Binding of nuclear factors to a satellite DNA of retroviral origin with marked differences in copy number among species of the rodent Ctenomys.

The major satellite DNA of the subterranean rodent Ctenomys, named RPCS, contains several consensus sequences characteristic of the U3 region of retroviral long terminal repeats (LTRs), such as a polypurine tract, CCAAT boxes, binding sites for the CCAAT/enhancer-binding protein (C/EBP), a TATA box and putative polyadenylation signals. RPCS presents an enormous variation in abundance between species of the same genus: while C. australis or C. talarum have approximately 3 x 10(6) copies per genome, C. opimus has none. A sequence (RPCS-I) with identity to the SV40-enhancer core element, present in all the repeating units of the satellite is specifically protected in DNase I footprintings. Competitions of band-shift assays with different transcription factor binding sites indicate that binding to RPCS-I is specific and involves CCAAT proteins related to NF-1, but not to C/EBP. By the use of quantitative protein/DNA binding assays we determined that, despite of their conspicuous difference in RPCS copy number, C. talarum and C. opimus have equivalent amounts and identical quality of RPCS-binding proteins. These results are consistent with the observation, by in situ hybridization, that RPCS is clustered in heterochromatic regions, where it might have restricted accessibility to transcription factors in vivo. This is the first report of the binding of transcription factors to a satellite DNA of retroviral origin.

The evolutionary history of Drosophila buzzatii. XXVII. Thorax length is positively correlated with longevity in a natural population from Argentina.

The correlation between body size and longevity was tested in an Argentinian natural population of Drosophila buzzatii. Mean thorax length of flies newly emerging from rotting cladodes of Opuntia vulgaris was significantly smaller than that of two samples of flies caught at baits. The present results which might be interpreted as directional selection for longevity favoring larger flies are in agreement with previous results achieved in a Spanish natural population of D. buzzatii. Flies emerging from different substrates showed significant differences in thorax length, suggesting that an important fraction of phenotypic variance can be attributed to environmental variability. However, laboratory and field work in different populations of D. buzzatii showed a significant genetic component for thorax length variation.

Retroviral-like features in the monomer of the major satellite DNA from the South American rodents of the genus Ctenomys.

It is well known that uninfected mammalian cells contain DNA sequences which are closely related to retroviral genomic segments. However, these sequences seldom (if ever) have been found associated to highly repetitive (satellite) DNA. RPCS is a 348 bp monomer of a major satellite DNA from the South American rodents of the genus Ctenomys. It was found that RPCS contains several elements which are typical of the U3 region of retroviral LTRs. These elements are: a) a polypurine tract; b) two enhancer core sequences; c) two NF1 binding sites; d) two C/EBP binding sites; e) two CCAAT-motifs; f) a TATA box, and g) two putative polyadenylation motifs. Furthermore, the relative positions of these elements are as in the U3 retroviral regions.

The evolutionary history of Drosophila buzzatii. XXIV. Second chromosome inversions have different average effects on thorax length.

We demonstrate a genetic correlation between rearrangements of the second chromosome of D. buzzatii and thorax length, as a measure of body size. The results indicate that 2j and 2jz3 arrangements are correlated with large size, whereas 2st arrangement is correlated with small size. Some inversions (2st and 2jz3) show dominant effects and others (2j/jz3) exhibit overdominance. These results show that at least 25 per cent of body size variation may be accounted for by the studied karyotypes. The possible integration of the genotypic, phenotypic and fitness levels, and also the possible implications to life-history evolution theories, are discussed. These results suggest that, under moderate to high heritability values, some kinds of chromosomal endocyclic and/or balancing selection may be valuable mechanisms for maintenance of body size variation.

Cytogenetics and karyosystematics of South American oryzomyine rodents (Cricetidae: Sigmodontinae). IV. Karyotypes of Venezuelan, Trinidadian, and Argentinian water rats of the genus Nectomys.

Bone marrow chromosomes were studied in South American water rats of the genus Nectomys from Venezuela, Trinidad, and Argentina. Specimens of N. squamipes from western and southern Venezuela showed a 2n = 52-53 karyotype, whereas a 2n = 56-57 karyotype was found in specimens from northeastern Argentina. In both cases, odd karyotypes can be explained by the presence of a supernumerary chromosome. In contrast, water rats from northeastern Venezuela and Trinidad showed a strikingly reduced 2n = 16-17 polymorphic chromosome complement. Six different karyomorphs were found among the latter, which may have resulted from a combination of pericentric inversions in two pairs of autosomes and a centromeric fusion in another autosomal pair. It is proposed that the new 2n = 16-17 cytotypes belong to a species of its own, for which the name N. palmipes is suggested.

DNA hybridization evidence for the Australasian affinity of the American marsupial Dromiciops australis.

DNA hybridization was used to compare representatives of the major groups of marsupials and a eutherian outgroup. Because of the large genetic distances separating marsupial families, trees were calculated from normalized percentages of hybridization; thermal-melting statistics, however, gave identical topologies for the well-supported clades. The most notable results were the association of the only extant microbiotheriid, Dromiciops australis, an American marsupial, with the Australasian Diprotodontia, and of both together with the Dasyuridae. Estimates of the rate of divergence among marsupial genomes suggest that the Dromiciops-Diprotodontia split occurred approximately 50 million years ago, well after the establishment of the major clades of marsupials but before deep oceanic barriers prohibited dispersal among Australia, Antarctica, and South America. Because Dromiciops is nested within an Australasian group, it seems likely that dispersal from Australia accounts for its present distribution.

Evidence for rolling-circle replication in a major satellite DNA from the South American rodents of the genus Ctenomys.

A major PvuII satellite DNA has been cloned from a South American octodontid rodent of the genus Ctenomys (C. porteousi). The satellite monomer, termed RPCS, is 337 bp in size and 42% G + C. Analysis of the nucleotide sequence demonstrates that RPCS is not composed of a series of shorter repeats. RPCS-related sequences were found in 11 of 12 Ctenomys species analyzed by hybridization under high-stringency conditions. The only negative species, C. opimus, was reactive under low-stringency conditions. RPCS-related sequences were not found under high- or low-stringency conditions in Calomys musculinus and Mus musculus. However, under low-stringency conditions, RPCS-related sequences were found in the octodontid Octodontomys gliroides, which is thought to have diverged from the genus Ctenomys more than 10 Myr ago. The pattern of periodicities observed, by restriction analysis, between Ctenomys species in the satellite array can be mainly accounted for by a rolling-circle amplification mechanism but cannot be solely accounted for by unequal crossing-over.

Cytogenetics and karyosystematics of phyllotine rodents (Cricetidae, Sigmodontinae). I. Chromosome multiformity and gonosomal-autosomal translocation in Reithrodon.

The chromosomes of 14 specimens of the genus Reithrodon from three different localities of Argentina and two localities of Uruguay were studied using G- and C-banding techniques. Specimens of Uruguay showed a karyotype of 2n = 28 chromosomes having a large metacentric X, and a telocentric Y chromosome. This karyotype is very similar to that recently described in a sample from southern Brazil, differing only in the nature of the Y chromosome, which is metacentric in the Brazilian form. All specimens from Argentina showed a 2n = 34 karyotype, differing from the Brazilian karyotype by two centric fusions, an acquisition of chromosome material, and at least one pericentric inversion, and by the telocentric nature of both the X and the Y chromosomes. G- and C-banding suggest that the metacentric gonosomes in the Brazilian form resulted from a double autosomal-X-Y Robertsonian translocation. The Uruguayan cytotype is interpreted as derived from a hypothetical neo-X/Y1Y2 ancestral form by the secondary loss of the Y1 chromosome. The karyotypic differences between the Brazilian-Uruguayan and the Argentinian forms afford evidence of species differentiation. It is proposed to assign the former to Reithrodon typicus, and the later to R. auritus.

Archosaurian reptiles: a new hypothesis on their origins.

The characteristics of the first archosaurs, the proterosuchian thecodonts, show that neither of the supposed common ancestors of archosaurs and lepidosaurs could actually be an ancestor of archosaurs. Instead, the evidence seems to indicate that the archosaurian ancestors are probably in the ophiacodont-varanopsid group of the pelycosaurian synapsids. In particular, the Varanopsidae are strongly indicative of proterosuchian relationships, as they have evolved some characters which are elsewhere found only in archosaurs. Archosaurs and lepidosaurs apparently have different origins; the former come from the pelycosaurs, and the latter come from the captorhinomorph cotylosaurs through the Millerettiformes.