花瓣通常是花中最显眼的器官，具有吸引传粉者的功能。但是，在有些被子植物中，花瓣是不存在的，形成了无瓣花。导致花瓣缺失的原因尚不清楚，但在毛茛科被认为与一个基因（即APETALA3-3，简称AP3-3）的“不表达”有关。然而，由于只关注了少数几个物种，前人的研究还存在一些问题，如：花瓣的缺失是否真的与AP3-3的“不表达”相关？如果是，导致AP3-3“不表达”的原因是什么？AP3-3的“不表达”究竟是不是导致花瓣缺失的原因？
毛茛科9种植物的花部器官（左）、AP3类基因的表达模式（中）和AP3-3基因的结构比较（右）。中图中蓝、黄和红色柱子分别表示AP3-1、AP3-2和AP3-3的表达量。
针对上述问题，中科院植物研究所孔宏智研究组与其合作者对更多的毛茛科植物开展了研究。研究发现，在毛茛科中，花瓣的多次丢失是平行进化的结果，且的确与AP3-3基因的“不表达”密切相关。研究还发现，导致AP3-3“不表达”的原因在不同的分支中是不同的：在大马士革黑种草（Nigella damascena）的一个无花瓣园艺品种中，一个转座子的插入导致了该基因的完全沉默以及花瓣向花萼的同源异型转变；在野生的瓣蕊唐松草（Thalictrum petaloideum）中，该基因已经完全丢失；在铁破锣（Beesia calthifolia）和拟扁果草（Enemion raddeanum）中，发生在编码区和调控区的碱基缺失使得该基因不能正常表达；在铁线莲属（Clematis）中，该基因具有假基因的特征，表达量极低。这些结果表明，AP3-3是毛茛科中控制花瓣发育的关键基因，但在不同的分支中以不同的方式被沉默或者下调。同时，尽管AP3-3的沉默的确能够导致花瓣的缺失，但它在野生无花瓣物种中的结构缺陷和表达量下降并不是导致花瓣缺失的原因，而可能是其结果。
该研究澄清了花瓣缺失与AP3-3基因表达之间的关系，也为阐明花瓣在毛茛科及其他类群中多次丢失的分子机制问题奠定了基础。
研究结果于3月11日在线发表在PNAS上，博士生张睿、助理研究员国春策、博士生张文根为该文的并列第一作者。该研究得到了国家科技部和国家自然科学基金委项目的资助。(生物谷Bioon.com)
doi: 10.1073/pnas.1219690110
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Disruption of the petal identity gene APETALA3-3 is highly correlated with loss of petals within the buttercup family (Ranunculaceae)
Rui Zhang, Chunce Guo, Wengen Zhang, Peipei Wang, Lin Li, Xiaoshan Duan, Qinggao Du,Liang Zhao, Hongyan Shan, Scott A-Hodges, Elena M. Kramer, Yi Ren, and Hongzhi Kong.
Absence of petals, or being apetalous, is usually one of the most important features that characterizes a group of flowering plants at high taxonomic ranks (i.e., family and above). The apetalous condition, however, appears to be the result of parallel or convergent evolution with unknown genetic causes. Here we show that within the buttercup family (Ranunculaceae), apetalous genera in at least seven different lineages were all derived from petalous ancestors, indicative of parallel petal losses. We also show that independent petal losses within this family were strongly associated with decreased or eliminated expression ofa single floral organ identity gene, APETALA3-3 (AP3-3), apparently owing to species-specific molecular lesions. In an apetalousmutant of Nigella, insertion of a transposable element into the second intron has led to silencing of the gene and transformation of petals into sepals. In several naturally occurring apetalous genera, such as Thalictrum, Beesia, and Enemion, the gene has either been lost altogether or disrupted by deletions in coding or regulatory regions. In Clematis, a large genus in which petalous species evolved secondarily from apetalous ones, the gene exhibits hallmarks of a pseudogene. These results suggest that, as a petal identity gene, AP3-3 has been silenced or down-regulated by different mechanisms in different evolutionary lineages. This also suggests that petal identity did not evolve many times independently across the Ranunculaceae but was lost in numerous instances.The genetic mechanisms underlying the independent petal
losses, however, may be complex, with disruption of AP3-3 being either cause or effect.
