Data from: Clade-specific positive selection on a developmental gene: branchless trichome and the evolution of stellate trichomes in Physaria (Brassicaceae)

Positive selection is known to drive the evolution of genes involved in evolutionary arms races, but what role does it play in the evolution of genes involved in developmental processes? We used the single-celled epidermal trichomes of Brassicaceae as a model to uncover the molecular evolutionary processes that contributed to the transition from dendritic trichomes, as seen in most species of Brassicaceae, to the distinctive stellate trichomes of the genus Physaria. We explored the role of positive selection on the evolution of BRANCHLESS TRICHOME (BLT), a candidate gene for changes in trichome branching pattern. Maximum likelihood models of codon evolution point to a shift in selective pressure affecting the evolution of BLT across the entire Physaria clade, and we found strong evidence that positive selection has acted on a subset of Physaria BLT codons. Almost all of the 10 codon sites with the highest probability of having evolved under positive selection are clustered in a predicted coiled-coil domain, pointing to changes in protein-protein interactions. Thus, our findings suggest that selection acted on BLT to modify its interactions with other proteins. The fact that positive selection occurred throughout the radiation of Physaria could reflect selection to stabilize development in response to an abrupt switch from the dendritic form to the stellate form, divergent selection for diversification of the stellate form, or both. These results point to the need for evolutionary developmental studies of BLT and its interacting proteins in Physaria.众所周知，正向选择（positive selection）驱动参与进化军备竞赛的基因的演化，但正向选择在参与发育过程的基因演化中扮演何种角色？我们以十字花科（Brassicaceae）的单细胞表皮毛状体（trichome）为模型，旨在揭示驱动从十字花科多数物种所见的树突状毛状体（dendritic trichome），向泡果荠属（Physaria）独特星状毛状体（stellate trichome）演化的分子进化过程。我们探究了正向选择在无分枝毛状体基因（BRANCHLESS TRICHOME，BLT）演化中的作用——该基因为毛状体分枝模式改变的候选基因。密码子（codon）演化的极大似然模型（maximum likelihood model）显示，整个泡果荠属演化支（clade）内，作用于BLT演化的选择压力发生了转变；同时我们发现了强有力的证据，表明正向选择曾作用于泡果荠属BLT的部分密码子位点。在10个最有可能经正向选择演化的密码子位点中，几乎全部都聚集在一个预测的卷曲螺旋结构域（coiled-coil domain）内，这提示蛋白质-蛋白质相互作用（protein-protein interaction）发生了改变。由此，我们的研究结果提示，自然选择通过作用于BLT，改变了其与其他蛋白质的相互作用。正向选择贯穿泡果荠属的辐射演化（radiation）全过程，这一现象可能反映了两种选择压力：一是为响应从树突状毛状体到星状毛状体的突然转变而稳定发育过程的选择，二是为推动星状毛状体形态多样化的定向选择，亦或是两者兼具。上述研究结果表明，有必要在泡果荠属中开展BLT及其互作蛋白的进化发育生物学研究。