Predictability and irreversibility of genetic changes associated with flower color evolution in Penstemon barbatus

Two outstanding questions in evolutionary biology are whether, and how often, the genetic basis of phenotypic evolution is predictable; and whether genetic change constrains evolutionary reversibility. We address these questions by studying the genetic basis of red flower color in Penstemon barbatus. The production of red flowers often involves the inactivation of one or both of two anthocyanin pathway genes, Flavonoid 3',5'-hydroxylase (F3'5'h) and Flavonoid 3'-hydroxylase (F3'h). We used gene expression and enzyme function assays to determine that redundant inactivating mutations to F3'5'h underlie the evolution of red flowers in P. barbatus. Comparison of our results to previously characterized shifts from blue to red flowers suggests that the genetic change associated with the evolution of red flowers is predictable: when it involves elimination of F3'5'H activity, functional inactivation or deletion of this gene tends to occur; however, when it involves elimination of F3'H activity, tissue-specific regulatory substitutions occur and the gene is not functionally inactivated. This pattern is consistent with emerging data from physiological experiments indicating that F3'h may have pleiotropic effects and is thus subject to purifying selection. The multiple, redundant inactivating mutations to F3'5'h suggest that reversal to blue-purple flowers in this group would be unlikely.

进化生物学领域存在两个核心悬而未决的问题：其一，表型演化的遗传基础是否具备可预测性，以及其可预测的频率如何；其二，遗传变化是否会限制演化的可逆性。本研究以红花钓钟柳（Penstemon barbatus）的红色花性状为研究对象，通过解析其遗传基础来解答上述问题。红色花的形成通常涉及两条花青素合成通路基因的失活，这两个基因分别为黄烷酮3',5'-羟化酶（Flavonoid 3',5'-hydroxylase，F3'5'h）与黄烷酮3'-羟化酶（Flavonoid 3'-hydroxylase，F3'h）。本研究通过基因表达检测与酶功能实验证实，红花钓钟柳红色花性状的演化，源于F3'5'h基因的多重冗余失活突变。将本研究结果与此前已报道的蓝花向红花的花色转变案例进行比对后发现，与红色花演化相关的遗传变化具备可预测性：当花色转变需要消除F3'5'H的酶活性时，往往会发生该基因的功能失活或完全缺失；而当需要消除F3'H的酶活性时，则仅会出现组织特异性的调控序列变异，该基因并不会发生功能失活。这一规律与近期生理学实验获得的新数据高度契合——相关实验表明，F3'h基因可能存在多效性，因此会受到纯化选择的作用。F3'5'h基因存在多重冗余失活突变这一现象，说明该类群的物种几乎不可能逆向演化回蓝紫色花性状。