Experimental analysis of ecological and genetic effects upon native rare species by the invasion of exotic related species

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Mechanism of native species extinction by the invasion of exotic related species was analyzed by ecological and ethological methods, together with that of population genetics, under the rearing experiments of Rhode us ocellatus kurumeus (Rok) and R. o. ocellatus (Roo). Mating behavior and reproductive success were examined in aquaria (6 pairs of Rok and 3 pairs of Roo), and changes in genotype and gene frequencies were monitored in an artificial pond (50 pairs of Rok and 5 pairs of Roo). In addition, the condition of Rok extinction caused by the invasion of Roo was theoretically inferred by using computer simulation.In aquaria, Roo was superior to Rok in mating behavior of males and reproductivity of females. Mating success was lower between subspecies, especially between Roo (♂) X Rok (♀). Mating frequency was the highest in Rok, and the next between subspecies. However, genotype of larvae was the highest in hybrids between Roo ( ♀) X Rok (♂). In pond, genotype of larvae was the highest in hybrids, while it was the lowest in Rok, which has not been observed since the next year of experiments. Hybrids increased to 96% in the third year, and gene frequency of Roo in hybrids amounted to about 70% in all the loci examined.In conclusion, it was suggested that the factors causing the extinction of Rok by the invasion of Roo are different between individuals and genes. The extinction of Rok individuals is caused by mating behavior, reproductivity, heterosis in embryogenesis, and lacks of premating and postmating isolations, while that of genes is caused by the difference in gene fitness. The validity of this suggestion was confirmed by the computer simulation that incorporated the above-mentioned factors under the various initial conditions.