Data from: Multiple evolutionary pathways to decreased lateral plate coverage in freshwater threespine sticklebacks

Adaptation to novel environments can be based either on standing genetic variation or variation attributable to new mutations. When standing genetic variation for a functional adaptation is lacking, and variation due to new mutations is not yet available, adaptation is possible only through alternative functional solutions. Reduction in the number of bony lateral plates as an adaptation to freshwater colonization by marine threespine sticklebacks (Gasterosteus aculeatus) has occurred in numerous independent cases through allelic substitution in the ectodysplasin-a (Eda) gene. Studying the phenotypic and genetic variation in plate number and size in five marine and six freshwater threespine stickleback populations, we found that when variation in Eda was limiting (i.e. alleles associated with the low-plate morph were missing or in extremely low frequency), plate number reduction did not take place in freshwater populations, but reduced lateral plate coverage was achieved by a reduction in the size of lateral plates. Our results suggest that this phenotypically and genetically discrete “small-plated” threespine stickleback – which is the dominant form in three northern European freshwater populations – may be functionally equivalent to the low-plated morph and hence, serve as an example of convergent evolution towards functional similarity in the face of genetic constraints.

生物对新生环境的适应性演化，既可基于留存遗传变异（standing genetic variation），也可源自新发突变所带来的遗传变异。当针对某一功能性适应的留存遗传变异缺失，且新发突变尚未产生可用变异时，生物仅能通过其他功能性解决方案实现适应性演化。海洋三棘刺鱼（Gasterosteus aculeatus）为适应淡水定居而发生的骨质侧骨板数目减少现象，已在多个独立演化事件中被观测到，其分子机制为外胚层发育蛋白a（ectodysplasin-a, Eda）基因的等位基因替换。本研究对5个海洋种群与6个淡水种群的三棘刺鱼侧骨板数目与大小的表型及遗传变异进行分析后发现：当Eda基因的可用变异受限（即与低板表型（low-plate morph）相关的等位基因缺失或频率极低）时，淡水种群无法通过减少侧骨板数目实现适应，转而通过缩小侧骨板大小来降低侧骨板的覆盖范围。本研究结果表明，这种在表型与遗传上均独立的“小侧板型”三棘刺鱼——其为欧洲北部3个淡水种群的优势类群——在功能上可能等同于低板表型个体，因此可作为遗传约束下实现功能趋同的趋同演化（convergent evolution）实例。