花瓣通常是花中最顯眼的器官,，具有吸引傳粉者的功能,。但是，在有些被子植物中,，花瓣是不存在的,，形成了無瓣花。導(dǎo)致花瓣缺失的原因尚不清楚,，但在毛茛科被認(rèn)為與一個(gè)基因（即APETALA3-3,，簡稱AP3-3）的“不表達(dá)”有關(guān)。然而,，由于只關(guān)注了少數(shù)幾個(gè)物種,，前人的研究還存在一些問題，如：花瓣的缺失是否真的與AP3-3的“不表達(dá)”相關(guān),？如果是,，導(dǎo)致AP3-3“不表達(dá)”的原因是什么？AP3-3的“不表達(dá)”究竟是不是導(dǎo)致花瓣缺失的原因,？
毛茛科9種植物的花部器官（左）,、AP3類基因的表達(dá)模式（中）和AP3-3基因的結(jié)構(gòu)比較（右）。中圖中藍(lán),、黃和紅色柱子分別表示AP3-1,、AP3-2和AP3-3的表達(dá)量。
針對(duì)上述問題,，中科院植物研究所孔宏智研究組與其合作者對(duì)更多的毛茛科植物開展了研究,。研究發(fā)現(xiàn)，在毛茛科中,，花瓣的多次丟失是平行進(jìn)化的結(jié)果,，且的確與AP3-3基因的“不表達(dá)”密切相關(guān)。研究還發(fā)現(xiàn),，導(dǎo)致AP3-3“不表達(dá)”的原因在不同的分支中是不同的：在大馬士革黑種草（Nigella damascena）的一個(gè)無花瓣園藝品種中,，一個(gè)轉(zhuǎn)座子的插入導(dǎo)致了該基因的完全沉默以及花瓣向花萼的同源異型轉(zhuǎn)變,；在野生的瓣蕊唐松草（Thalictrum petaloideum）中，該基因已經(jīng)完全丟失,；在鐵破鑼（Beesia calthifolia）和擬扁果草（Enemion raddeanum）中,，發(fā)生在編碼區(qū)和調(diào)控區(qū)的堿基缺失使得該基因不能正常表達(dá)；在鐵線蓮屬（Clematis）中,，該基因具有假基因的特征,，表達(dá)量極低。這些結(jié)果表明,，AP3-3是毛茛科中控制花瓣發(fā)育的關(guān)鍵基因,，但在不同的分支中以不同的方式被沉默或者下調(diào)。同時(shí),，盡管AP3-3的沉默的確能夠?qū)е禄ò甑娜笔?，但它在野生無花瓣物種中的結(jié)構(gòu)缺陷和表達(dá)量下降并不是導(dǎo)致花瓣缺失的原因，而可能是其結(jié)果,。
該研究澄清了花瓣缺失與AP3-3基因表達(dá)之間的關(guān)系,，也為闡明花瓣在毛茛科及其他類群中多次丟失的分子機(jī)制問題奠定了基礎(chǔ)。
研究結(jié)果于3月11日在線發(fā)表在PNAS上,，博士生張睿,、助理研究員國春策、博士生張文根為該文的并列第一作者,。該研究得到了國家科技部和國家自然科學(xué)基金委項(xiàng)目的資助,。(生物谷Bioon.com)
doi: 10.1073/pnas.1219690110
PMC:
PMID:
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.
