Abstract
BackgroundThe frogs of the Tribe Cophomantini present, in general, 2n = 24 karyotype, but data on Aplastodiscus showed variation in diploid number from 2n = 24 to 2n = 18. Five species were karyotyped, one of them for the first time, using conventional and molecular cytogenetic techniques, with the aim to perform a comprehensive comparative analysis towards the understanding of chromosome evolution in light of the phylogeny.ResultsAplastodiscus perviridis showed 2n = 24, A. arildae and A. eugenioi, 2n = 22, A. callipygius, 2n = 20, and A. leucopygius, 2n = 18. In the metaphase I cells of two species only bivalents occurred, whereas in A. arildae, A. callipygius, and A. leucopygius one tetravalent was also observed besides the bivalents. BrdU incorporation produced replication bands especially in the largest chromosomes, and a relatively good banding correspondence was noticed among some of them. Silver impregnation and FISH with an rDNA probe identified a single NOR pair: the 11 in A. perviridis and A. arildae; the 6 in A. eugenioi; and the 9 in A. callipygius and A. leucopygius. C-banding showed a predominantly centromeric distribution of the heterochromatin, and in one of the species distinct molecular composition was revealed by CMA3. The telomeric probe hybridised all chromosome ends and additionally disclosed the presence of telomere-like sequences in centromeric regions of three species.ConclusionsBased on the hypothesis of 2n = 24 ancestral karyotype for Aplastodiscus, and considering the karyotype differences and similarities, two evolutionary pathways through fusion events were suggested. One of them corresponded to the reduction of 2n = 24 to 22, and the other, the reduction of 2n = 24 to 20, and subsequently to 18. Regarding the NOR, two conditions were recognised: plesiomorphy, represented by the homeologous small-sized NOR-bearing pairs, and derivation, represented by the NOR in a medium-sized pair. In spite of the apparent uniformity of C-banding patterns, heterogeneity in the molecular composition of some repetitive regions was revealed by CMA3 staining and by interstitial telomeric labelling. The meiotic tetravalent might be due to minute reciprocal translocations or to non-chiasmatic ectopic pairing between terminal repetitive sequences. The comparative cytogenetic analysis allowed to outline the chromosome evolution and contributed to enlighten the relationships within the genus Aplastodiscus.
Highlights
The frogs of the Tribe Cophomantini present, in general, 2n = 24 karyotype, but data on Aplastodiscus showed variation in diploid number from 2n = 24 to 2n = 18
The 15 known species of Aplastodiscus are currently distributed into three groups: the A. albofrenatus group (A. albofrenatus, A. arildae, A. ehrhardti, A. eugenioi, A. weygoldti, and A. musicus), the A. albosignatus group (A. albosignatus, A. callipygius, A. cavicola, A. flumineus, A. ibirapitanga, A. leucopygius, and A. sibilatus), and the A. perviridis group (A. cochranae and A. perviridis) [7]
The slides were standard stained with Giemsa, and submitted to nucleolar organiser region marked by silver impregnation (Ag-Nucleolar organiser region (NOR)) technique [16], C-banding [17], fluorochrome staining with AT-specific DAPI and GC-specific Chromomycin A3 (CMA3)
Summary
Many questions regarding the taxonomy of this genus remained, because the traits used for its characterisation were shared with representatives of the genus Hyla [1]. This fact led to the assignment of the name Hyla perviridis [2], and this species was included in the H. albomarginata group, along with H. albomarginata, H. albosignata, and H. albofrenata, due to, among other characters, the green colour typical of the species [3]. Based on morphological and bioacoustics data, as well as breeding behaviour of Hyla cochranae and H. perviridis [1], the genus Aplastodiscus was re-characterised, but the authors emphasised that further taxonomic studies were still necessary. The 15 known species of Aplastodiscus are currently distributed into three groups: the A. albofrenatus group (A. albofrenatus, A. arildae, A. ehrhardti, A. eugenioi, A. weygoldti, and A. musicus), the A. albosignatus group (A. albosignatus, A. callipygius, A. cavicola, A. flumineus, A. ibirapitanga, A. leucopygius, and A. sibilatus), and the A. perviridis group (A. cochranae and A. perviridis) [7]
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