Capoeta, a genus of hybrid origin?

It has long been suspected that the relationship of Capoeta is with the Oriental algae scrapers of the genera Onychostoma, Semiplotus, and Scaphiodonichthys. Since, mitochondrial molecular markers have demonstrated that Capoeta is closely related to the genera Aulopyge, Barbus and Luciobarbus. All mitochondrial phylogenetic analyses indicate that Capoeta is nested within Luciobarbus. As is the case with the majority of Palearctic barbels, all Luciobarbus studied are tetraploid, with 100 chromosomes. Conversely, all Capoeta studied are hexaploid, with 150 chromosomes. The shift from tetraploidy to hexaploidy is a pivotal event in the evolution of Capoeta. Polyploidisation in fish is often associated with hybridisation, and it can be postulated that this was also the case in the origin of Capoeta. If a tetraploid mother (chromosomes in the egg: n = 2) and a diploid father (chromosome in the sperm cell: n = 1) (or vice versa) hybridise, the offspring is triploid (n = 3) and is likely to be sterile. It can be postulated that triploids cannot produce viable gametes.

It is plausible that these hybrids were sufficiently abundant in a certain situation to mate with each other and were occasionally able to produce mitotic gametes, i.e., gametes without meiotic division. These eggs and sperm should have been triploid (n = 3), and after fertilisation, a new fish with an even number of chromosomes, here six, would result. This hexaploid fish could then spawn normally with the other hexaploid hybrids, which might have constituted the initial step of the new lineage, the genus Capoeta. Support for this hybridisation event is further strengthened by nuclear DNA evidence from the RAG1 gene which, together with mitochondrial markers, suggests a biparental genomic contribution. While the mitochondrial data confirm Luciobarbus as the maternal source, the RAG1 tree indicates a mixed ancestry, consistent with an ancient Cyprinion-like paternal lineage, and reinforces the role of polyploidisation in the evolution of Capoeta. These hybridisation and polyploidisation events may have enabled the hybrids to occupy a different ecological niche than their parents, resulting in the evolution of reproductive isolation from their parents.

Further reading. Yang et al. 2015 (hybrid origin).