圆唇苣苔属(Gyrocheilos)花柱侧偏弯折现象及其传粉适应机制

圆唇苣苔属(Gyrocheilos)是苦苣苔科的中国特有属,有5种,全部狭域分布在我国西南及广东的高海拔山区。圆唇苣苔属所有物种的花柱侧偏且花柱顶端呈90°弯折,使得柱头位于花开口的中央位置。这种独特的侧偏弯折花柱结构,说明圆唇苣苔属可能有着特殊的演化历史和适应机制。为揭示这种特殊的花柱侧偏弯折现象的发生范围、发育过程及其传粉适应机制,该研究在圆唇苣苔(Gyrocheilos chorisepalus)、折毛圆唇苣苔(G. retrotrichus)和微毛圆唇苣苔(G. microtrichus)3个物种中开展了花部综合征观察,并研究了广东大雾岭保护区内的折毛圆唇苣苔花发育过程、花部特征和繁育系统以及传粉过程。结果表明:(1)微毛圆唇苣苔只有花柱左偏弯折现象,而圆唇苣苔和折毛圆唇苣苔虽然大部分花是花柱左偏弯折,但在部分个体中出现了少量的花柱右偏弯折现象(占种群总花数的2%～3%)。(2)传粉观察发现,折毛圆唇苣苔在花蕾期即出现了花柱弯折现象,2个可育雄蕊的花药合生、位于花冠筒喉部中央位置,与侧偏花柱不存在左右镜像对称关系。(3)折毛圆唇苣苔的花粉胚珠比(P/O)为456.98±15.55,属于兼性异交繁育系统。折毛圆唇苣苔存在一定的传粉限制,自交授粉可以结实,但异交种子萌发率更高,可能存在近交衰退。(4)折毛圆唇苣苔的访花昆虫较少,访花频率较低,主要访花昆虫有隧蜂、熊蜂、食蚜蝇等; 熊蜂体型较大,访花时降落在弯折花柱和花瓣下唇,胸部侧面及下部能有效接触到柱头。(5)反射率结果显示,折毛圆唇苣苔花瓣反射波长范围集中在紫光和蓝紫光区域,花冠的反射波长范围与蜂类视觉范围一致且花冠筒外侧和花瓣下唇的反射强度最大,更容易吸引蜂类落置在花冠宽大的下唇; 圆唇苣苔属的花柱侧偏弯折现象可能来自近缘的长蒴苣苔属(Didymocarpus)的花柱下弯现象或镜像花(mirror-image flowers)。综上认为,这种侧偏弯折的花柱,可能通过提供昆虫降落平台,使得柱头位于花开口中央和花瓣下唇的上方位置,提高了柱头接触访花昆虫的概率,是适应高海拔地区低频率访花者的一种机制。

Style lateral bending and its pollination adaptation in Gyrocheilos(Gesneriaceae)

Gyrocheilos is a small genus of Gesneriaceae endemic to hight altitude mountains in Southwest China and Guangdong Province, with only five species. This genus is characterized by laterally bending style, which curves 90° at the top of the style and the stigma right at the mouth of floral tube. This unusual floral trait may have special evolutionary and adaptive mechanisms. In this paper, three Gyrocheilos species, i.e. G. chorisepalus, G. retrotrichus, G. microtrichus were examined to figure out the developmental patterns of floral lateral bending. Breeding systems, floral syndrome and pollination processes were also studied in G. retrotrichus at Dawuling Mountain, Guangdong Province, to explore its pollination adaptations. The results were as follows:(1)G. microtrichus had only left-bending style, while right-bending style were observed in several flowers(2%-3% of the total flowers)of G. chorisepalus and G. retrotrichus, although the individuals and populations were dominated by left-bending style.(2)Pollination observation found that style lateral bending occurred at the early stage of floral bud in G. retrotrichus, with two fertile stamens were anther-united and hidden at the middle of the floral throat, and there was no left and right mirror symmetry relationship with the lateral style.(3)The pollen-ovule ratio(P/O)was( 456.98±15.55), belonging to facultative outcross breeding system. G. retrotrichus had a certain pollen limitation and self-pollination, but the germination rate of outcross seeds was higher, and there might be inbreeding decline.(4)There were few floral visitors and the frequency was low of G. retrotrichus. The main floral visitors were Halictidae, Bombus and Syrphidae. Bombus were large, and they landed on the bent style and the lower lip of the petals when visiting flowers. The side and lower part of the chest could effectively contact the stigma.(5)The detected reflected lights of flowers were composed of purple and blue-purple lights and the lower lips and outer floral tube with the highest reflection intensity, more likely to attract bees to land on the corolla''s lower lip. Molecular phylogeny indicated that the closest genus of Gyrocheilos was Didymorcarpus, which was characterized by style downward bending and mirror-image flowers, suggesting the style lateral bending in Gyrocheilos probably evolved from either of these conditions. In conclusion, the lateral bending style might provide landing platform for pollinators and the stigma above the lower lips increase contact probability the floral visitors, which is an adaptation to the very low insect visitation due to fragmented and foggy habitats in high altitude areas.