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 i...生物对新生环境的适应既可依托现存遗传变异（standing genetic variation），亦可源于新突变产生的遗传变异。若某功能性适应对应的现存遗传变异缺失，且尚未产生新突变带来的遗传变异，则物种仅能通过替代性功能路径实现适应。以海生三棘刺鱼（Gasterosteus aculeatus）淡水定殖的适应性演化为例，其骨化侧板数量的减少已在诸多独立演化事件中，通过ectodysplasin-a（Eda）基因的等位基因替换得以实现。本研究对5个海生种群与6个淡水种群的三棘刺鱼的侧板数量及大小的表型与遗传变异展开分析，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减少的演化，但可通过……