Phylogeographic relationships and the evolutionary history of the Carassius auratus complex with a newly born homodiploid raw fish (2nNCRC).

BACKGROUND: An important aspect of studying evolution is to understand how new species are formed and their uniqueness is maintained. Hybridization can lead to the formation of new species through reorganization of the adaptive system and significant changes in phenotype. Interestingly, eight stable strains of 2nNCRC derived from interspecies hybridization have been established in our laboratory. To examine the phylogeographical pattern of the widely distributed genus Carassius across Eurasia and investigate the possible homoploid hybrid origin of the Carassius auratus complex lineage in light of past climatic events, the mitochondrial genome (mtDNA) and one nuclear DNA were used to reconstruct the phylogenetic relationship between the C. auratus complex and 2nNCRC and to assess how demographic history, dispersal and barriers to gene flow have led to the current distribution of the C. auratus complex. RESULTS: As expected, 2nNCRC had a very close relationship with the C. auratus complex and similar morphological characteristics to those of the C. auratus complex, which is genetically distinct from the other three species of Carassius. The estimation of divergence time and ancestral state demonstrated that the C. auratus complex possibly originated from the Yangtze River basin in China. There were seven sublineages of the C. auratus complex across Eurasia and at least four mtDNA lineages endemic to particular geographical regions in China. The primary colonization route from China to Mongolia and the Far East (Russia) occurred during the Late Pliocene, and the diversification of other sublineages of the C. auratus complex specifically coincided with the interglacial stage during the Early and Mid-Pleistocene in China. CONCLUSION: Our results support the origin of the C. auratus complex in China, and its wide distribution across Eurasia was mainly due to natural Pleistocene dispersal and recent anthropogenic translocation. The sympatric distribution of the ancestral area for both parents of 2nNCRC and the C. auratus complex, as well as the significant changes in the structure of pharyngeal teeth and morphological characteristics between 2nNCRC and its parents, imply that homoploid hybrid speciation (HHS) for C. auratus could likely have occurred in nature. The diversification pattern indicated an independent evolutionary history of the C. auratus complex, which was not separated from the most recent common ancestor of C. carassius or C. cuvieri. Considering that the paleoclimate oscillation and the development of an eastward-flowing drainage system during the Pliocene and Pleistocene in China provided an opportunity for hybridization between divergent lineages, the formation of 2nNCRC in our laboratory could be a good candidate for explaining the HHS of C. auratus in nature.

Spatiotemporal characteristics of the pharyngeal teeth in interspecific distant hybrids of cyprinid fish: Phylogeny and expression of the initiation marker genes.

As an important feeding organ and taxonomical characteristic, the pharyngeal teeth of cyprinid fish have very high morphological diversity and exhibit species-specific numbers and arrangements. Many genes have been verified to regulate the pharyngeal teeth development and act as the initiation marker for teeth. Six initiation marker genes for pharyngeal teeth were used as RNA probes to investigate the expression pattern, and these genes were further used to construct a phylogenetic tree for cyprinid fish including some distant hybrids. The results from in situ hybridization showed that similarities and differences existed in the expression of dlx2b, dlx4b, dlx5a, pitx2, fth1b, and scpp5 in the pharyngeal region of the hybrids (BT) by the crosses of blunt snout bream (BSB, ♀) × topmouth culter (TC, ♂). Particularly, we found a high specificity marker gene scpp5 for the early development of pharyngeal teeth. The Scpp5 expression pattern established a clear graphic representation on the spatiotemporal characteristics of the early morphogenesis of pharyngeal teeth in BT and BSB. Our results suggested that the scpp5 expression in 4V1, 3V1, and 5V1 in BT occurred earlier than that in BSB, while the replacement rate of pharyngeal teeth (4V2, 3V2, and 5V2) was faster in BSB. Phylogenetic analysis revealed that the six marker genes were highly conserved and could be used as the molecular marker for identifying the parents of the distant hybrids in cyprinid fish. The expression patterns of the scpp5 gene was examined in various tissues, including the brain, gill, heart, liver, muscle, skin, fins, gonad, eye, and kidney, showing that the scpp5 gene was ubiquitously expressed, indicating its important role in cyprinid fish.