Abstract
BackgroundSynbranchidae or swamp eels are fishes belonging to the order Synbranchiformes that occur in both freshwater and occasionally in brackish. They are worldwide distributed in tropical and subtropical rivers of four different continents. A large degree of chromosomal variation has been found in this family, mainly through the use of conventional cytogenetic investigations. Inside this group, a still almost unexplored species under the cytogenetic point of view is the Asian swamp eel Monopterus albus, a widely distributed species throughout Asia. Here, we tested the hypothesis of chromosomal speciation, where a case of sympatric speciation may occur as the primary consequence of chromosomal rearrangements. We performed a comparative chromosomal analysis of M. albus from 22 different localities in Thailand, using distinct staining methods (C-banding, Ag-NO3, and Chromomycin A3), and FISH with repetitive DNA probes (5S rDNA, 18S rDNA, Rex1 element and microsatellite repeats).ResultsThis approach evidenced two contrasting karyotypes (named karyomorphs A and B) that varied concerning their 2n and repetitive DNAs distribution, where chromosomal fusions and pericentric inversions were involved in such differentiation. While the karyomorph A has 2n = 24 chromosomes, the karyomorph B has only 2n = 18, both with NF = 24. In addition, karyomorph A contains only acrocentric chromosomes, while karyomorph B contains three unique metacentric pairs. These features highlight that M. albus has already gone through a significant genomic divergence, and may include at least two cryptic species.ConclusionsThis marked chromosomal differentiation, likely linked to the lifestyle of these fishes, point to the occurrence of a chromosomal speciation scenario, in which fusions and inversions had a prominent role. This highlights the biodiversity of M. albus and justifies its taxonomic revision, since this nominal species may constitute a species complex.
Highlights
Synbranchidae or swamp eels are fishes belonging to the order Synbranchiformes that occur in both freshwater and occasionally in brackish
This study presents a comparative chromosomal analysis of Monopterus albus from 22 different localities in Thailand (Fig. 2), using distinct staining methods (C-banding, Ag-NO3, and chromomycin A3) as well as fluorescence in situ hybridization (FISH) with repetitive DNA probes (5S ribosomal DNA (rDNA), 18S rDNA, Rex1 element and microsatellite repeats)
Distinguishable through cytogenetic analysis, specimens from both karyomorphs have the same morphology, making difficult the identification of such probable new species. The integration of both conventional and molecular cytogenetic approaches allowed the proposal of some chromosomal rearrangements probably related to the differentiation of both M. albus karyomorphs, where centric fusions appear as the main evolutionary sources shaping such a process (Fig. 3c)
Summary
Synbranchidae or swamp eels are fishes belonging to the order Synbranchiformes that occur in both freshwater and occasionally in brackish. A large degree of chromosomal variation has been found in this family, mainly through the use of conventional cytogenetic investigations Inside this group, a still almost unexplored species under the cytogenetic point of view is the Asian swamp eel Monopterus albus, a widely distributed species throughout Asia. The chromosomal rearrangements promote the reorganization of the genetic structure, and the evolutionary impact and the consequences at the speciation level can vary according to the rearrangement type, that is, inversion, fusion, fission or translocation [7,8,9,10,11,12] Such chromosomal rearrangements can facilitate adaptation to heterogeneous environments by limiting genomic recombination [10]. Repetitive DNAs, widely distributed in the eukaryotic genomes, are generally divided into two classes, one comprising tandem sequences (satellite DNAs, minisatellites and microsatellites), and the other comprising interspersed sequences, such as transposons and retrotransposons [14]
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