Reproductive Isolation Among Lateral Plate Phenotypes (Low, Partial, Complete) of the Threespine Stickleback, Gasterosteus Aculeatus, From the White Sea Basin and the Kamchatka Peninsula, Russia

Reproductive isolation was investigated among sympatric lateral plate morphs of threespine stickleback from the White Sea basin and also among phenotypically similar morphs from the distant Kamchatka River basin (Lake Azabachije). Female choice tests show that gene flow is restricted among the completely plated and low plated morphs at both locations; behavioural isolation between these morphs is complete among Lake Azabachije fish, and nearly so (93% positive assortative mating) among White Sea basin fish. However, the experiments also demonstrate that there are no barriers to reproduction among the Azabachije and White Sea complete morphs, among the Azabachije low and White Sea complete morphs, nor among the Azabachije complete and White Sea low morphs. In addition, there is no evidence of barriers to gene flow among the low and partially plated morphs. Therefore, although gene flow is restricted among the extreme morphs within each locality, nevertheless gene exchange is possible, either directly or secondarily, among all phenotypes. The reproductive isolation between the complete and low morphs from the White Sea basin developed in situ no more than eight generations after the sticklebacks were introduced into an isolated freshwater pond. Therefore behavioural isolation can evolve very rapidly among the lateral plate phenotypes of Gasterosteus aculeatus.