台灣原生秋海棠之雜交起源與多倍體種化

Abstract

臺灣原生秋海棠屬(Begonia L.)植物已知共有18種，其中包含4種天然雜交種，種間雜交頻繁，物種特有率達70％以上。在分類上，臺灣原生秋海棠屬於秋海棠組(Sect. Diploclinium)、扁果組(Sect. Platycentrum)及無翅組(Sect. Sphenanthera)。先前的細胞學研究發現，臺灣產本屬植物相較亞洲地區之分類群(多為2n=22, 30)擁有多樣的染色體數：2n = 22, 26, 36, 38, 52, 60, 64, 82，其中僅圓果秋海棠(B. longifolia, 2n=22)、裂葉秋海棠(B. palmata, 2n=22)、蘭嶼秋海棠(B. fenicis, 2n=26)為二倍體。這樣的現象暗示雜交及多倍體化事件在臺灣產秋海棠多樣性的形成中可能扮演重要角色。本研究利用二個低拷貝核基因PI和RPB2片段，以及合併的三個葉綠體DNA片段(trnL-trnF、trnC-ycf6、accD-psaI)進行分子親緣分析，藉由基因樹的比較來推論臺灣原生多倍體秋海棠的網狀演化歷史，並加入鄰近地區之物種樣本以探討臺灣種類之起源。 研究結果發現，兩個核基因與葉綠體DNA分別建構之基因樹皆支持岩生秋海棠(B. ravenii)及蘭嶼秋海棠與臺灣其他秋海棠產的親緣關係較遠，裂葉秋海棠亦與臺灣多倍體物種的形成無關。臺灣原生多倍體秋海棠應屬於扁果組-無翅組支序，在葉綠體基因樹中共有三個支序：Longifolia clade、Chitoensis clade、Formosana clade；而PI和RPB2核基因樹中，多倍體秋海棠分別都形成ABCD四個主要支序，其中A支序包含二倍體的圓果秋海棠，其餘三個支序皆由多倍體物種組成。 由核基因樹中各物種的支系組成以及葉綠體基因樹的母系來源，我們假設共有四個支系參與臺灣原生多倍體秋海棠的演化：2n=38的溪頭複合群(溪頭秋海棠、坪林秋海棠、九九峰秋海棠、南臺灣秋海棠)來自B和D支系的雜交，二倍體親本未知；水鴨腳可能是溪頭複合群之祖先(BD)與圓果秋海棠(A)雜交形成的獨立支系，且為藤枝秋海棠的親本之一；臺灣秋海棠則由未知親本(C)與圓果秋海棠(A)雜交而來，並可能參與了2n=52的鹿谷複合群(鹿谷秋海棠、出雲山秋海棠、霧台秋海棠)及藤枝秋海棠等高倍體物種的形成。 分子證據支持雜交與多倍體化在台灣原生秋海棠的演化上扮演重要角色，而此機制也可能在秋海棠屬的多樣性形成具重要性，值得進一步探討。

With only 14 species, Taiwanese begonias are exceptional for their extensive variation in chromosome numbers (2n=22, 26, 36, 38, 52, 60, 64, 82), implying that mechanisms such as interspecific hybridization, polyploidization, and subsequent aneuploidization should have been crucial in generating the diversity of begonias in Taiwan. To untangle the origins of Taiwanese begonias and their complicated evolutionary history, DNA sequences of two low-copy nuclear genes (PI, RPB2) and cpDNA intergenic spacers (trnL-trnF, trnC-ycf6, and accD-psaI) of Taiwanese and representative begonias from adjacent Asian regions were analyzed. The nuclear gene trees and cpDNA trees all support that B. ravenii (sect. Diploclinium) and B. fenicis (sect. Diploclinium) are two distinct species and belong to different clades of sect. Diploclinium. Based on cpDNA and nuclear gene trees, we hypothesize the following：Four putative diploid progenitors probably had involved in the formation of Taiwanese endemic polyploid begonias, as suggested by the four clades (A, B, C, and D) in both nuclear gene trees. With the exception of B. longifolia, the exact identity of the other three putative progenitors remains unclear. Two 2n=38 groups (B. taiwaniana and Chitoensis group) were formed from A × C and B × D hybrids, respectively, with A and B as maternal progenitors. B. formosana was derived from the hybrid between the progenitor of Chitoensis group (BD) and B. longifolia. The 2n=52 group are hypothesized to have originated from polyploidizations and subsequent aneuploidization between the two 2n=38 groups. Begonia tengchiana (2n=82) might have derived from the hybrids between B. taiwaniana and B. formosana.