Reyes-Prieto Adrian, Moustafa Ahmed, Bhattacharya Debashish
Department of Biological Sciences and Roy J. Carver Center for Comparative Genomics, University of Iowa, Iowa City, Iowa 52242-1324, USA.
Curr Biol. 2008 Jul 8;18(13):956-62. doi: 10.1016/j.cub.2008.05.042.
Plantae (as defined by Cavalier-Smith, 1981) plastids evolved via primary endosymbiosis whereby a heterotrophic protist enslaved a photosynthetic cyanobacterium. This "primary" plastid spread into other eukaryotes via secondary endosymbiosis. An important but contentious theory in algal evolution is the chromalveolate hypothesis that posits chromists (cryptophytes, haptophytes, and stramenopiles) and alveolates (ciliates, apicomplexans, and dinoflagellates) share a common ancestor that contained a red-algal-derived "secondary" plastid. Under this view, the existence of several later-diverging plastid-lacking chromalveolates such as ciliates and oomycetes would be explained by plastid loss in these lineages. To test the idea of a photosynthetic ancestry for ciliates, we used the 27,446 predicted proteins from the macronuclear genome of Tetrahymena thermophila to query prokaryotic and eukaryotic genomes. We identified 16 proteins of possible algal origin in the ciliates Tetrahymena and Paramecium tetraurelia. Fourteen of these are present in other chromalveolates. Here we compare and contrast the likely scenarios for algal-gene origin in ciliates either via multiple rounds of horizontal gene transfer (HGT) from algal prey or symbionts, or through endosymbiotic gene transfer (EGT) during a putative photosynthetic phase in their evolution.
植物界（按照卡瓦利埃-史密斯1981年的定义）的质体是通过初级内共生进化而来的，即一个异养原生生物奴役了一个光合蓝细菌。这种“初级”质体通过次级内共生扩散到其他真核生物中。藻类进化中一个重要但有争议的理论是色素体藻类假说，该假说认为色素体藻类（隐藻、定鞭藻和不等鞭毛藻）和囊泡藻类（纤毛虫、顶复门原虫和甲藻）有一个共同的祖先，这个祖先含有一个源自红藻的“次级”质体。按照这种观点，一些后来分化出的缺乏质体的色素体藻类，如纤毛虫和卵菌的存在，可以用这些谱系中质体的丢失来解释。为了验证纤毛虫光合祖先的观点，我们利用嗜热四膜虫大核基因组中预测的27446种蛋白质，对原核生物和真核生物基因组进行了查询。我们在纤毛虫嗜热四膜虫和双小核草履虫中鉴定出16种可能源自藻类的蛋白质。其中14种在其他色素体藻类中也存在。在这里，我们比较并对比了纤毛虫中藻类基因起源的可能情况，这些情况要么是通过从藻类猎物或共生体进行多轮水平基因转移（HGT），要么是在其进化过程中的一个假定光合阶段通过内共生基因转移（EGT）。
