魚類の種分化プロセスにおける交雑と遺伝子浸透

抄録

Hybridization and introgression have been considered important aspects in the speciation process and evolution of reproductive barriers. Although most hybrid zones of animals have been consideredas ‘tension zones, ’ maintained by dispersal/selection balance of hybrids, many of the hybrid zones of freshwater fishes may lead to hybrid swarm. On the other hand, some of the hybrid zones of diadromous, brackishwater and maritime fishes may, infact, be ‘tensionzones’ and widespread introgression of genes. These differences may be dependent upondispersal abilities, because migration from ‘pure’ populations to a hybrid zone (and vise versa) can be expected in widely dispersible species, but not in drainage-restricted freshwater fishes. Although many of the examples of fish hybrid zones may have originated fbllowing secondary contact of geographically-isolated populations, reinfbrcement of reproductive barriers have not appeared. In particular, hybrid swarms of freshwater fishes have often emerged as hybrid-origin species. In some cases in brackishwater species, mtDNA trees have suggested long term hybridization, there being no evidence for reinfbrcement. One of the reasons fbr the continuing existence of hybrid zones (and genetic differences among populations) may be environmental selective pressures in ecotones, because ‘tensionzones’ of fishes originally fbrmed in a transitional zone between different environments.