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

Abstract

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.