Abstract:

Andromonoecy, i.e. presence of both male and bisexual (hermaphroditic) flowers on the same individuals of a species, is quite common in both wind- and animal-pollinated seed plants, while its ecological and evolutionary implications are still not fairly understood. In this paper we demonstrate an andromonoecious mating system in a native perennial herb, Globba lancangensis (Zingiberaceae), found in Xishuangbanna, SW China. Firstly, we describe the floral biology of this species: pollen viabilities, fruit-set ratios, behaviors of visiting insects and so forth, and then compare the morphological differences between male and hermaphroditic flowers. Secondly, we present the results of experiments designed to determine its breeding system, and to test whether this system has a self-incompatibility mechanism. Thirdly, we show the variations of sexual expression in this species with emphasis on the change of male : hermaphrodite ratios. Finally, we discuss the ecological and evolutionary implications of the variations in reproductive characteristics.Male and hermaphroditic flowers differed in several dimensions. Hermaphroditic flowers have significantly larger ovaries, longer corolla tubes, marginally wider staminodes and longer labellums. Hermaphroditic flowers have 48.4±12.2 (n=32) ovules in each ovary while male flowers have almost no viable ovules in their much smaller ovaries. Male flowers have 1 372±312 (n=32) pollen grains per anther, which is marginally more than that in hermaphroditic flowers with 1 099±346 (n=32). Male flowers also have a significantly longer anther appendage than hermaphroditic flowers. In natural populations, both male and hermaphroditic flowers open at 2∶30-3∶00 and the anthers dehisce at 5∶30-6∶00. There is no significant difference between male and hermaphroditic flowers in flower-opening and anther-dehiscing time. However, the male flowers shed at 13∶30-14∶00 while hermaphroditic flowers may stay until 4∶00 next morning. The pollen viabilities of hermaphrodite flowers were higher until 16∶00 in contrast to male flowers whose pollen viabilities decreased abruptly after noon. A ramet may present either a single male or a single hermaphrodite flower for many days during flowering, making itself temporally androdioecious, and may thus promote outcrossing. The existence of male flowers may offer offsets of pollen for the perfect flowers, which have relatively low P/O ratios (23±7, n=32). Thus, G. lancangensis may achieve its seed set by regulating the ratios of male to hermaphroditic flowers in community. There were no significant differences in fruit-set ratios between hand-selfing (17.2% or 17.7%) and hand-crossing (16.4% or 15.3%) but significant differences in seed-set ratios (seed/ovule ratios) were seen (5.71% or 6.89% vs. 42.76% or 42.99%), which suggested that this plant might maintain the mechanism of self-incompatibility. In contrast to previous reports that butterflies were visitors for Globba, two species of bees, Megapis dorstata and Nomia strigata, are the main visitors for G. lancangensis, while the former is the effective pollinator. G. lancangensis may encourage out-crossing by means of both andromonoecy and self-incompatibility.