Abstract
BackgroundThe combination of classical cytogenetics with molecular techniques represents a powerful approach for the comparative analysis of the genome, providing data for the systematic identification of chromosomal homologies among species and insights into patterns of chromosomal evolution within phylogenetically related groups. Here, we present cytogenetic data on four species of Neotropical treefrogs of the genus Phyllomedusa (P. vaillantii, P. tarsius, P. distincta, and P. bahiana), collected in Brazil and Ecuador, with the aim of contributing to the understanding of the chromosomal diversification of this genus.ResultsWith the exception of P. tarsius, which presented three telocentric pairs, all the species analyzed had conservative karyotypic features. Heterochromatic patterns in the genomes of these species revealed by C-banding and fluorochrome staining indicated the presence of a large number of non-centromeric blocks. Using the Ag-NOR method and FISH with an rDNA 28S probe, we detected NOR in the pericentromeric region of the short arm of pair 7 in P. vaillantii, pair 1 in P. tarsius, chromosomes 1 and 9 in P. distincta, and in chromosome 9 in P. bahiana, in addition to the presence of NOR in one homologue of chromosome pair 10 in some individuals of this species. As expected, the telomeric probe detected the terminal regions of the chromosomes of these four species, although it also detected Interstitial Telomeric Sequences (ITS) in some chromosomes of the P. vaillantii, P. distincta and P. bahiana karyotypes.ConclusionA number of conservative chromosomal structures permitted the recognition of karyotypic homologies. The data indicate that the presence of a NOR-bearing chromosome in pair 9 is the plesiomorphic condition in the P. burmeisteri group. The interspecific and intraspecific variation in the number and location of rDNA sites reflects the rapid rate of evolution of this character in Phyllomedusa. The ITS detected in this study does not appear to be a remnant of structural chromosome rearrangements. Telomeric repeats were frequently found in association with heterochromatin regions, primarily in the centromeres, which suggests that (TTAGGG)n repeats might be an important component of this heterochromatin. We propose that the ITSs originated independently during the chromosomal evolution of these species and may provide important insights into the role of these repeats in vertebrate karyotype diversification.
Highlights
The combination of classical cytogenetics with molecular techniques represents a powerful approach for the comparative analysis of the genome, providing data for the systematic identification of chromosomal homologies among species and insights into patterns of chromosomal evolution within phylogenetically related groups
Karyological features have been widely accessed by classical methods, and advances in molecular cytogenetics based on Fluorescence in situ hybridization (FISH) experiments have resulted in improved chromosomal mapping of large numbers of sequences and permitted the study of chromosomal variation
Secondary constrictions was observed in the pericentromeric region of the short arm of pair 7 in P. vaillantii karyotype, in the pair 1 in P. tarsius, pairs 1 and 9 in P. distincta, and in chromosomes 9 in P. bahiana karyotype, besides secondary constriction in one homologue of the pair 10
Summary
The combination of classical cytogenetics with molecular techniques represents a powerful approach for the comparative analysis of the genome, providing data for the systematic identification of chromosomal homologies among species and insights into patterns of chromosomal evolution within phylogenetically related groups. The ribosomal RNA gene, which is a repetitive DNA sequence that is organized in tandem, is widely used in chromosomal investigations and provides a good chromosomal marker for comparative cytogenetic studies [2]. This sequence shows several features of ‘hotspots’ of chromosomal recombination because it consists of a clustered organization of repeats and is frequently located in pericentromeric and subtelomeric regions [3]. The association of the NOR repositioning events due to the presence of transposable elements has already been noted [8,9]
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