The role of the chromosomal rearrangements in the evolution and speciation of Elopiformes fishes (Teleostei; Elopomorpha)

Abstract Elopiformes is one of the most ancient orders of Teleostei fishes and includes the Megalopidae and Elopidae families. Although relevant to scientific research, cytogenetic and molecular data are lacking for both families. Here, we compared Elopiformes species from the perspective of genetic divergence and cytogenetics to elucidate the chromosomal diversification and evolutionary processes of Elopiformes. The cytogenetic results showed that Megalops atlanticus and Elops smithi are 2n = 50, with differences in their chromosomal formulas. The distributions of heterochromatin and repetitive DNA sequences revealed many characteristics shared between Elopiformes and other basal lineages of Teleostei fishes. In addition, the results suggested that different chromosomal rearrangements have occurred in the analysed Elopiformes species. The species tree indicated that Megalopidae and Elopidae were monophyletic groups, but the genetic divergence was higher for Megalops species than for Elops species. The small divergence and sympatric distribution of Elops species may not prevent hybridization. However, the karyotype differentiation may be an effective post-zygotic mechanism preventing continuous hybridization, and it could be the key factor in this speciation process. In conclusion, this is the first report of Elopiformes species that integrates molecular and cytogenetic data. Our results emphasize the importance of cytogenetic data for further defining Elopidae species.