CLINAL VARIATION OF LATERAL PLATES IN THREESPINE STICKLEBACK FISH

The Ventura River contains threespine sticklebacks and rainbow trout, a stickleback predator that selects for lateral plate (LP) number. Lateral plates are modified scales that occur as single bilateral rows and their number is highly heritable. Lateral plate counts of Ventura River sticklebacks form a step cline with two homogeneous areas separated by a partial barrier (ephemeral stream) to dispersal. The upstream area has high mean LP counts and a mode at 14 plates per fish, characteristic of populations subject to fish predation. All stations in this area except the most downstream one are sympatric with trout. The downstream area has lower LP counts and a mode at 10 plates per fish, common in California populations that lack predatory fishes. Within most of this area LP numbers tend to increase going upstream. This spatial variation is interpreted to indicate an interaction between different selection regimes in upstream and downstream areas and asymmetrical gene flow with a downstream bias. Threespin...