Biogeographic history of Cichlasoma sanctifranciscense Kullander (Cichliformes: Cichlidae) in the Northeastern Mata Atlântica and São Francisco freshwater ecoregions

The genus Cichlasoma currently comprises 13 valid species, occurring mainly in the Amazon river basin and adjacent areas. Among them, Cichlasoma sanctifranciscense is the only representative of the genus found in both the Northeastern Mata Atlântica (NMAF) and São Francisco (SF) freshwater ecoregions. Within the NMAF, its distribution is concentrated primarily in the Northern Group, an area recognized as having the highest level of ichthyofaunal exchange with the São Francisco river basin. This distribution pattern is consistent with the hypothesis that the São Francisco river was historically connected to the Itapicuru and Vaza-Barris rivers (NMAF). To evaluate this hypothesis, this study aimed to investigate, describe, and analyze the phylogeographic pattern of C. sanctifranciscense across the Northeastern Mata Atlântica and São Francisco freshwater ecoregions. To this end, mitochondrial Cytochrome Oxidase I (COI) sequences (618 bp) were obtained and analyzed from 35 specimens (NMAF = 21; SF = 14) collected from eight localities in the SF and seven in the NMAF. Bayesian phylogenetic inference recovered the monophyly of the species with strong statistical support. The haplotype network analysis revealed 15 haplotypes, including 11 exclusive and four shared haplotypes. Furthermore, haplotypes were shared both among populations within the NMAF and between the NMAF and São Francisco ecoregions. Interestingly, this inter-ecoregional haplotype sharing involved the Salitre river population, a right-bank tributary of the São Francisco, and populations from the NMAF. The AMOVA results revealed very low genetic differentiation between the two ecoregions (FST = 0.00178; p>0.05), suggesting recent gene flow. Although preliminary, these findings provide additional support for the hypothesis of historical connectivity between the SF and NMAF freshwater ecoregions. Nevertheless, additional studies, particularly those incorporating more molecular markers (nuDNA) and additional analytical approaches, are still needed to refine our understanding of how these historical connections were established.