Ibrahim Douglas G, Burke Terry, Ripley Brad S, Osborne Colin P
Department of Animal and Plant Sciences, University of Sheffield, Sheffield, UK.
Ann Bot. 2009 Jan;103(1):127-36. doi: 10.1093/aob/mcn204. Epub 2008 Oct 30.
The grass Alloteropsis semialata is the only plant species with both C(3) and C(4) subspecies. It therefore offers excellent potential as a model system for investigating the genetics, physiology and ecological significance of the C(4) photosynthetic pathway. Here, a molecular phylogeny of the genus Alloteropsis is constructed to: (a) confirm the close relationship between the C(3) and C(4) subspecies of A. semialata; and (b) infer evolutionary relationships between species within the Alloteropsis genus.
The chloroplast gene ndhF was sequenced from 12 individuals, representing both subspecies of A. semialata and all four of the other species in the genus. ndhF sequences were added to those previously sequenced from the Panicoideae, and used to construct a phylogenetic tree.
The phylogeny confirms that the two subspecies of A. semialata are among the most recently diverging lineages of C(3) and C(4) taxa currently recognized within the Panicoideae. Furthermore, the position of the C(3) subspecies of A. semialata within the Alloteropsis genus is consistent with the hypothesis that its physiology represents a reversion from C(4) photosynthesis. The data point to a similar evolutionary event in the Panicum stenodes-P. caricoides-P. mertensii clade. The Alloteropsis genus is monophyletic and occurs in a clade with remarkable diversity of photosynthetic biochemistry and leaf anatomy.
These results confirm the utility of A. semialata as a model system for investigating C(3) and C(4) physiology, and provide molecular data that are consistent with reversions from C(4) to C(3) photosynthesis in two separate clades. It is suggested that further phylogenetic and functional investigations of the Alloteropsis genus and closely related taxa are likely to shed new light on the mechanisms and intermediate stages underlying photosynthetic pathway evolution.
两耳草（Alloteropsis semialata）是唯一同时拥有C3和C4两个亚种的植物物种。因此，它作为一个模型系统，在研究C4光合途径的遗传学、生理学及生态意义方面具有巨大潜力。在此，构建了两耳草属的分子系统发育树，以：（a）确认两耳草C3和C4亚种之间的密切关系；（b）推断两耳草属内物种间的进化关系。
对代表两耳草两个亚种及该属其他四个物种的12个个体的叶绿体基因ndhF进行测序。将ndhF序列添加到先前从黍亚科测序得到的序列中，并用于构建系统发育树。
系统发育分析证实，两耳草的两个亚种是目前在黍亚科中公认的C3和C4类群中最近分化的谱系之一。此外，两耳草C3亚种在两耳草属中的位置与以下假设一致，即其生理特征代表了从C4光合作用的逆转。数据表明在细茎黍-类雀稗-梅氏黍分支中发生了类似的进化事件。两耳草属是单系的，并且出现在一个光合生物化学和叶片解剖结构具有显著多样性的分支中。
这些结果证实了两耳草作为研究C(3)和C(4)生理学的模型系统的实用性，并提供了与两个独立分支中从C(4)到C(3)光合作用逆转相一致的分子数据。建议对两耳草属及近缘类群进行进一步的系统发育和功能研究，这可能会为光合途径进化的机制及中间阶段带来新的启示。
