Evolutionary cytogenetics of the Hoplias lacerdae, Miranda Ribeiro, 1908 group. A particular pathway concerning the other Erythrinidae fish

High Genetic Diversity despite Conserved Karyotype Organization in the Giant Trahiras from Genus Hoplias (Characiformes, Erythrinidae)

In the fish genus Hoplias, two major general groups can be found, one of which is formed by the “common trahiras” (Hoplias malabaricus group) and the other by the “giant trahiras” (Hoplias lacerdae group, in addition to Hoplias aimara), which usually comprises specimens of larger body size. Previous investigations from the giant trahiras group recovered 2n = 50 meta/submetacentric chromosomes and no sex chromosome differentiation, indicating a probable conservative pattern for their karyotype organization. Here, we conducted comparative cytogenetic studies in six giant trahiras species, two of them for the first time. We employed standard and advanced molecular cytogenetics procedures, including comparative genomic hybridization (CGH), as well as genomic assessments of diversity levels and phylogenetic relationships among them. The results strongly suggest that the giant trahiras have a particular and differentiated evolutionary pathway inside the Hoplias genus. While these species share the same 2n and karyotypes, their congeneric species of the H. malabaricus group show a notable chromosomal diversity in number, morphology, and sex chromosome systems. However, at the same time, significant changes were characterized at their inner chromosomal level, as well as in their genetic diversity, highlighting their current relationships resulting from different evolutionary histories.

Comparative Cytogenetics of Hoplerythrinus unitaeniatus (Agassiz, 1829) (Characiformes, Erythrinidae) Species Complex from Different Brazilian Hydrographic Basins

Chromosomal characteristics of Hoplerythrinus unitaeniatus populations from 5 Brazilian river basins, namely Arinos (Amazonas basin), Araguaia, Paraguai, Alto Paraná, and São Francisco were analyzed by conventional Giemsa staining, C-banding, silver nitrate impregnation, and fluorescence in situ hybridization (FISH) with 18S and 5S rDNA and telomeric sequence (TTAGGG)n probes. The diploid chromosome number was 2n = 48 in representatives of the populations from Paraguai and Alto Paraná River basins and 2n = 52 for those from the Arinos and Araguaia River basins. The São Francisco population had individuals with 2n = 50 and 52 occurring in sympatry. C-banding showed heterochromatic blocks mainly located at interstitial and pericentromeric positions in most of the chromosomes. Silver nitrate impregnation demonstrated simple NORs for representatives from Arinos and Araguaia River populations and multiple NORs for specimens from Paraguai, Alto Paraná, and São Francisco River populations. FISH with 18S and 5S rDNA probes revealed many chromosomes carrying these cistrons, with up to 21 chromosomes bearing 18S rDNA sites (Alto Rio Paraná basin) and up to 12 chromosomes with 5S rDNA sites (Paraguai basin), besides the occurrence of colocalization in all populations. FISH with telomeric sequence (TTAGGG)n detected sites in the terminal portion of the chromosomes in all populations. These data reinforce the idea that H. unitaeniatus is a species complex. Evolutionary and biogeographical aspects of the group in the Neotropical region are discussed.

Comparative cytogenetics in the genus Hoplias (Characiformes, Erythrinidae) highlights contrasting karyotype evolution among congeneric species

BACKGROUND

The Erythrinidae fish family contains three genera, Hoplias, Erythrinus and Hoplerythrinus widely distributed in Neotropical region. Remarkably, species from this family are characterized by an extensive karyotype diversity, with 2n ranging from 39 to 54 chromosomes and the occurrence of single and/or multiple sex chromosome systems in some species. However, inside the Hoplias genus, while H. malabaricus was subject of many studies, the cytogenetics of other congeneric species remains poorly explored. In this study, we have investigated chromosomal characteristics of four Hoplias species, namely H. lacerdae, H. brasiliensis, H. intermedius and H. aimara. We used conventional staining techniques (C-banding, Ag-impregnation and CMA3 -fluorescence) as well as fluorescence in situ hybridization (FISH) with minor and major rDNA and microsatellite DNAs as probes in order to analyze the karyotype evolution within the genus.

RESULTS

All species showed invariably 2n = 50 chromosomes and practically identical karyotypes dominated only by meta- and submetacentric chromosomes, the absence of heteromorphic sex chromosomes, similar pattern of C-positive heterochromatin blocks and homologous Ag-NOR-bearing pairs. The cytogenetic mapping of five repetitive DNA sequences revealed some particular interspecific differences between them. However, the examined chromosomal characteristics indicate that their speciation was not associated with major changes in their karyotypes.

CONCLUSION

Such conserved karyotypes contrasts with the extensive karyotype diversity that has been observed in other Erythrinidae species, particularly in the congeneric species H. malabaricus. Nevertheless, what forces drive such particularly different modes of karyotype evolution among closely related species? Different life styles, population structure and inner chromosomal characteristics related to similar cases in other vertebrate groups can also account for the contrasting modes of karyotype evolution in Hoplias genus.

Can DNA barcoding accurately discriminate megadiverse Neotropical freshwater fish fauna?

BACKGROUND

The megadiverse Neotropical freshwater ichthyofauna is the richest in the world with approximately 6,000 recognized species. Interestingly, they are distributed among only 17 orders, and almost 80% of them belong to only three orders: Characiformes, Siluriformes and Perciformes. Moreover, evidence based on molecular data has shown that most of the diversification of the Neotropical ichthyofauna occurred recently. These characteristics make the taxonomy and identification of this fauna a great challenge, even when using molecular approaches. In this context, the present study aimed to test the effectiveness of the barcoding methodology (COI gene) to identify the mega diverse freshwater fish fauna from the Neotropical region. For this purpose, 254 species of fishes were analyzed from the Upper Parana River basin, an area representative of the larger Neotropical region.

RESULTS

Of the 254 species analyzed, 252 were correctly identified by their barcode sequences (99.2%). The main K2P intra- and inter-specific genetic divergence values (0.3% and 6.8%, respectively) were relatively low compared with similar values reported in the literature, reflecting the higher number of closely related species belonging to a few higher taxa and their recent radiation. Moreover, for 84 pairs of species that showed low levels of genetic divergence (<2%), application of a complementary character-based nucleotide diagnostic approach proved useful in discriminating them. Additionally, 14 species displayed high intra-specific genetic divergence (>2%), pointing to at least 23 strong candidates for new species.

CONCLUSIONS

Our study is the first to examine a large number of freshwater fish species from the Neotropical area, including a large number of closely related species. The results confirmed the efficacy of the barcoding methodology to identify a recently radiated, megadiverse fauna, discriminating 99.2% of the analyzed species. The power of the barcode sequences to identify species, even with low interspecific divergence, gives us an idea of the distribution of inter-specific genetic divergence in these megadiverse fauna. The results also revealed hidden genetic divergences suggestive of reproductive isolation and putative cryptic speciation in some species (23 candidates for new species). Finally, our study constituted an important contribution to the international Barcoding of Life (iBOL.org) project, providing barcode sequences for use in identification of these species by experts and non-experts, and allowing them to be available for use in other applications.