Genomic markers confirm introgressive hybridization in Cichla (Teleostei) in an invaded coastal system

Hybridization seems frequent among Cichla species (peacock bass), as studies have reported hybrids in populations within and outside their natural range (the Amazon and Orinoco river basins). Cichla kelberi and C. monoculus were introduced to the São João river (Southeast Brazil) more than 20 years ago, where they successfully colonized different environments. In this system, specimens have coloration and morphology that differ from patterns observed in their native range, indicating that the contact between C. monoculus and C. kelberi eliminated reproductive isolation, leading to introgressive hybridization. In this context, we used the LWS and dlx2 genes and the random amplified polymorphic DNA technique to characterize Cichla species in the São João river and to detect possible hybridization processes. The genetic markers identified here showed mismatches between mitochondrial and nuclear DNA in C. monoculus and C. kelberi, sampled both in the reservoir and in the estuarine stretch. Fragments of nuclear DNA from both species, as well as the presence of mitochondrial DNA from one species associated with the nuclear genome of the other, provide strong evidence of hybridization associated with bidirectional genetic introgression. Because hybrids may be more resistant, competitive, and aggressive than their parental species, these invaders may pose a significant threat to native biodiversity in this and other coastal rivers of the region.