No evidence for temperature-dependent selection on lateral plate variation in the three-spined stickleback: a single-replicate mesocosm experiment

Reduction of lateral plates in the three-spined stickleback (Gasterosteus aculeatus Linnaeus, 1758) is a well-known example of parallel evolution. However, the environmental factors driving this variation remain unclear. Sticklebacks with reduced plates tend to occur more frequently in warmer habitats, whereas completely plated populations are more common in colder environments, suggesting a possible role of winter temperature in shaping lateral plate variation. We conducted a mesocosm experiment in two locations with contrasting winter temperatures and associated environmental conditions to examine whether lateral plate number influenced survival, growth, and maturation. Hybrids with varying plate numbers were released into outdoor mesocosms. Survival and growth were substantially reduced in the colder environment, particularly in males, and gonadal development was also delayed. In contrast, no significant difference in lateral plate number was detected between survivors from the two mesocosms, and lateral plate number showed no significant association with growth-related traits or condition factor. In conclusion, we found no detectable evidence that differences in winter environmental conditions generated differential selection on lateral plate number under the conditions of this experiment. Because each temperature treatment was represented by a single mesocosm, we cannot exclude the possibility that site-specific environmental factors contributed to the observed patterns.