An experimental study on strain combinations in heterosis in salinity tolerance of the guppy Poecilia reticulata

As an experimental model of heterosis in fish, the present study examined t he amount of heterosis in salinity tolerance in four crosses between guppy strains having various kinships. Salinity tolerance was measured as surviva l time after transfer from fresh water to 35 ppt seawater. The amount of he terosis, expressed by the difference between the means of the parents and t heir F-1 hybrids in every pair, was larger in the strain combinations havin g distant kinship than in those having close kinship. The result indicates that the average amount of heterosis is dependent on the magnitude of kinsh ip between the strains. The average survival times of the F-1 hybrids did n ot correlate with those of female parents, male parents, or mid-parent valu es but correlated closely with the average amount of heterosis in the four strain combinations, indicating that the average phenotypic value of the F- 1 hybrids is not influenced by that of their parents but is dependent on th e average amount of heterosis brought by the strain combination. Within eac h strain combination, variance in the amount of heterosis among pairs was l arger in the strain combinations having close kinship than in those having distant kinship. These results indicate that the amount of heterosis was la rger and more uniform in the strain combinations having distant kinship tha n in those having close kinship. The present study experimentally demonstra tes that the strain combination having distant kinship is more useful for g enetic improvement utilizing heterosis in fish.