Evidence for type assortative mating between the freshwater and Omono types of nine-spined stickleback in natural fresh water

A total of 123 adult specimens of the nine-spined stickleback Pungitius pungitius were identified as either the freshwater or Omono types, based on diagnostic allozyme loci, and their allozyme patterns compared with restriction fragment length polymorphism patterns of polymerase chain reaction-amplified mtDNA segments (mtDNA PCR-RFLP pattern). All specimens of the two types were clearly identified by the mtDNA PCR-RFLP patterns, except for a single putative hybrid progeny. The mtDNA PCR-RFLP patterns of 133 clutches from 21 nests were also compared with those of their parental males, and 126 out of 127 clutches were found to be consistent with the parental male pattern, the remaining six clutches being guarded by an F₁ hybrid male. The results demonstrated that type assortative mating had occurred between the two types in nature, indicating the existence of a premating isolating mechanism between them.