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中国科研团队解开兰花进化之谜

2017-09-14 15:16:23   央广网

研究团队在二代测序基础上,结合三代测序和相关数据对深圳拟兰、小兰屿蝴蝶兰和铁皮石斛进行基因组组装,使测序对象的基因组组装质量更高。研究团队还对兰科5个亚科的代表性物种进行取样,从而获取大量转录组数据用于基因功能分析和验证。

研究还发现所有现存兰花的祖先曾在6600万年前的第三次生物大灭绝事件前发生了一次全基因组复制(即基因组加倍,WGD)事件,由此开启了现生兰花的起源。全基因组复制事件后使兰花的祖先多了一套基因,成功躲过了此次大灭绝。生物大灭绝后地球生态系统遭到严重破坏,但兰花随即通过基因的扩张和收缩致使基因组出现了分化而形成了5个亚科,产生更多的多样性而迅速适应了新的生态系统。因此所有现存于地球上的兰花都是恐龙灭绝时期幸存的兰花后代。

研究团队通过基因组比较,发现兰花有474个特有基因家族,因此得以重建一个兰花祖先的基因工具包和基因池,并可从中窥视兰花新的基因家族及其扩张和收缩的进化历史,从而揭示了兰花相关性状和习性进化的分子机制。深圳拟兰有36个MADS-box功能基因,它们调控兰花的花部器官发育。研究发现,拟兰的花没有唇瓣和完整的蕊柱是由于B-AP3和E类基因丢失所造成的。而兰花的祖先通过WGD后,复制了B-AP3基因调控花瓣唇瓣化,使兰花呈现出两侧对称,形成了特异性传粉机制,促进了兰花新物种产生和多样性形成,这也是两侧对称兰花种类比辐射对称种类多的原因。这一发现颠覆了人们对兰花的进化是由辐射对称向两侧对称演化的认知,修正了达尔文关于兰花演化的假说。

研究发现,拟兰的花粉呈现散生状态是由P-和S-subclades基因调控的结果,而花粉粘合成花粉块正是P-subclade基因丢失所造成的。花粉块的形成,是兰科进化史上的一项关键创新,这是花粉块形成后产生了花朵间一对一的传粉模式,促成了生殖隔离从而有助于新物种形成。不像拟兰那样一朵花的花粉可以散播给许多朵花传粉而造成后代同质化。

兰科进化的一个关键性状是种子缺乏胚乳。研究发现,植物种子的胚乳发育是由typeI Mβ MADS-box基因调控的,而兰花的种子没有胚乳是由于缺少了这一类基因。没有胚乳种子使自身体积变得非常细小而可以远距离传播和降落到树上或石头上。

研究发现,拟兰之所以地生是由AGL12基因控制了它的根只能在地下生长和吸收养分。而当兰花缺乏AGL12-like基因时,兰花则生长出气生根,这些根发育出一种海绵状表皮,能在空气中吸收养分和储存水分,致使兰花能附着在树上或石上生长。这种附生特性的形成,可使兰花利用其它植物所不能利用的生境进行生长。这些生境之间存在很大差异,容易隔断兰花物种的基因交流(即生境隔离),有助于促进新物种的产生。

本研究得到国家林业局、广东省林业厅、广东省科技厅、深圳市城管局(林业局)、深圳市人居环境委、深圳市科创委、深圳市财委、深圳市工贸信息委、深圳市国资委以及深圳市投资控股有限公司的支持。

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