O. flavescens, Oligoryzomys magellanicus, Oligoryzomys griseolus, Oligoryzomys victus, Oligoryzomys andinus and Oligoryzomys arenalis need exhaustive revision. The sister species to all the remaining entities of the genus was O. microtis, suggesting an Amazonian origin for the genus.     

Chromosome homology in the climbing rats,genus tylomys (Rodentia: Cricetidae)

The climbing rats (Tylomys spp.) have diploid numbers of 52 (T. panamensis), 42 (T. nudicaudus), 40 (T. n. gymnurus) and 36 (T. n. villai). Using G-band analysis we found that the variations are mainly of the Bobertsonian type, and practically all changes can be traced. G-banding also revealed that biarmed chromosomes with similar morphology may be composed of different components. Such conclusions were verified also by analysis of the chromosomes of interspecific hybrids. C-band staining showed that the constitutive heterochromatin of Tylomys is mainly located in the centromeric regions and the sex chromosomes, a situation similar to that of Microtus agrestis. In one specimen of T. panamensis, however, an additional terminal heterochromatic segment was found in one member of the large metacentric pair. Our data underline that in mammalian cytotaxonomy studies both C- and G-band (or C- and Q-band) techniques must be applied to gain maximal information.     

New karyotypes and somatic and germ-cell banding in Akodon arviculoides (Rodentia, Cricetidae).

Chromosomal polymorphism resulting from three autosomal pericentric inversions and a complex rearrangement involving the largest chromosome of the complement (pair 1) in Akodon arviculoides (2n= 14, 15) is reported. G- and C-banding patterns in somatic and meiotic cells allowed the precise identification of all chromosomes and rearrangements. In meiosis of male specimens with 2n = 15, a large trivalent reflecting the complex rearrangement in autosomal pair 1 was observed. Two possible explanations for it are discussed. G- and C-bands in diplotene cells in heterozygotes for the inversions showed different configurations depending on the pairing in the inverted segments. Chiasma frequency data fro three specimens are analyzed.     

Chromosomal evolution of the brown mice,genus Scotinomys (Rodentia: Cricetidae)

Early taxonomic investigations associated the genus Scotinomys with South American akodontine rodents, but more recent morphological analyses based in large part on the glans penis have linked brown mice with North American peromyscines, specifically the genus Baiomys. Differentially stained chromosomal preparations of S. xerampelinus were compared with other cricetine taxa. Chromosomally, golden mice (Ochrotomys) were found to be most similar to Scotinomys, this association based on two apparent synapomorphic G-banded chromosomes. The majority of banded chromosomes possessed by Scotinomys were found to be either uniquely derived or to appear unaltered, and therefore presumably ancestral, when compared to the G-band patterns of other cricetids. These results fail to support the morphological hypotheses that closely unite Scotinomys with Baiomys, and instead support a hypothesis that treats Scotinomys, Ochrotomys, and perhaps Baiomys as a rather loosely associated assemblage of genera that are phylogenetically intermediate between the relatively complex pene, primarily South American cricetine rodents and the more simple pene North American neotomine-peromyscines.     

Detection of two morphologically cryptic species from the cursor complex (Akodon spp; Rodentia, Cricetidae) through the use of RAPD markers

The cursor complex is a group within the Akodon genus of South American rodents, formed by Akodon cursor and A. montensis. Correct distinction between these two species is of great importance since they can harbor different Hantavirus strains. These species are only distinguishable by means of karyotypic or internal anatomic features, requiring dissection; recently, some other genetic methods have become available. We developed RAPD markers capable of distinguishing between A. cursor and A. montensis. Samples included 42 individuals of A. cursor from four localities and 16 individuals of A. montensis from two localities. Fifty-five bands, 41 of which were polymorphic, were analyzed. A principal component analysis showed that this set of markers could successfully distinguish between the two species, mainly based on three RAPD bands. The number of bands in each population was compared within a 95% confidence interval as a measure of intraspecific variability. The A. cursor populations were found to have marked genetic structure across the study area (AMOVA; F(ST?)= 0.21), which in part might be because of the relatively limited dispersal capabilities of this species. Species-specific bands, with potential for species identification, were identified.     

DNA of Akodon (Rodentia, Cricetidae). II. Molecular hybridization of repetitive DNA sequences

Interspecies repetitive DNA homology was studied in akodont rodents related at generic and suprageneric levels. The homology was determined by taking the species Akodon molinae as the reference species. The 3H-DNA/DNA hybridization on filters showed a closer relationship between A. molinae and A. azarae, A. dolores and A. mollis than between A. molinae and Bolomys obscurus. These data agree with the taxonomical ranking of the species. The quantity and quality of the hybrid DNAs were measured by investigating their thermal stabilities and subsequent comparison to the results obtained on the reference species. These data indicate high similitude between the repetitive DNA of A. dolores and A. molinae. Increasing differences were shown to occur in the repetitive DNA of A. mollis, B. obscurus and A. azarae, respectively. Since these results coincide with the G-banding homologies and differ slightly from the taxonomical rank, it is speculated that the divergency between the DNA of A. molinae and A. azarae is the result of a differential process of DNA amplification which is not related to the phylogenetical distance separating the two species.     

An assessment of species limits of the South American mouse genus Oligoryzomys (Rodentia,Cricetidae) using unilocus delimitation methods

Oligoryzomys, as currently understood is formed by 25 living species, is the most diverse genus of the tribe Oryzomyini of the New World subfamily Sigmodontinae of cricetid rodents. Nonetheless, the species richness of Oligoryzomys seems to be an underestimate, given some species complex has been proposed in previous studies, at the time that large geographic areas remain to be sampled, and several taxonomic forms have not been assessed with contemporary approaches. In this study, we present a new assessment of the species diversity of Oligoryzomys based on multiple unilocus species delimitation methods (ABGD, BPP, PTP, GMYC and b GMYC), using 665 cytb gene sequences as evidence (532 gathered from Genbank and 133 obtained in this study). We sampled representatives of almost all currently known species of Oligoryzomys, at the time that extending the geographic coverage to the Central Andes, a large area that was largely unrepresented in previous studies. Phylogenetic relationships, based on a non‐redundant alignment, were inferred via maximum likelihood and Bayesian inference; an ultrametric tree, used in species delimitation analyses, was obtained using multiple secondary calibration points. Results of species delimitation methods are discussed at the light of previous knowledge (e.g., taxonomic history and geographic provenance of samples in relation to type localities) and the morphological assessments of some specimens. Results of the distinct delimitation methods are mostly congruent, being BPP and PTP the most sensible to estimate species delimitation, allowing us to suggest that Oligoryzomys is composed of 30 lineages of species level. Of these, 22 correspond to forms currently considered species; some of these include in their synonymy some forms currently considered valid species (e.g., yatesi would be a synonym of longicaudatus). The remaining eight lineages are candidate species that need to be further evaluated. This study, by advancing taxonomic hypothesis that should be further tested in future studies, constitutes a stepping‐stone for upcoming taxonomic and biogeographic studies centred on Oligoryzomys.     

Chromosomal localization of telomeric sequences in three species of Akodon (Rodentia, Sigmodontinae)

The distribution of the vertebrate telomeric sequence T2AG3 in three species of the rodent genus Akodon was examined by FISH with a peptide nucleic acid probe. In addition to the expected telomeric hybridization, non-telomeric signals were observed in the three species. In A. dolores, centromeric signals were visible in two of the four biarmed autosome pairs featuring Robertsonian polymorphism, indicating the retention of at least part of the telomeric sequences during the fusion process, and an interstitial signal of lower intensity was observed in the short arm of another. In A. boliviensis, a strong signal was observed near the centromeric end of the first chromosome pair. The first pair of A. azarae (homologous to the first pair of A. boliviensis) showed a similar but markedly amplified signal, and a subcentromeric signal in the X chromosome corresponding to a heterochromatic region; additionally, interstitial signals of lower intensity were present in one to four chromosomes in the majority of cells examined.