Basic cytogenetics and physical mapping of ribosomal genes in four Astyanax species (Characiformes, Characidae) collected in Middle Paraná River, Iguassu National Park: considerations on taxonomy and systematics of the genus

Species delimitation in Psalidodon fasciatus (Cuvier, 1819) complex (Teleostei: Characidae) from three hydrographic basins

Psalidodon fasciatus is a complex of several fish species widely distributed in Brazilian hydrographic systems that share morphological characteristics and show a high taxonomic complexity and genetic variability. Cytogenetic and molecular analyses were carried out in populations from three Brazilian hydrographic basins, aiming to contribute to systematic and biogeographical knowledge of the group. The chromosomal markers verified the occurrence of species-specific characters and indicated the existence of six distinct operational taxonomic units (OTUs): P. fasciatus (São Francisco River basin), Psalidodon sp. 1, Psalidodon sp. 2 and Psalidodon eigenmanniorum (Uruguay River basin); and Psalidodon sp. 3 and Psalidodon sp. 4 (Paraná River basin). The chromosomal markers showed more similarities among species from the Uruguay River and São Francisco River basins. DNA barcoding analyses (assemble species by automatic partitioning, neighbour-joining, maximum likelihood and maximum parsimony) indicated the existence of at least three distinct OTUs. The chromosomal evolution rates were demonstrated to be higher than the molecular evolution rates, reinforcing the importance of using chromosomal markers to delimit OTUs in integrative taxonomy studies. These results suggest that the São Francisco River Basin population (the type locality) should be considered as true P. fasciatus, and the others, until now treated as Psalidodon aff. fasciatus, are cryptic species.

B chromosomes of multiple species have intense evolutionary dynamics and accumulated genes related to important biological processes

BACKGROUND

One of the biggest challenges in chromosome biology is to understand the occurrence and complex genetics of the extra, non-essential karyotype elements, commonly known as supernumerary or B chromosomes (Bs). The non-Mendelian inheritance and non-pairing abilities of B chromosomes make them an interesting model for genomics studies, thus bringing to bear different questions about their genetic composition, evolutionary survival, maintenance and functional role inside the cell. This study uncovers these phenomena in multiple species that we considered as representative organisms of both vertebrate and invertebrate models for B chromosome analysis.

RESULTS

We sequenced the genomes of three animal species including two fishes Astyanax mexicanus and Astyanax correntinus, and a grasshopper Abracris flavolineata, each with and without Bs, and identified their B-localized genes and repeat contents. We detected unique sequences occurring exclusively on Bs and discovered various evolutionary patterns of genomic rearrangements associated to Bs. In situ hybridization and quantitative polymerase chain reactions further validated our genomic approach confirming detection of sequences on Bs. The functional annotation of B sequences showed that the B chromosome comprises regions of gene fragments, novel genes, and intact genes, which encode a diverse set of functions related to important biological processes such as metabolism, morphogenesis, reproduction, transposition, recombination, cell cycle and chromosomes functions which might be important for their evolutionary success.

CONCLUSIONS

This study reveals the genomic structure, composition and function of Bs, which provide new insights for theories of B chromosome evolution. The selfish behavior of Bs seems to be favored by gained genes/sequences.

Detection of natural hybridization and delimitation of two closely related operational taxonomic units of the Astyanax fasciatus (Teleostei: Characidae) complex through integrative approaches

Astyanax is a species-rich, non-monophyletic genus composed of several supraspecific taxa that are poorly delimited. The Astyanax fasciatus complex is one of these taxa and shows high taxonomic complexity. To elucidate the evolutionary history of the A. fasciatus complex from southern South America, we conducted cytogenetic, molecular and morphological analyses in specimens from the Uruguay River basin. Cytogenetic characters demonstrated two closely related operational taxonomic units: Astyanax sp. 1 (8m+22sm+10st+6a), Astyanax sp. 2 (8m+24sm+10st+4a) and natural hybrids (8m+23sm+8st+5a). 5S ribosomal DNA sites were found in two pairs of m chromosomes and one pair of a chromosomes in Astyanax sp. 1, two pairs of a chromosomes and one pair of m chromosomes in Astyanax sp. 2, and three m chromosomes and three a chromosomes in hybrids. As51 sites were found in three chromosomes in Astyanax sp. 1 and in five chromosomes in Astyanax sp. 2 and hybrids. Mitochondrial sequence analyses did not separate the two units and hybrids. Morphological analyses revealed differences between Astyanax sp. 2 and hybrids. This secondary contact with gene flow between lineages that diverged long ago might slow or reverse the differentiation/speciation process. These results help us to understand the evolutionary history of this highly complex clade of Astyanax in southern South America.

Molecular Clock as Insight to Estimate the Evolutionary History and Times of Divergence for 10 Nominal Astyanax Species (Characiformes, Characidae): An Evolutionary Approach in Species with 2n = 36, 46, 48, and 50 Chromosomes

Astyanax is a genus with a wide distribution ranging from the south United States to north of Patagonia (Argentina). The available cytogenetic data on Astyanax indicate a high karyotypic diversity, with diploid number of 36-52 chromosomes, presence of B chromosomes, heterochromatin polymorphism, and variations with respect to the number and localization of nucleolar organizer regions (NORs) and 18S and 5S ribosomal DNA sites. In the present study, we estimated the evolutionary history and times of divergence for 10 nominal Astyanax species from the South and Central/North American (Cna) continents, which present distinct chromosomal characteristics, based on molecular clocks inferred from mitochondrial DNA sequence. The molecular clock results indicate the origin of three distinct clades (Humeral dark spot [Hds]; Diffuse humeral spot [Dhs]; Cna group) during the late Miocene about 11.2 million years ago (Mya). Thus, Astyanax mexicanus (Cna) represent a species that diverged a long time ago (∼8.6 Mya) from the Hds group, and Astyanax schubarti is the oldest species (∼6.5 Mya) among the Dhs species.

Trends in Karyotype Evolution in Astyanax (Teleostei, Characiformes, Characidae): Insights From Molecular Data

The study of patterns and evolutionary processes in neotropical fish is not always an easy task due the wide distribution of major fish groups in large and extensive river basins. Thus, it is not always possible to detect or correlate possible effects of chromosome rearrangements in the evolution of biodiversity. In the Astyanax genus, chromosome data obtained since the 1970s have shown evidence of cryptic species, karyotypic plasticity, supernumerary chromosomes, triploidies, and minor chromosomal rearrangements. In the present work, we map and discuss the main chromosomal events compatible with the molecular evolution of the genus Astyanax (Characiformes, Characidae) using mitochondrial DNA sequence data, in the search for major chromosome evolutionary trends within this taxon.

Chromosomal characterization in two species of an Astyanax bimaculatus complex (Characidae, Characiformes) using different techniques of chromosome banding

Astyanax has been the subject of extensive cytogenetic studies due to its wide karyotypic diversity. This genus comprises species complexes, namely groups of fish of difficult morphological differentiation, such as the bimaculatus complex, which includes the characids with a rounded humeral spot. Thence, the present study proposed to accomplish a cytogenetic characterization of two species of this complex: A. asuncionensis and A. altiparanae, aiming to find chromosomal markers that differentiate these species, as well as achieve a better understanding of the karyotype evolution in the genus. For this we used different techniques of chromosome banding as C-banding, impregnation by silver nitrate, fluorochrome staining and FISH with 18S rDNA probe. This is the first cytogenetic study in A. asuncionensis, from Miranda river, which presented 2n = 50 and 18 m + 22sm + 6st + 4a (FN = 96) and single NORs. The populations of A. altiparanae also presented 2n = 50, but with different karyotypic formulae: the population of the Quexada river presented 16 m + 24sm + 4st + 6a (FN = 94) and the Esperança stream and Jacutinga river showed 16 m + 20sm + 4st + 10a (FN = 90). All analyzed populations showed an interindividual variation in the number and location of the nucleolar organizer regions (NORs). Single and multiple NORs were detected either by impregnation with silver nitrate or by FISH with 18S rDNA probe. After C-banding, the two species differed in relation to the composition and heterochromatin distribution. The meiotic cells of A. altiparanae male individuals were also analyzed, showing that, despite the high karyotype variability, chromosome pairing occurs normally. The data show that A. altiparanae and A. asuncionensis share some characteristics with other species of the bimaculatus complex, suggesting a close phylogenetic relationship among those species. However, some features can be used as differentiation chromosomal markers in altiparanae/asuncionensis morphotypes, which could result from a natural speciation process.