Ongoing Bidirectional Introgression and the Maintenance of Species Boundaries in Hemiculter Hybrid Zone

The proper delimitation of species plays an important role in taxonomy as well as in studies related to speciation. However, introgressive hybridization can confuse our ability to accurately construct phylogenetic relationships, particularly when two closely related species coexist and crossbreed. Studies have shown that Hemiculter leucisculus and H. tchangi coexist widely in the upper reaches of the Yangtze River, having similar life history and morphological characteristics, which may lead to the occurrence of hidden hybridization. However, natural hybridization has not been reported in the system. Here, we focus on the sympatric sister-species H. leucisculus and H. tchangi , using mitochondrial and nuclear loci to verify the occurrence of hybridization and introgression, and to explore the hybridization pattern and its evolutionary significance. Our results clearly revealed that H. leucisculus and H. tchangi showed extensive introgressive hybridization and provided evidence of ongoing hybridization. Genetic data showed that strength of the introgression varied by geographic locality, the number of hybrids shows a gradual increase from upstream to downstream, which is mainly influenced by the abundance of species. Morphological results displayed that hybrids did not exhibit the intermediate type, so it isn’t easy to distinguish hybrids through morphological and there were mismatching in morphological and genetic assignment results. The present study suggests that species boundaries are maintained in the face of ongoing introgression due to the differences in parental species. Our research will help to understand the role of interspecific gene flow in shaping species boundaries and provide insights to better understand closely related species interactions and to develop timely strategies regarding species conservation in the future.