Asymmetric hybridization and introgression between sibling species of the pufferfish Takifugu that have undergone explosive speciation

Pufferfishes belonging to the genus Takifugu are a prominent example of recent adaptive radiations of marine fishes. Using amplified fragment length polymorphism (AFLP) and mitochondrial DNA (mtDNA) markers, the genetic characteristics of natural hybrids between two sibling species, Takifugu snyderi and Takifugu stictonotus, were investigated to gain insights into the role of hybridization in rapid diversification. Numerous early generations of hybrids (131 F-1 hybrids and 18 first-generation back-crosses) were screened by Bayesian assignment procedures from samples collected at three sites off the Pacific coast of eastern Honshu, Japan (Ibaraki: 36 degrees 21'N, 140 degrees 37'E; Fukushima: 37 degrees 03'N, 141 degrees 03'E; Iwate: 40 degrees 02'N, 141 degrees 59'E), during 2012-2014. Analysis of mtDNA indicated that hybridization is highly directional, as the majority of the F-1 hybrids (75.6%) were offspring between T. stictonotus females and T. snyderi males. Among the 18 backcrosses, 17 were toward T. snyderi and one was toward T. stictonotus. Two of 118 individuals classified as genetically pure T. snyderi based on AFLP markers were affected by mtDNA introgression from T. stictonotus. These results suggest that interspecific gene flow has been highly asymmetrical toward T. snyderi, which may partly explain the marked difference in intraspecific genetic diversity between the two species. The proportion of F-1 hybrids in the Ibaraki and Fukushima areas is exceptionally high compared with that of other marine fishes, indicating the need for continuous monitoring of hybridization and its impact on integrity of each parental species under the changing marine environment.