Wolfe A D, dePamphilis C W
Department of Biology, Vanderbilt University, USA.
Mol Biol Evol. 1998 Oct;15(10):1243-58. doi: 10.1093/oxfordjournals.molbev.a025853.
The photosynthetic gene rbcL has been lost or dramatically altered in some lineages of nonphotosynthetic parasitic plants, but the dynamics of these events following loss of photosynthesis and whether rbcL has sustained functionally significant changes in photosynthetic parasitic plants are unknown. To assess the changes to rbcL associated with the loss of functional constraints for photosynthesis, nucleotide sequences from nonparasitic and parasitic plants of Scrophulariales were used for phylogeny reconstruction and character analysis. Plants in this group display a broad range of parasitic abilities, from photosynthetic ("hemiparasites") to nonphotosynthetic ("holoparasites"). With the exception of Conopholis (Orobanchaceae), the rbcL locus is present in all parasitic plants of Scrophulariales examined. Several holoparasitic genera included in this study, including Boschniakia, Epifagus, Orobanche, and Hyobanche, have rbcL pseudogenes. However, the holoparasites Alectra orobanchoides, Harveya capensis, Harveya purpurea, Lathraea clandestina, Orobanche corymbosa, O. fasciculata, and Striga gesnerioides have intact open reading frames (ORFs) for the rbcL gene. Phylogenetic hypotheses based on rbcL are largely in agreement with those based on sequences of the nonphotosynthetic genes rps2 and matK and show a single origin of parasitism, and loss of photosynthesis and pseudogene formation have been independently derived several times in Scrophulariales. The mutations in rbcL in nonparasitic and hemiparasitic plants would result in largely conservative amino acid substitutions, supporting the hypothesis that functional proteins can experience only a limited range of changes, even in minimally photosynthetic plants. In contrast, ORFs in some holoparasites had many previously unobserved missense substitutions at functionally important amino acid residues, suggesting that rbcL genes in these plants have evolved under relaxed or altered functional constraints.
光合基因rbcL在一些非光合寄生植物谱系中已经丢失或发生了显著改变，但光合作用丧失后这些事件的动态变化以及rbcL在光合寄生植物中是否持续发生功能上的显著变化尚不清楚。为了评估与光合作用功能限制丧失相关的rbcL变化，玄参目的非寄生植物和寄生植物的核苷酸序列被用于系统发育重建和特征分析。该类群中的植物表现出广泛的寄生能力，从光合（“半寄生植物”）到非光合（“全寄生植物”）。除了肉苁蓉属（列当科）外，rbcL基因座存在于所有被检测的玄参目寄生植物中。本研究中包括的几个全寄生属，如草苁蓉属、岩寄生属、列当属和肉苁蓉属，都有rbcL假基因。然而，全寄生植物小花兔丝子、卡氏哈维草、紫花哈维草、地下肉草、伞房列当、簇生列当和独脚金属的rbcL基因具有完整的开放阅读框（ORF）。基于rbcL的系统发育假说在很大程度上与基于非光合基因rps2和matK序列的假说一致，表明寄生现象有单一的起源，并且光合作用的丧失和假基因的形成在玄参目中已经独立发生了几次。非寄生植物和半寄生植物中rbcL的突变将导致氨基酸替换在很大程度上较为保守，这支持了即使在最低限度光合的植物中，功能蛋白也只能经历有限范围变化的假说。相比之下，一些全寄生植物的开放阅读框在功能重要的氨基酸残基处有许多以前未观察到的错义替换，这表明这些植物中的rbcL基因是在宽松或改变的功能限制下进化的。
