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Ecological genetics of threespine sticklebacks. (Gasterosteus) Hay, Douglas Edward


The threespine stickleback (Gasterosteus aculeatus L.) is a highly varied and widely distributed species of small fish. Throughout much of its range there are two distinct forms: a marine or anadromous form, trachurus, with 30-35 lateral bony plates (or scales), and a freshwater form, leiurus, with 7, or fewer, plates. In some areas another form, called semiarmatus, with intermediate plate numbers, is abundant. Previous studies have emphasized the number of lateral plates as a criterion of phenotypic assessment. However, this criterion is arbitrary and can be misleading. Therefore, in this study, new criteria are described that permit distinction of phenotypes by the presence or absence of certain, individual plates. Leiurus and semiarmatus, previously defined by the number of plates, are re-defined: all semiarmatus possess a particular plate that is absent in leiurus. All semiarmatus can be unequivocally distinguished from leiurus, although there is considerable phenotypic variation within each group. Phenotypic frequencies were assessed by periodic collections from two populations: a leiurus population and a mixed population of leiurus and semiarmatus. The relative frequency of two different leiurus phenotypes changed markedly between generations but were constant within generations (about one year). In the mixed population, the semiarmatus phenotypes increased in frequency, within generations. Field and laboratory tests indicate that the changes in frequency are not due to error in the sampling methods, error in assessment of phenotype, or differential dispersal among the phenotypes. Changes in the frequency of semiarmatus can be explained by differential predation by size-selective predators such as the cutthroat trout (Salmo clarki) and the water scorpion (Ranatra fusca). Both of these predators tend to selectively prey on the smallest sticklebacks available. Early in each generation, leiurus phenotypes are smaller than the semiarmatus phenotypes, but with time, the size difference decreases, and the frequency of semiarmatus increases. This suggests that size-selective predators initially eliminate more leiurus than semiarmatus, and the effect of this is an increase on the frequency of semiarmatus each generation. Differences in certain aspects of reproductive biology were examined as causes of the changes between generations. These included: (1) inheritance of plates; (2) viabilities of crosses; (3) phenotypic composition of breeding fish; (3) fecundity; (4) assortive mating. The phenotypic composition of breeding fish, and the inheritance of plates are the most important factors that affect changes in phenotypic frequencies between generations. In the pure leiurus populations; the phenotypic composition of breeders was significantly different from the population at large. This suggests that leiurus phenotypes may differ in their reproductive potential. The inheritance of the plates (or phenotypes) is especially important as a factor that changes the frequency of semiarmatus, Crosses among leiurus never produce any semiarmatus progeny, but crosses among semiarmatus, and between semiarmatus and leiurus, produce many leiurus progeny. Consequently, the frequency of semiarmatus is higher among breeding fish than it is among the young-of-the-year in the next generation. Leiurus x trachurus crosses produce semiarmatus progeny but semiarmatus is often abundant in populations where trachurus is absent. In these areas semiarmatus must consistently replicate itself. Therefore, contrary to previous suggestions, it is not a phenotype that arises only by introgression or hybridization between leiurus and trachurus. In mixed populations where trachurus is absent, semiarmatus must increase in frequency within generations or gradually go extinct. Recurrent extinction of semiarmatus seems a reasonable explanation for the origin of leiurus populations.

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