Clinal Variation of Lateral Plates in Threespine Stickleback Fish

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Abstract
The Ventura River [California, USA] 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 2 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/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/fish, common in California populations that lack predatory fishes. Within most of this area, LP numbers tend to increase going upstream. This spatial variation indicates an interaction between different selection regimes in upstream and downstream areas and asymmetrical gene flow with a downstream bias. Threespine sticklebacks actively disperse, and in the Ventura River system, dispersal appears to accomplish gene flow. In the upstream area, trout prediation causes high LP counts at all but the most downstream station, where trout are absent. Gene flow from above causes high LP counts at this station. Gene flow from the upstream area causes the weak cline for LP number within the downstream area. Homogeneity of allozymes among samples is consistent with the view that the cline is a result of population differentiation, not secondary contact between divergent populations, and that there is gene flow within the system. Comparison of LP numbers of samples made at 1 site in different years indicates that the cline is unstable. Allozyme variability is unusually low in Ventura River sticklebacks. Predatory fishes probably have the same selective effect on stickleback LP number in California as they do in the Pacific northwest. It tends to corroborate the importance of natural selection in geographical variation of threespine sticklebcks, but demonstrates that gene flow can influence variation within a drainage.

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