Cytogenetic analysis of the genus Thoropa Cope, 1865 (Anura-Cycloramphidae) with evolutionary inferences based on repetitive sequences

Abstract

Cytogenetics can be a useful tool to assist in taxonomic problems by adding information to the widely used morphological and molecular approaches. These taxonomic problems are especially common in anurans, once they are very diverse, highly polymorphic, and present many cryptic species. The genus Thoropa Cope, 1865 is composed of six specialist species that reproduce in rocky outcrops and are distributed throughout the Atlantic Forest and Cerrado ecotones. Phylogenetic studies point to possible cryptic species within the T. miliaris group. To assist in the evolutionary and taxonomic understanding of this group, classical cytogenetic techniques were used to find possible molecular markers for the genus through rDNA5S, rDNA18S, and U2snDNA probes and analyze their chromosome distribution in the group of T. miliaris. Despite the well conserved karyotype under conventional staining and classical techniques, such as Ag-NOR, our C-banding results showed differences in the centromeric heterochromatin concentration between two populations of T. miliaris. Furthermore, some differences among the populations and species were found for rDNA5S and U2snDNA. This study contributes to a better understanding of the evolutionary relationships within the genus; however, the use of different probe sequences, such as satDNA, is essential for a more robust cytogenetic analysis.

Cytogenetic characterization and mapping of the repetitive DNAs in Cycloramphus bolitoglossus (Werner, 1897): More clues for the chromosome evolution in the genus Cycloramphus (Anura, Cycloramphidae)

“…The karyotype of C . bolitoglossus , described here, presents a highly conserved diploid number of 2n = 26 chromosomes, as reported previously in other cycloramphids [ 5 – 8 , 53 ]. Despite this conserved diploid number, interspecific variation in the fundamental number (FN = 50 or 52) indicates that intrachromosomal rearrangements may have played a central role in this genus chromosomal diversification.…”

“… The hypothesis of the evolutionary chromosomal changes in the Cycloramphus and Zachaenus species, as inferred from the available cytogenetic data [ 5 – 8 , 53 ] in the phylogeny of the genus presented by de Sá et al [ 9 ]. Events of character transformation are indicated: (a) centromere repositioning in chromosome pair 1 of C .…”

“…acangatan and C . fuliginosus , all the Cycloramphus species that have been karyotyped to date [ 7 , 53 ], including C . bolitoglossus , have rDNA 45S sites in pair 6, detected by Ag-NOR method.…”

“…bolitoglossus . In the genus Thoropa (which is closely related to Cycloramphus ) the large metacentric chromosome 1 also showed to harbor the 5S rDNA sites in most of the species [ 53 ]. Based on this, we may infer that chromosome 1 of C .…”

Evolutionary insights into the genomic organization of major ribosomal DNA in ant chromosomes

“…Species with multiple rDNA clusters do not usually exhibit silver‐staining in all clusters (reviewed by Sumner, 2003; Vicari et al ., 2008). However, it must be assumed that if a species has only a single NOR (or single 45S rDNA gene site), then it will be transcriptionally active, as evidenced for different organisms (Dobigny et al ., 2002; Barros et al ., 2015; Falcione et al ., 2018; Cholak et al ., 2020; Malimpensa et al ., 2020).…”