Impact of asymmetrical hybridization followed by sterile F1 hybrids on species replacement in Pseudorasbora

Pseudorasbora pumila, one of the endangered freshwater minnows in eastern Japan, has been largely replaced by the accidentally introduced species, P. parva, which originated from western Japan. In the contact zone, P. pumila and P. parva have hybridized intensively, producing sterile F-1 hybrids. The present study determined the maternal parent of F-1 hybrids using mtDNA haplotypes to investigate the mating system between P. parva and P. pumila in the hybrid zone. We also pursued the successive changes in the genetic structures of hybridizing populations over a 5-year period using allozymes. A total of 100 natural F-1 hybrids collected from six different populations had P. pumila mtDNA without exception, suggesting that sterile F-1 hybrids resulted from mating only between P. pumila females and P. parva males. Such asymmetrical hybridization implies that P. pumila females waste considerably greater reproductive efforts compared with P. parva males. The data suggest that the rapid replacement of P. pumila by P. parva has been promoted by asymmetrical hybridization, resulting in sterile F-1 hybrids.