Prevalent Introgression Underlies Convergent Evolution in the Diversification of Pungitius Sticklebacks

New mutations and standing genetic variations contribute significantly to repeated phenotypic evolution in sticklebacks. However, less is known about the role of introgression in this process. We analyzed taxonomically and geographically comprehensive genomic data from Pungitius sticklebacks to decipher the extent of introgression and its consequences for the diversification of this genus. Our results demonstrate that introgression is more prevalent than suggested by earlier studies. Although gene flow was generally bidirectional, it was often asymmetric and left unequal genomic signatures in hybridizing species, which might, at least partly, be due to biased hybridization and/or population size differences. In several cases, introgression of variants from one species to another was accompanied by transitions of pelvic and/or lateral plate structures—important diagnostic traits in Pungitius systematics—and frequently left signatures of adaptation in the core gene regulatory networks of armor trait development. This finding suggests that introgression has been an important source of genetic variation and enabled phenotypic convergence among Pungitius sticklebacks. The results highlight the importance of introgression of genetic variation as a source of adaptive variation underlying key ecological and taxonomic traits. Taken together, our study indicates that introgression-driven convergence likely explains the long-standing challenges in resolving the taxonomy and systematics of this small but phenotypically highly diverse group of fish.