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The relationships of salinity tolerance to immunolocalization of Na+,K(+)-ATPase in the gill epithelium were examined during seawater and freshwater adaptation of the guppy. In fresh water, immunoreactivity for Na+,K(+)-ATPase appeared in two types of chloride cells, which are located on the primary lamellae of the gills. Immunoreactivity was strong in the chloride cells located at the base of the secondary lamellae and weak in the chloride cells located at the interlamellar region. During seawater adaptation, the strongly-immunoreactive chloride-cells increased in number and size while the weakly-immunoreactive chloride-cells decreased in number with an increase in salinity tolerance. In the fish of the seawater-adapted strain, on the other hand, most of the chloride cells were located at the base of the secondary lamellae and showed strong immunoreactivity. During freshwater adaptation, the strongly-immunoreactive chloride-cells decreased in number and size while the weakly-immunoreactive chloride-cells increased in number with a decrease in salinity tolerance. A positive correlation was observed between the salinity tolerance and the occupying area of the strongly-immunoreactive chloride-cells while a negative correlation was observed between the salinity tolerance and the occupying area of the weakly-immunoreactive chloride-cells during the seawater and freshwater adaptation. These results directly suggested that not only the occupying area of chloride cells but also the expression of Na+,K(+)-ATPase protein in the cells is important with respect to the osmoregulatory function in the gills and hypoosmoregulatory ability at the individual level.

Wild and cultured populations of the Pacific oyster, Crassostrea gigas, were analysed by starch gel electrophoresis for catalase (CAT). The comparison of genotypic frequencies at the Cat locus revealed an apparent advantage of heterozygotes in the wild and cultured populations.The observed frequency of heterozygotes in each population was higher than the expected frequency.This result suggests a case of balanced polymorphism.The genotypes of the Cat locus were associated with marked differences in early mortality during the cultured period.The survival rate of each homozygote was lower than the mean survival rate in the population.

SUMMARY: 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.