风筝果镜像花的雌雄异位变化及传粉生态型的形成

亚洲特有植物风筝果(Hiptage benghalensis)镜像花(mirror-image flowers)的雌雄异位变化极大, 柱头与可育花药的空间距离在海南岛与云南西双版纳种群仅有2-4 mm, 而在贵州及云南东南部种群可达10-12 mm。为验证 “镜像花雌雄异位的变化是各个地理种群受到传粉昆虫体型大小选择的结果”假说, 本文开展了传粉生态学与分子生物学研究, 探讨风筝果不同地理种群的雌雄异位变化是否与当地传粉者体型相关, 是否有着遗传基础, 以及是否形成了“传粉生态型”(pollination ecotype)。结果表明, 雌雄异位程度大的贵州种群和云南东南部种群, 其传粉者也是体型较大的木蜂(Xylocopa nasalis)和熊蜂一种(Bombus sp.); 雌雄异位程度小的海南岛种群和云南西双版纳种群的传粉者则是体型较小的大蜜蜂(Apis dorsata)、西方蜜蜂(A. mellifera)。除了雌雄异位的明显分化(F = 53.69, P < 0.01)之外, 花柱长度也在这两类种群之间差异显著(F = 12.5, P < 0.05), 与传粉昆虫身体长度密切相关。通过叶绿体ycf1b、rbcL和细胞核ITS 3个DNA片段的序列分析, 由大型蜂传粉的雌雄异位程度较大的3个种群(贵州和云南东南部种群)遗传关系极近, 与小型蜂传粉种群(雌雄异位较小的海南岛种群和云南西双版纳种群)存在明显的遗传分化。这些结果显示, 风筝果不同地理种群的镜像花雌雄异位程度的变化可能是适应局域不同体型大小的传粉昆虫的选择结果, 而且种群间出现了基因流的隔断, 可能与大型蜂、小型蜂传粉的两种生态型有关。镜像花通过雌雄异位的变化适应不同地域的传粉者而形成传粉生态型, 这种传粉生态型及其传粉隔离机制可能是风筝果属物种形成与维持的一个重要原因。

Pollination ecotypes and herkogamy variation of Hiptage benghalensis (Malpighiaceae) with mirror-image flowers

Herkogamy, the spatial separation of sexual organs within flowers, has long been thought to be a floral adaptation of reducing self pollination in hermaphrodites. Herkogamy of mirror-image flowers in the Asian endemic Hiptage benghalensis (Malpighiaceae) varies greatly between populations on Hainan Island (smaller than 4 mm) and Guizhou Province (larger than 10 mm). Such divergence in herkogamy could be adaptive to pollinator body size, which relates to pollen placement and stigma contact position. Recipient selection between local pollinators and plants may facilitate pollinator assembly in isolated populations, even forming pollination ecotypes. Pollinators were investigated in six geographic populations of H. benghalensis to study whether variations in herkogamy correspond to pollinator size. Molecular markers with nuclear ITS 1/4, chloroplast ycf1b and rbcL were also used to detect genetic differentiation between populations with large or small herkogamy. Our data showed small herkogamy in two populations from Hainan Island and one population from Xishuangbanna (southern Yunnan Province) as they were pollinated by small bees including Apis dorsata and Apis mellifera, with an abdomen width varying from 2 to 4 mm. Two populations from Guizhou Province and one population from Southeast Yunnan with larger herkogamy were pollinated by carpenter bees Xylocopa nasalis and bumble bees (Bombus sp.), with an obvious larger body size with a mean abdomen width of 11 mm. The style length (F = 12.5, P < 0.05) was also significantly different between large versus small herkogamous populations. Genetic data based on nuclear (ITS 1/4) and chloroplast (rbcL and ycf1) DNA also supported the differentiation between small and large herkogamous populations. These results indicated associations between flower and pollinator size, suggesting pollination ecotypes in H. benghalensis. This study indicated that the pollination ecotype in mirror-image flowers was associated with variations in herkogamy.