Abstract
As an experimental model of heterosis in fish, the present study examined the amount of heterosis in salinity tolerance in four crosses between guppy strains having various kinships. Salinity tolerance was measured as survival time after transfer from fresh water to 35 ppt seawater. The amount of heterosis, expressed by the difference between the means of the parents and their F1 hybrids in every pair, was larger in the strain combinations having distant kinship than in those having close kinship. The result indicates that the average amount of heterosis is dependent on the magnitude of kinship between the strains. The average survival times of the F1 hybrids did not correlate with those of female parents, male parents, or mid-parent values but correlated closely with the average amount of heterosis in the four strain combinations, indicating that the average phenotypic value of the F1 hybrids is not influenced by that of their parents but is dependent on the average amount of heterosis brought by the strain combination. Within each strain combination, variance in the amount of heterosis among pairs was larger in the strain combinations having close kinship than in those having distant kinship. These results indicate that the amount of heterosis was larger and more uniform in the strain combinations having distant kinship than in those having close kinship. The present study experimentally demonstrates that the strain combination having distant kinship is more useful for genetic improvement utilizing heterosis in fish.
