石蓯蓉（藍雪科）柱頭與花粉兩形性之研究

Abstract

藍雪科中大部份的種類皆具兩型花柱（distyly），即在一物種內同時存在兩種不同花部形態的個體，於柱頭與花藥高度、及花粉與柱頭形態等方面皆出現差異，並且只有不同花部形態個體之間的他花異型授粉 (intermorph pollination) 才能成功產生後代，此現象稱為異型自交不親和性（heteromorphic self-incompatibility）。前人研究推測，藍雪科內異型自交不親和性的演進過程為，先出現穩定的自交不親和性，接著柱頭與花粉形態方面產生差異，即出現柱頭與花粉兩形性，最後自交不親和性與兩形性結合，形成異型自交不親和性。然而，在藍雪科中不具兩型花柱卻呈現柱頭與花粉兩形性的石蓯蓉（Limonium sinense），其是否亦具有異型自交不親和性的現象，以及其在柱頭與花粉形態發育方面的差異皆尚未釐清。 本研究經野外實地調查石蓯蓉的柱頭與花藥高度之關係發現，石蓯蓉A型個體的柱頭位置較B型個體者高，但A型個體的雄蕊高度與B型個體者之間不具顯著差異，因此石蓯蓉A型與B型個體的柱頭高度之間已有分化的趨勢。石蓯蓉的A型與B型個體，經掃描式電子顯微鏡的觀察，其在早期花部發育的過程，以及花器始原的形態皆相似，但在晚期花部發育則產生不同形態的花粉與柱頭。由穿透式電子顯微鏡的觀察，顯示A型與B型個體之間，於小孢子形成 (microsporogenesis) 過程中四分體晚期開始出現差異。A型個體的原圓柱體 (procolumella)，在四分體小孢子細胞膜上的距離大、密集程度低，使成熟花粉粒呈現突緣網紋（lophate）的花粉外壁；柱頭始原則發育出穗狀（cob）的構造。B型個體的原圓柱體，在四分體小孢子細胞膜表面的距離小、密集程度高，形成細網紋（reticulate）的花粉外壁結構；柱頭則呈現乳突狀（papillate）的柱頭結構。由人工授粉試驗顯示，進行A型與B型個體之間的他花異型授粉處理後，花粉可正常萌發，花粉管亦延伸至花柱內，但A型個體的結實率較B型個體低；相同花部形態個體之間的他花同型授粉（intramorph pollination），偶爾會觀察到花粉萌發的情形，但只有極少數花粉管能延伸到花柱內，並且這些少數延伸的花粉管最後亦出現結實的現象；A型與B型個體的自花授粉（selfing）處理，則觀察不到花粉萌發與花粉管的形成。 本研究結果指出，（1）石蓯蓉具已分化的柱頭高度；（2）石蓯蓉A型與B型個體之間的柱頭與花粉形態差異皆發生於花部發育晚期，此時A型與B型個體的柱頭始原開始各自分化，而A型與B型個體的四分體小孢子於原外壁 (primexine) 發育情形亦出現差異；（3）石蓯蓉在進行他花同型授粉處理之後，偶爾會出現結實現象，而自花授粉處理完全不會產生後代，此情形代表石蓯蓉具強烈自交不親和性，但在異型自交不親和性方面則呈現不穩定的狀況，符合前人研究所推測之藍雪科內先演化出自交不親和性，再出現柱頭與花粉兩形性，最後形成異型自交不親和性的演進過程。

Many species of Plumbaginaceae are distyly, with long-styled morph and short-styled morph occurred in the same species. Furthermore, some distylous species have dimorphic stigmatic cells and pollen exine. Only pollination between two morphs (intermorph) is compatible, and this circumstance terms heteromorphic self-incompatibility. Previous researches in Plumbaginaceae hypothesized that the species with monomorphic stigma and pollen exine as well as self-incompatibility appeared first, and then the species with dimorphic stigma and pollen exine associated with self-incompatibility, forming heteromorphic self-incompatibility later. Limonium sinense, a perennial herb in Plumbaginaceae, is non-distylous but dimorphic in stigma and pollen exine. However, there was no report indicating the existence of hetermorphic self-incompatibility in this species, and the differentiation of morphological development between two morphs remained unclear. In this study, SEM observation indicated that the number and shape of floral primodia and the developmental processes in early floral development stage of Morph A and Morph B in L. sinense were similar. Dimorphic stigmas and pollen grains appeared in later stage of floral development. Through observation of TEM, sparse procolumellae formed on tetraspore membrane of Morph A in the later tetrad stage of microsporogenesis and then developed into lophate exine in mature stage, while dense procolumellae deposited on tetraspore membrane of Morph B in the later tetrad stage of microsporogenesis and then forming reticulate exine in mature stage. At the same time, stigma primordial of Morph A developed into cob shape, while stigma primordia of Morph B differentiated into papillate shape. Field investigation of L. sinense revealed the positions of stigmas in Morph A were higher than that in Morph B, but the positions of anthers between two morphs were similar. The intermorph pollination had pollen grains germinated and pollen tube elongated normally, but the fruit set produced in Morph A was lower than that in Morph B. Pollination in the same morph (intramorph) displayed few pollen grains germinated and pollen tube elongated seldom into styles. The fruit set produced by intramorph pollination was low in both Morph A and Morph B. Selfing in L. sinense showed that no pollen grain germinated and no fruit produced. The results of this study on L. sinense revealed that the stigma positions of Morph A and Morph B are different, that stigmas and pollen grains of these two Morphs had their differentiated developments starting from the late tetrad stage of microsporogenesis showed differentiated development, and that self-incompatibility is strong in intrafloral selfing but unstable in monomorphic interfloral pollination. The results were coinciding with the presumption of previous research on the evolutionary sequences of heteromorphic self-incompatibility system in Plumbaginaceae, with self-incompatibility first, then dimorphic stigmas and pollen grains, and finally heteromorphic self-incompatibility.