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The genus Gymnotus is a large monophyletic group of freshwater weakly-electric fishes, with wide distribution in Central and South America. It has 46 valid species divided into six subgenera (Gymnotus, Tijax, Tigre, Lamontianus, Tigrinus and Pantherus) with large chromosome plasticity and diploid numbers (2n) ranging from 34 to 54. Within this rich diversity, there is controversy about whether Gymnotus (Gymnotus) carapo species is a single widespread species or a complex of cryptic species. Cytogenetic studies show different diploid numbers for G. carapo species, ranging from 40 to 54 chromosomes with varied karyotypes found even between populations sharing the same 2n. Whole chromosome painting has been used in studies on fish species and recently has been used for tracking the chromosomal evolution of Gymnotus and assisting in its cytotaxonomy. Comparative genomic mapping using chromosome painting has shown more complex rearrangements in Gymnotus carapo than shown in previous studies by classical cytogenetics. These studies demonstrate that multiple chromosome pairs are involved in its chromosomal reorganization, suggesting the presence of a complex of cryptic species due to a post zygotic barrier. In the present study, metaphase chromosomes of G. carapo occidentalis “catalão” (GCC, 2n = 40, 30m/sm+10st/a) from the Catalão Lake, Amazonas, Brazil, were hybridized with whole chromosome probes derived from the chromosomes of G. carapo (GCA, 2n = 42, 30m/sm+12st/a). The results reveal chromosome rearrangements and a high number of repetitive DNA sites. Of the 12 pairs of G. carapo chromosomes that could be individually identified (GCA 1–3, 6, 7, 9, 14, 16 and 18–21), 8 pairs (GCA 1, 2, 6, 7, 9, 14, 20, 21) had homeology conserved in GCC. Of the GCA pairs that are grouped (GCA [4, 8], [5, 17], [10, 11] and [12, 13, 15]), most kept the number of signals in GCC (GCA [5, 17], [10, 11] and [12, 13, 15]). The remaining chromosomes are rearranged in the GCC karyotype. Analysis of both populations of the G. carapo cytotypes shows extensive karyotype reorganization. Along with previous studies, this suggests that the different cytotypes analyzed here may represent different species and supports the hypothesis that G. carapo is not a single widespread species, but a group of cryptic species.

Although crocodilians have attracted enormous attention in other research fields, from the cytogenetic point of view, this group remains understudied. Here, we analyzed the karyotypes of eight species formally described from the Alligatoridae family using differential staining, fluorescence in situ hybridization with rDNA and repetitive motifs as a probe, whole chromosome painting (WCP), and comparative genome hybridization. All Caimaninae species have a diploid chromosome number (2n) 42 and karyotypes dominated by acrocentric chromosomes, in contrast to both species of Alligatorinae, which have 2n = 32 and karyotypes that are predominantly metacentric, suggesting fusion/fission rearrangements. Our WCP results supported this scenario by revealing the homeology of the largest metacentric pair present in both Alligator spp. with two smaller pairs of acrocentrics in Caimaninae species. The clusters of 18S rDNA were found on one chromosome pair in all species, except for Paleosuchus spp., which possessed three chromosome pairs bearing these sites. Similarly, comparative genomic hybridization demonstrated an advanced stage of sequence divergence among the caiman genomes, with Paleosuchus standing out as the most divergent. Thus, although Alligatoridae exhibited rather low species diversity and some level of karyotype stasis, their genomic content indicates that they are not as conserved as previously thought. These new data deepen the discussion of cytotaxonomy in this family.

Farlowella schreitmuelleri Arnold, 1936. Farlowella schreitmuelleri also had a diploid chromosome number 58 and karyotype composed of 10m + 30m +14st+4a, and FN = 112 (Fig. 1d). A secondary constriction was observed in the distal region of the long arms of the acrocentric pair No. 28, together with size heteromorphism, corresponding to the Ag-NOR (Fig. 1f), the ribosomal 18S DNA (Fig. 2e), and positive C-banding sites. The genome of this species possessed a much larger quantity of heterochromatin compared to Farlowella cf. amazonum, located in the centromeric region of all chromosomes, where some bands were more conspicuous than others. Interstitial bands also occurred in the long arms of pairs Nos. 7, 9, 14, 22, and 23. Pairs Nos.8, 24, 25, and 28 also had conspicuous bands in the terminal region of the long arms (Fig. 1e). Multiple 5S rDNA sites were observed, in sm pair No. 16 (in only one of the homologs) and st pair No. 26. Both these sites were positive for C-banding (Fig. 2f). Hybridiza- tion with telomeric probes revealed terminal signals in all chromosomes and Interstitial Telomeric Sequences (ITSs) in pairs Nos. 7, 14, 25, and 28, all of which corresponded with heterochromatin blocks. In pair No. 28, the ITS was observed in only one of the homologs (Fig. 2b)., Published as part of Marajó, Leandro, Viana, Patrik F., Ferreira, Milena, Rapp Py-Daniel, Lúcia H. & Feldberg, Eliana, 2018, Cytogenetics of two Farlowella species (Loricariidae: Loricariinae): implications on the taxonomic status of the species, pp. 1-8 in Neotropical Ichthyology 16 (4) on pages 2-3, DOI: 10.1590/1982-0224-20180029, http://zenodo.org/record/3710253