Species limits and introgression in Pimelodus from the Magdalena-Cauca River basin.

Low morphological differentiation among taxa hampers their appropriate identification, basic biological studies and promotion of any conservation effort. Aiming to provide the first insight into the evolution and speciation among north-western South American members of Pimelodus, this study tested the hypothesis that Pimelodus yuma, Pimelodus grosskopfii and Pimelodus crypticus represent three independently evolving species and explored signals of interspecific hybridisation. The outcomes based on mitochondrial (cox1) and nuclear (RADseq, microsatellites and rag2) markers combined with coalescence-based and allele-frequency methods confirmed that each studied species represents an independently evolving unit. Contrary to expectations, P. yuma was found as a sister clade of P. blochii, while P. crypticus (formerly confused with P. blochii) was phylogenetically closer to P. grosskopfii. We also found strong evidence of ancient introgression (0.66-3.32 mya) between the non-sibling species P. yuma and P. grosskopfii, breaking the absence of interbreeding and the independent evolutionary trajectory among north-western South American Pimelodus during their diversification history, a pre-requisite to define species limits. However, there were non-significant values of current gene flow between them, supporting the hypothesis of full isolation.