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.

生物对新生境的适应可依托两类遗传基础：一是现有遗传变异（standing genetic variation），二是新突变产生的遗传变异。若种群缺乏支撑该功能性适应的现有遗传变异，且尚未获得新突变带来的遗传变异，则仅能通过替代性功能性方案实现适应。海生三棘刺鱼（Gasterosteus aculeatus）通过外胚层发育蛋白A（ectodysplasin-a，Eda）基因的等位基因替换，独立演化出骨侧板数量减少的表型以适应淡水定居环境，该现象已在多起独立案例中被观测到。本研究对5个海生种群与6个淡水种群的三棘刺鱼展开研究，分析其骨侧板数量与大小的表型及遗传变异，结果显示：当Eda基因的变异受限（即与低板形态型相关的等位基因缺失或频率极低）时，淡水种群未出现侧板数量的减少，但可通过缩小侧板尺寸实现侧部骨板覆盖范围的缩减。研究结果表明，这种表型与遗传特征均独立的“小侧板型”三棘刺鱼——其在3个北欧淡水种群中占据优势类群——在功能上与低板形态型等效，因此可作为遗传约束下趋同演化出功能相似表型的典型案例。