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At least 379 records · Page 21Linked to original sources
PubMed1978

Chromosomal polymorphism caused by supernumerary chromosomes in Rattus rattus ssp. frugivurus (Rafinesque, 1814) (Rodentia, Muridae).

A chromosomal numeric polymorphism 2n = 38, 39, 40 and 41 in the species Rattus rattus ssp. frugivurus (Rafinesque, 1814) is reported for the first time for this subspecies. The numbers 2n = 39, 40 and 41 are new for the species. The polymorphism is due to the presence of 1, 2 or 3 B-chromosomes, which are all small metacentrics of the size and shaped vary close to the other autosomes of the normal complement, and whose character of being supernumeraries is shown in Meiosis.

Animals↗
PubMed1995

Body size variability and water balance: a comparison between mainland and island populations of Mastomys huberti (Rodentia: Muridae) in Senegal.

Generally rodents are found to be larger on islands than on the mainland. However, there are some exceptions to this rule, and the aim of this paper is to examine one of them. On the mainland of Senegal, Mastomys huberti occupies humid habitats. However, it occurs also on dry and sandy islands (Saloum delta), where its representatives are dwarf. Since water availability appeared to be the limiting factor in these islands when compared to the mainland, we studied water turnover characteristics in relation to body size, in mainland and island populations at the end of the dry season, under both field and laboratory conditions. All populations were found to be water balanced in their natural habitats. They presented similar rates of water turnover, even though island animals were subjected to stronger constraints than mainland ones. Laboratory experiments suggested that the physiological plasticity of one of the island populations may be reduced. Island populations have a higher kidney size to body weight ratio than those from the mainland. We propose that smaller size in the islands allows the maintenance of water balance with a smaller amount of water, and that a higher ratio of kidney filtration surface to body size may help Mastomys huberti to survive in dry islands. We discuss the factors responsible for body size variability and variation in water exchange characteristics and conclude that different factors could explain body size variation among island populations, depending on the species considered and the ecological constraints met within the islands.

Animals↗
PubMed1983

Evolution of the genome size in Akodon (Rodentia, Cricetidae).

Nuclear DNA contents were estimated by microdensitometry in five species of Akodon rodents: Arodon molinae, A. dolores, A. mollis, A. azarae, Bolomys obscurus) and in three chromosomal varieties of A. molinae (2n = 42; 2n = 43, 2n = 22). The data obtained showed that the species with the highest DNA content was B. obscurus, followed in order of decreasing genome size by A. molinae, A. mollis, A. dolores and A. azarae. In A. molinae the forms with 2n = 42 chromosomes had the lowest and the forms with 2n = 44 the highest amount of DNA, while the forms with 2n = 43 had intermediate DNA contents. The variation in DNA amount detected in A. molinae was interpreted as a phenomenon of amplification occurring in the chromosomal areas involved in the chromosomal rearrangement giving rise to the polymorphism exhibited by this species. The DNA contents of shared chromosomes (chromosomes with similar size, morphology and G banding pattern, which are found in two or more phylogenetically related species), were compared and correlated with values of total nuclear DNA. The information obtained indicates that: (a) shared chromosomes have variable amounts of DNA: (b) in a given species there is a correlation between the amount of nuclear and chromosomal DNA in most shared chromosomes (and perhaps in most of the chromosomal complement), e.g., the higher the amount of nuclear DNA, the higher the content of DNA in shared chromosomes; (c) some chromosomes may undergo processes of amplification or deletion restricted to certain regions and usually related with mechanisms of chromosomal rearrangements.(ABSTRACT TRUNCATED AT 250 WORDS)

Animals↗
PubMed1995

Allozymic characterization and evolutionary relationships in the Brazilian Akodon cursor species group (Rodentia-Cricetidae).

The present study involved an electrophoretic survey of 22 protein loci in 269 individuals belonging to three species of the genus Akodon, A. aff. cursor (2n = 16), A. cursor (2n = 14/15), and A. montensis (2n = 24/25/26), collected in Eastern Brazil. The joint results of gene diversity, genetic distances, phenetic analyses, and phylogenetic trees suggested that A. aff. cursor has recently separated from A. cursor and that the three species have experienced a recent chromosomal divergence followed by low allozyme differentiation. These data are in agreement with their classification as sibling species.

Alleles↗
PubMed1997

Determination of the evolutionary relationships in Rattus sensu lato (Rodentia : Muridae) using L1 (LINE-1) amplification events.

We determined approximately 215 bp of DNA sequence from the 3'-untranslated region (UTR) of 240 cloned L1 (LINE-1) elements isolated from 22 species of Rattus sensu lato and Rattus sensu stricto murine rodents. The sequences were sorted into different L1 subfamilies, and oligonucleotides cognate to them were hybridized to genomic DNA of various taxa. From the distribution of the L1 subfamilies in the various species, we inferred the partial phylogeny of Rattus sensu lato. The four Maxomys species comprise a well-defined clade separate from a monophyletic cluster that contains the two Leopoldamys and four Niviventer species. The Niviventer/Leopoldamys clade, in turn, shares a node with the clade that contains Berylmys, Sundamys, Bandicota, and Rattus sensu stricto. The evolutionary relationships that we deduced agree with and significantly extend the phylogeny of Rattus sensu lato established by other molecular criteria. Furthermore, the L1 amplification events scored here produced a unique phylogenetic tree, that is, in no case did a character (a given L1 amplification event) appear on more than one branch. The lack of homoplasy found in this study supports the robustness of L1 amplification events as phylogenetic markers for the study of mammalian evolution.

Animals↗
PubMed1999

A translocated mitochondrial cytochrome b pseudogene in voles (Rodentia: Microtus)

A full-length cytochrome b pseudogene was found in rodents; it has apparently been translocated from a mitochondrion to the nuclear genome in the subfamily Arvicolinae. The pseudogene (psi cytb) differed from its mitochondrial counterpart at 201 of 1143 sites (17.6%) and by four indels. Cumulative evidence suggests that the pseudogene has been translocated to the nucleus. Phylogenetic reconstruction indicates that the pseudogene arose before the diversification of M. arvalis/M. rossiaemeridionalis from M. oeconomus, but after the divergence of the peromyscine/sigmodontine/ arvicoline clades some approximately 10 MYA. Published rates of divergence between mitochondrial genes and their nuclear pseudogenes suggest that the translocation of this mitochondrial gene to the nuclear genome occurred some 6 MYA, in agreement with the phylogenetic evidence.

Animals↗
PubMed2000

Effects of opioid receptor blockade on defensive behavior elicited by electrical stimulation of the aversive substrates of the inferior colliculus in Rattus norvegicus (Rodentia, Muridae).

RATIONALE: Electrical or chemical stimulation of some structures of the midbrain tectum, such as the dorsal periaqueductal gray matter, deep layers of the superior colliculus and inferior colliculus induce fear and flight behavior. These structures constitute the main neural substrates commanding defensive behavior in brainstem. Many neurotransmitters are implicated in the modulation of aversion at the mesencephalic level. OBJECTIVE: The aim of this work is to investigate the involvement of opioid mechanisms in modulation of defensive behavior in dorsal mesencephalon. METHODS: Male Wistar rats were fixed in a stereotaxic frame and a chemitrode was implanted into the midbrain, targeted to the central nucleus of the inferior colliculus. In the present study, the effects of peripheral and central administration of naloxone, naltrexone or naloxonazine on aversive thresholds (freezing and escape reactions) elicited by electrical stimulation of the midbrain tectum were determined. RESULTS: Peripherally and centrally administered naloxone caused a significant increase in the freezing and flight thresholds elicited by electrical stimulation of the aversive substrates of the inferior colliculus. These effects were confirmed by peripheral and central administration of naltrexone and by microinjections of naloxonazine in inferior colliculus. CONCLUSIONS: These findings suggest that endogenous opioids are involved in the modulation of the aversive behavior elicited by midbrain tectum stimulation. Since microinjections of naloxonazine in the central nucleus of the inferior colliculus caused a significant increase in the aversive thresholds elicited by electrical stimulation of this structure, it is possible that micro1 opioid receptor located in this nucleus may be critically implicated in this neural circuitry.

Animals↗
PubMed2004

Evolution of MHC-DRB class II polymorphism in the genus Apodemus and a comparison of DRB sequences within the family Muridae (Mammalia: Rodentia).

Allelic diversity at major histocompatibility complex (MHC) genes is thought to be maintained by balancing selection over long periods of time, even across multiple speciation events. Trans-species sharing of MHC alleles among genera has been supported by many studies on mammals and fish, but in rodents, the results are ambiguous. We investigated natural levels of MHC- DRB variability and evolutionary processes in the wood mouse (Apodemus sylvaticus) and the yellow-necked mouse (Apodemus flavicollis), which are common, sympatric murid rodents in European forests. Using single-strand conformation polymorphism analysis and DNA sequencing, 38 DRB exon 2 alleles were detected among 162 A. sylvaticus from nine different locations in Germany and Switzerland, and 15 DRB exon 2 alleles were detected among 60 A. flavicollis from three different locations in northern Germany. There was evidence for balancing selection in both species. Phylogenetic analysis, including additional murid taxa, showed that the DRB exon 2 sequences did not separate according to species, consistent with trans-species evolution of the MHC in these taxa.

Alleles↗
PubMed1999

Allelic diversity at the Mhc-DQA locus in cotton rats (Sigmodon hispidus) and a comparison of DQA sequences within the family muridae (Mammalia: Rodentia).

The cotton rat (Sigmodon hispidus) is a common murid rodent of the southern United States, Mexico, and Central America. Using single-stranded conformation polymorphism analysis and DNA sequencing techniques, 11 DQA exon 2 alleles were detected among 180 S. hispidus from Caddo County, Oklahoma, USA. The alleles represent a single locus exhibiting a high level of polymorphism. Nucleotide and amino acid distance values among DQA alleles of S. hispidus were higher than those within Mus musculus and species of Rattus. Although the distribution of polymorphic amino acid residues among alleles of S. hispidus was similiar to that of Mus and Rattus, some residues of the alpha-helix region were more variable in S. hispidus. Comparisons of nonsynonymous and synonymous substitutions indicated a trend toward higher numbers of nonsynonymous substitutions; however, this difference was not significant statistically among S. hispidus alleles. To examine evolution of DQA alleles within Muridae, we performed a phylogenetic analysis that included DQA alleles from S. hispidus, Peromyscus leucopus, M. musculus, R. norvegicus, and six Australian species of Rattus. Results depicted monophyly for each genus, and this concordance between species and gene trees represents a lack of evidence for trans-species persistence of alleles among these genera.

Alleles↗