非洲菫花對稱性轉換的分子發育機制探討

Abstract

花的對稱性在被子植物的演化中是一個非常重要的性狀。而苦苣苔科的非洲菫原本花形為兩側對稱，但栽培品種因為人為的偏好，栽培出的許多花形轉變為輻射對稱的品系。由於此轉變發生在同屬的不同品系間，使我們可以在基因背景相似的情形下探討花對稱形態如何從兩側對稱轉變為輻射對稱。野生型非洲菫 (Wild type) 的腹側花瓣大於背側花瓣且只能看到兩枚雄蕊位於腹側；另對比兩種人擇後不同輻射對稱性品系花發育過程發現：(1) ‘ Little rick’ 品系：五枚花瓣皆一樣大，類似野生型的腹側花瓣，有五枚可見雄蕊；(2) ‘No-stamen’ 品系：花瓣大小皆一樣小，類似野生型的背側花瓣，雄蕊皆敗育不可見。由於前人研究指出在金魚草中， CYCLOIDEA (CYC) 和 DICHOTOMA (DICH) 基因皆表現在花的背側，共同促進 RADIALIS (RAD) 的表現，來抑制可能促進花腹側發育的 DIVARICATA (DIV) 擴散至背側，進而使花的背腹側形態分化。因此我們推測非洲菫在人擇栽培過程中，花對稱性轉變跟上述基因的表現模式改變有關。Real time PCR 基因表現結果顯示在發育早期，Wild type 非洲菫的 SaCYC1A 及 SaCYC1B 集中在背側花瓣表現；對比在 ‘No-stamen’ 品系的 SaCYC1A 及 SaCYC1B 卻在所有花瓣皆增量表現；而 ‘ Little rick’ 品系中的 SaCYC1A 及 SaCYC1B 在花瓣背側的表現量較 Wild type 少。由於非洲菫在花瓣及雄蕊花原基形成的時期便已建立其對稱形態，因此 ‘No-stamen’ 品系可能在花原基發育時期 SaCYC1A 及 SaCYC1B 擴張至全花表現使所有雄蕊敗育，並使所有花瓣相對於野生型縮小，形成腹側形態背側化的輻射對稱；而 ‘Little rick’ 品系可能因為 SaCYC1A 及 SaCYC1B 在花原基發育時期於花背側的表現減少而使五枚雄蕊都能發育，使所有花瓣小增大，形成背側形態腹側化的另一種輻射對稱型；但由於仍有 CYC 在花背側表現，所以 ‘Little rick’ 花的雄蕊間仍有殘存之兩側對稱性 (背側雄蕊明顯小於腹側四枚雄蕊)。本研究報導了花對稱相關基因表現在空間上 (Heterotopic) 及時間 (Heterochronic) 上的改變及表現量的改變，可能會引起花對稱性狀在兩側對稱及輻射對稱間轉變。

Flower bilateral symmetry (zygomorphy) is among the major evolutionary trends in angiosperms as it greatly facilitates the interaction with pollinators thus creates the floral diversity. The wild type flower of the African violet (Saintpaulia sp.; Gesneriaceae) is zygomorphic, but flowers of its domesticated cultivars are mostly actinomorphic, which appears to be favoured by humans. This provides us a great opportunity to study the developmental transition between zygomorphy and actinomorphy, given their similar genetic background. Wild type flowers show zygomorphy in that two dorsal petals are samller than lateral and ventral ones and only two stamens developed in ventral parts. Two independently domesticated cultivars showed drastic differences in flower morphology: (1) Ventralized effect, Saintpaulia sp. cf. Little rick: five petals are large and equally proportionate, like the ventral petals of the wild type, and five stamens developed instead of two. (2) Dorsalized effect, Saintpaulia sp. cf. No-stamen: five petals are equally small, comparable in relative size to the dorsal petals of the wild type, and no stamens develop. It is known that the dorsally expressed flower genes CYCLOIDEA (CYC) and the its downstream gene RADIALIS (RAD) together with the ventral identity flower genes DIVARICATA (DIV) interact to create zygomorphy in Antirrhinum majus. I examined their expressions via real time PCR throughout the duration of anthesis between the wild type and two cultivars. In the wild type, SaCYC1A and SaCYC1B were expressed in dorsal petals at early stages, In contrast, in ‘No-stamen’, SaCYC1A and SaCYC1B expressed in all petals in early stages. In ‘Little rick’, we found SaCYC1A and SaCYC1B only expressed dorsally in early stages but their expression amount is less than wild type. Our results revealed that these differences became pronounced as early as the initiation of petal and stamen primordia. When correlating to SEM pictures of the flower development pattern, in ‘No stamen’ cultivar over-expression of SaCYC1A and SaCYC1B to all petals started as early as primordia initiated, this perhaps create the actinomorphic flower through the dorsalized effect. On the other hand, in ‘Little rick’, reduction of SaCYC1A and SaCYC1B expression was observed when primordia initiated, perhaps causes ventralized effect to generate the actinomorphic flower. Our results demonstrate that altering of genes in aspects of heterochrony, heterotopy and expression amount could trigger developmental transition between floral zygomorphy and actinomorphy during African violet domestications.