Surprising genomic diversity in the Neotropical fish Synbranchus marmoratus (Teleostei: Synbranchidae): how many species?

Abstract

Chromosomes and nuclear DNA content variation in the eel Synbranchus marmoratus were studied in seven samples taken from rio Paraguay and rio Paraná basins to examine whether karyotype and nuclear DNA content variations could offer useful information for the characterization of independent evolutionary lineages in the genus and construction of evolutionary and biogeographical hypotheses. The occurrence of different karyotypes was already described for this species; however, a new cytotype from rio Miranda was discovered. Nuclear DNA content showed a wide variation among samples and individuals, ranging from 5.2 to 9.1 pg of DNA/nuclei. An analysis of variance confirmed the occurrence of significant differences among samples. In a deeper series of analyses, a multimodality was found in the distribution of nuclear DNA content, in which several, more or less discrete units arose. Finally, combining karyotype formulae with nuclear DNA content, a complex relationship among the rivers sampled was found. Based on the available data, we suggest that several independent evolutionary lineages of Synbranchus marmoratus occur in the rivers sampled. Biogeographical hypotheses are proposed and discussed.<br>Os cromossomos e a variação de conteúdo de DNA nuclear foram estudados em sete amostras de Synbranchus marmoratus das bacias dos rios Paraguai e Paraná com o objetivo de avaliar se diferenças no cariótipo e no conteúdo de DNA nuclear poderiam fornecer informações para a caracterização de linhagens evolutivas independentes no gênero e para a elaboração de hipóteses evolutivas e biogeográficas. A ocorrência de diferentes cariótipos já foi descrita para essa espécie, entretanto, um novo citótipo foi encontrado no rio Miranda. O conteúdo de DNA nuclear mostrou uma ampla variação entre as amostras e indivíduos, com valores entre 5,2 a 9,1 pg de DNA/núcleo. Uma análise de variância confirmou a ocorrência de diferenças significativas entre as amostras. Em uma série de análises, um padrão multimodal foi encontrado na distribuição do conteúdo de DNA nuclear, pelo qual várias unidades, mais ou menos discretas, foram identificadas. Combinando a fórmula cariotípica com o conteúdo de DNA nuclear, uma relação complexa entre os rios foi observada. Com base nos dados disponíveis, sugerimos que existem várias linhagens evolutivas independentes de Synbranchus marmoratus nos rios amostrados. Hipóteses biogeográficas são propostas e discutidas

Molecular and cytogenetic analyses of cryptic species within the Synbranchus marmoratus Bloch, 1795 (Synbranchiformes: Synbranchidae) grouping: species delimitations, karyotypic evolution and intraspecific diversification

“…However, many morphological traits can be subtle or ambiguous, which often makes it difficult to recognize and describe new species. Therefore, it is necessary to increase the number of tools and characteristics available for differentiating species from this group (Nakamoto et al, 1986;Foresti et al, 1992;Melilo et al, 1996;Sanchez & Fenocchio, 1996;Perdices et al, 2005;Torres et al, 2005;Nirchio et al, 2011;Carvalho et al, 2012). In the present study, several karyomorphs of S. marmoratus collected in distinct Brazilian river basins were described.…”

“…Hypotheses concerning the primitive diploid number in Synbranchus have already been proposed, but lead to opposite predictions (Melilo et al, 1996;Torres et al, 2005). Herein, based on the karyotypic information plotted on a phylogenetic tree of S. marmoratus an alternative hypothesis concerning chromosomal differentiation patterns in Synbranchus can be suggested.…”

“…The occurrence of species with distinct diploid numbers in Synbranchidae reveals that fusion/fission rearrangements are fundamental mechanisms in the karyoevolution of this group (Torres et al, 2005;Carvalho et al, 2012). Hypotheses concerning the primitive diploid number in Synbranchus have already been proposed, but lead to opposite predictions (Melilo et al, 1996;Torres et al, 2005).…”

“…In addition, natural or unnatural random events, such as headwater capture or changes in watercourses, can lead to secondary contacts between previously separated species (Blanco et al, 2009;Peres et al, 2009), providing ideal scenarios to study novel species complexes. Indeed, such complexes have already been studied in a variety of fish orders, including Characiformes, Synbranchiformes, and Gymnotiformes (Moreira-Filho & Bertollo, 1991;Torres et al, 2005;Milhomem et al, 2008).…”

“…Although S. marmoratus specimens appear to belong to a single taxonomic group, some populations may display an extensive karyotype diversity, with diploid numbers ranging from 42 to 46 chromosomes; furthermore, variations in chromosome morphology as well as in the distribution of constitutive heterochromatin and rDNA genes have also been observed (Foresti et al, 1992;Melilo et al, 1996;Sanchez & Fenocchio, 1996;Torres et al, 2005). Although distinguishing the intraspecific S. marmoratus groups is relatively easy with cytogenetic tools, distinguishing between these groups based solely on morphology is often impossible (Rosen & Rumney, 1972), making the identification of new species difficult.…”