Chromosomal aspects and inheritance of the XY female condition in Akodon azarae (Rodentia,Sigmodontinae)

The populations of several species of Akodon present, besides XX females, a variable proportion of fertile XY females. In Akodon azarae, a correspondence exists between the X-chromosome C-banding pattern and the sexual phenotype of XY individuals: males carry a determinate X-chromosome type, defined by its C-banding pattern, and XY females, any of two others. To confirm the relation between X-chromosome type and the XY female condition and to investigate the hereditary transmission of these different X-chromosomes, we analyzed 50 animals captured in the field and 95 individuals corresponding to the F₁ and F₂ offspring of 16 crosses.It was seen that the correlation between X type and the sexual phenotype of XY animals is retained, and that the three X types are transmitted to the progeny. It was also observed that the male offspring of XY females receive the X-chromosome from their male parents and the Y from their mothers. These results strongly support the causal role of an X-borne mutation in A. azarae XY sex reversal, and discard a mutation of the Y-chromosome as the sole basis of this phenomenon.     

Inheritance of litter size at birth in farmed arctic foxes (Alopex lagopus, Canidae, Carnivora)

Natural populations of the arctic fox (Alopex lagopus, Canidae, Carnivora) differ drastically in their reproductive strategy. Coastal foxes, which depend on stable food resources, produce litters of moderate size. Inland foxes feed on small rodents, whose populations are characterized by cycling fluctuation. In the years with low food supply, inland fox populations have a very low rate of reproduction. In the years with high food supply, they undergo a population explosion. To gain insight into the genetic basis of the reproductive strategy of this species, we performed complex segregation analysis of the litter size in the extended pedigree of the farmed arctic foxes involving 20,665 interrelated animals. Complex segregation analysis was performed using a mixed model assuming that the trait was under control of a major gene and a large number of additive genetic and random factors. To check the significance of any major gene effect, we used Elston-Stewart transmission probability test. Our analysis demonstrated that the inheritance of this trait can be described within the frameworks of a major gene model with recessive control of low litter size. This model was also supported by the pattern of its familial segregation and by comparison of the distributions observed in the population and that expected under our model. We suggest that a system of balanced polymorphism for litter size in the farmed population might have been established in natural populations of arctic foxes as a result of adaptation to the drastic fluctuations in prey availability.     

Reproduction, development and growth of Akodon lindberghi (Hershkovitz, 1990) (Rodentia, Muridae, Sigmodontinae) raised in captivity

The reproduction, development and growth of Akodon lindberghi were studied in captivity. The colony was derived from animals captured in Sim?o Pereira, Minas Gerais state, which represents a new area of geographical distribution known for this species. Twelve males and twelve females were crossed, producing 144 young in 53 litters. Post-partum oestrus was observed and gestation length was estimated in 23 days. Litter size ranged from 1 to 4 with a mean of 2.72 (SD = 0.97, n = 53) and modal size of 3. Sexual dimorphism was neither present in body mass at birth nor at weaning. There was a significant negative correlation between litter size and mass at birth or weaning. Permanent emergence of adult external appearance occurred at 15 days. Puberty for males and females was 43 and 42 days, respectively, and the first fecundation event for two females was recorded at 47 and 54 days of age. The weight growth was described by fitting a Gompertz model. No significant difference was found in any parameter of growth curves for males and females. Measurements (head-body, tail, hind foot and internal and external ear lengths) obtained for adult individuals also did not reveal the presence of sexual dimorphism.     

Phenotypic plasticity in response to low quality diet in the South American omnivorous rodent Akodon azarae (Rodentia: Sigmodontinae)

We studied the responses in the omnivorous rodent A. azarae submitted to a low quality diet at morphological, physiological and biochemical levels. At short term, a decrease in body mass occurred. A later increase in food consumption constituted a strategy that allowed a temporal recovery of physical condition. However, hyperphagia appeared not to be enough to maintain physical condition after 30 days of low quality diet consumption. At the morphological level, an increase in length (9%) of the anterior portion of the gut occurred, the part of the gut where digestion and absorption take place. A decrease in small intestine weight could be related with the long-term impairment of body condition. Inhibition of sucrase specific activity in small intestine would indicate a down-regulation of sucrase-isomaltase complex. Total maltase specific activity in small intestine was not affected suggesting an up-regulation of sucrase-independent maltase specific activity. A down-regulation of protease specific activity in small intestine occurred in response to low quality diet. The specific activity of disaccharidases in caecum and large intestine was down-regulated. The strategies and constraints at different levels of A. azarae upon low quality diet are discussed.     

Phenotypic flexibility of digestive morphology and physiology of the South American omnivorous rodent Akodon azarae (Rodentia: Sigmodontinae)

We have determined the occurrence of responses at different levels (morphological, physiological and biochemical) in the omnivorous rodent Akodon azarae upon cold acclimation (15 degrees C). A short-term enhancement in food consumption appeared to account for the maintenance of both mass and body composition. At the morphological level, the main response was an increase in the dimensions of small intestine, which constitutes the section of the gut where absorption and secretion take place. An increase in sucrase specific activity was only found in small intestine. Sucrose independent maltase activity was very low since 99.8% of total maltase activity was due to sucrase-isomaltase (SI) complex. Protease specific activities were not affected. The fact that resting metabolic rates determined at 15 and 23 degrees C were similar in cold acclimated animals suggests a change in lower critical temperature. In conclusion, our results show that A. azarae exhibits different strategies to support cold environment that could lead to an enhancement in digestion and absorption efficiency. Furthermore, this work suggests that low temperature is an independent cue of other environmental factors to trigger the strategies allowing the maintenance of body condition in A. azarae.     

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.     

Inter-male aggression with regard to polygynous mating system in Pampean grassland mouse, Akodon azarae (Cricetidae: Sigmodontinae)

Based on the hypothesis that, in Akodon azarae, polygyny operates through female defence, we studied inter-male aggression in order to test the following predictions: during the breeding period (1) resident males are more aggressive than intruder males in the presence of females (FP), and (2) aggressive behaviour is independent of male condition (resident or intruder) in the absence of females (FA). To test our predictions, we used the resident male behavioural response towards an intruder male in relation to FP or FA. We conducted 30 encounters in FP and 27 in FA in 0.79-m² round enclosures placed in the Espinal Reservation. Our results support the prediction that, in FP, the intensity of aggressive behaviour exhibited by males varied in relation to resident or intruder condition. Resident males showed high levels of aggression towards intruders, and intruders exhibited the greatest values of submissive behaviours with residents. In FA, the intensity of aggressive behaviour did not vary in relation to resident or intruder condition. Both resident and intruder males exhibited low aggressive behaviour and inter-male encounters resulted mainly in non interactive behaviours. Our results support the hypothesis that, in A. azarae, the polygynous mating system operates through female defence.     

Genetic studies in representatives of genusRhipidomys (Rodentia, Sigmodontinae) from Brazil

Karyotypic polymorphism of five taxa of the rodent genusRhipidomys from the Brazilian Amazon and Cerrado biomes was analysed.Rhipidomys nitela Thomas, 1901 from Amazon has 2n=48, FN=68. The other species, all have 2n=44, but can be separated into two groups, one with high FNs (76, 80) and the other with low FNs (48, 52). Two cytotypes ofR. mastacalis (Lund, 1840) with high FNs were trapped in four localities of the Cerrado, showing 19 and 17 biarmed autosomes, respectively. A low FN (48) was observed inR. leucodactylus (Tschudi, 1844) in two localities of the Cerrado and FN=52 in one locality in the Cerrado and the Amazon. All taxa with 2n=44 have a medium-sized acrocentric X chromosome and a small Y.Rhipidomys nitela is different from the species with 2n=44 by presenting a heterochromatic short arm of the X chromosome. In all karyotypes analysed, the nucleolus organizer regions were located in the short arms of two to six pairs and the (T₂AG₃)_n telomeric probes hybridizedin situ in both the short and long arms of all pairs of the karyotypes.     

A new karyotype of Oligoryzomys (Sigmodontinae, Rodentia) from central Brazil

A new karyotype of the genus Oligoryzomys was described for specimens collected in Brazilian Cerrado in Tocantins and Goiás States. Conventional staining, G-, C-banding, and Ag-NOR staining techniques were used for describe this karyotype with 2n=70, AN=74 or 76. The chromosome complement, with the highest diploid number known among Oligoryzomys species, differs from all others previously reported. This small sized Oligoryzomys species can be differentiated from other Brazilian Oligoryzomys not only by chromosomal complement, but also in some morphological attributes. The new species is apparently restricted in distribution and endemic of Brazilian Cerrado, occurring only in Rio Tocantins basin.     

Breeding season, litter size and nestling mortality of the Harvest mouse, Micromys minutus (Rodentia: Muridae), in Britain

Data are presented on the breeding biology of Micromys minutus in Britain. It is shown that although litters are produced over a protracted period (May-December), 74% of the litters are born in August and September. Mean litter size is 5·40±0·16( n = 62), which is a significant reduction on the mean litter size of 6·75±0·40( n = 16) calculated from pre-1917 literature. Litter size was found to be constant throughout the breeding season, and there was no difference in mean litter size between two samples collected close to the northern and southern limits of the known British range. By comparing the mean litter size of samples 1–5, 6–10 and 11+ days old, no significant loss of individual mice within litters could be detected, although 12·3% of all litters died prior to weaning. This loss was predominantly of autumnal litters, cold and wet being important climatic factors that terminate the breeding season. Comparisons are made with data from other parts of the animal's range.     

Inheritance of white head spotting in natural populations of South American water rat (Nectomys squamipes Rodentia: Sigmodontinae)

Specimens with white head spots are present at low frequency in the natural populations of South American water rat (Nectomys squamipes) and absent in the sibling species Nectomys rattus. We analyzed the pattern of inheritance of the phenotype using complex segregation analysis of pedigrees of a captive-bred population of N. squamipes. We found that the inheritance of the white head spot in this species can be described within the framework of the major gene recessive model with incomplete penetrance of genotypes.     

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.     

Colonization process of the Brazilian common vesper mouse, Calomys expulsus (Cricetidae, Sigmodontinae): a biogeographic hypothesis

Riverine barriers have been associated to genetic diversification and speciation of several taxa. The Rio S?o Francisco is one of the largest rivers in South America, representing the third largest river basin in Brazil and operating as a geographic barrier to gene flow of different taxa. To evaluate the influence of the Rio S?o Francisco in the speciation of small rodents, we investigated the genetic structure of Calomys expulsus with phylogenetic and network analyses of cytochrome b DNA. Our results suggested that C. expulsus can be divided into 3 subpopulations, 2 on the left and another one on the right bank of this river. The time of divergence of these subpopulations, using a Bayesian framework, suggested colonization from the south to the north/northeast. Spatial analysis using a clustering method and the Monmonier's algorithm suggested that the Rio S?o Francisco is a biogeographic barrier to gene flow and indicated that this river may play a role in the incipient speciation process of these subpopulations.     

Selection response for litter size at birth and litter weight at weaning in the first parity in mice

Summary Five 60-pair lines of mice were selected for seven generations for the following criteria: number born alive (LSO), random selection of litters (LC), number born alive divided by the weight of the dam at 9 weeks (LSO/DWT), total litter weight weaned divided by the weight of the dam at 9 weeks (LWT/DWT), and weight of litter weaned (LWT). All traits were measured in the first parity only and litters were not standardized. Realized heritabilities for LSO, LWT, LSO/DWT, and LWT/ DWT were 0.10± 0.06, 0.11±0.07, 0.22 ± 0.04, and 0.22 ± 0.08, respectively. Selection response for the two ratio lines was due to correlated responses in the respective numerators, LSO and LWT, as DWT did not decrease. In terms of improving LWT, selection for LWT/ DWT was three times as effective as selection for LSO/ DWT.