WUSCHEL與AGAMOUS基因複製 對俄氏草珠芽發育之影響

Abstract

開花植物如何演化並藉其有性或無性繁殖策略維繫物種存續，是植物學研究的重要議題之一。俄氏草具有包含「成花反轉」及「假性胎生」並形成無性繁殖體（珠芽）的獨特繁殖方式，先前研究顯示俄氏草花序特性基因ToCEN亦在珠芽序中表現，暗示珠芽序和花序在結構與遺傳調控上具有若干同源性。演化發育學理論敘及新形態發生常源自修改既有基因與調控機制而產生，因此本研究旨在探討是否也具有同源套組基因參與調控俄氏草花與珠芽分生組織這兩種同源構造的發育過程。研究中選擇了成花決定套組基因作為主要的探討對象，即幹細胞特性基因WUSCHEL (WUS)與其共同調控因子AGAMOUS (AG)。有趣的是，在大多數物種裡都只會有各一個WUS和AG的直系同源基因(ortholog)，然而這兩個基因在俄氏草裡都發現了基因複製的現象，並且在發育過程呈現分歧表現。其中ToWUS1、ToAG1、ToAG3主要表現在花序，而ToWUS2與ToAG2則主要表現在珠芽序。雖然這類保守的套組基因複製並不常發生，一旦發生了則通常會伴隨著天擇作用。而此現象正好與在俄氏草裡看到的結果相符，模式檢定亦支持ToWUS2有受到正向天擇的證據，或許和它複製後具有其時空表現與功能棲位有關，並提供珠芽分生組織無限生長之特性。總結本研究呈現在俄氏草遺傳組中複製的幹細胞特性基因ToWUS1與ToWUS2分別和同源之有性與無性分生組織的器官生成相關，而創新的WUS-AG套組基因表現模式可能和新的形態發生有關，且該新形態係構成在俄氏草繁殖策略中至關重要的珠芽序。

Reproductive strategy, including sexual or asexual, which directly related to the maintenance of one species, is one of the most important issues of angiosperms. Titanotrichum oldhamii exhibits a unique reproductive strategy, which consists of floral reversion and pseudoviviparous bulbils formation. Previous study revealed that the inflorescence meristem identity gene ToCEN participates in bulbiliferous shoot formation, suggesting that bulbiliferous shoot and inflorescence may share certain homology. As the evo-devo theory proposes that novel morphogenesis are largely evolved by recruiting the pre-existed genes, whether there exists homologous regulatory toolkits between the two homologous structures ̶ the floral meristem and the bulbiliferous meristem ̶ in Titanotrichum is concerned in this study. The key regulatory toolkits for floral determinacy, namely the stem cell identity gene WUSCHEL (WUS) and its co-operator AGAMOUS (AG) are therefore chosen as the major candidates. Interestingly, unlike most of the species have only one WUS ortholog and one euAG ortholog, both genes are duplicated in Titanotrichum and displaying divergent expression patterns. While ToWUS1, ToAG1, and ToAG3 are mainly expressed in inflorescence, ToWUS2 and ToAG2 are dominantly expressed in bulbiliferous shoot. Though duplication of conserved toolkit is rare, it is usually followed by selection. This is also consistent with the results that ToWUS2 was positively selected, maybe for occupying certain regulatory niches that provide the indeterminate growth of bulbiliferous meristem. In case of Titanotrichum, this study presents the scenario that the duplicated stem cell identity genes ToWUS1 and ToWUS2 are involved in organogenesis of homologous sexual and asexual meristems respectively. And innovation on WUS-AG toolkit is closely linked to the novel morphogenesis, the bulbiliferous development, which contributes to the alternative reproductive strategy of Titanotrichum.