Abstract
To investigate the origin of the freshwater form of the threespine stickleback, Gasterosteus aculeatus, genetic differentiation among local populations of the species was elucidated from isozyme genetic markers of skeletal muscle and liver tissue by means of an electrophoretic method. The mean value of the genetic distance between the freshwater and anadromous forms of this species was very large (D= 0.6746), almost at the level for congeneric species pairs in both freshwater and marine fishes, whereas the mean values of pairs within the respective forms were very small (D=0.0006-0.0015). The level of genetic variability of the freshwater form was distinctly lower than that of the anadromous form. Results suggest that local populations of the freshwater form from northern Japan have been derived monophyletically from a common ancestor which was established before the recent postglacial colonization.
