Floral Dimorphism and Gynodioecy in Thymus quinquecostatus (Lamiaceae)

Abstract

The first study of floral dimorphism and the sexual system in natural populations of a Japanese species of Thymus is presented. Thymus quinquecostatus was previously reported to show a dimorphism in flower size. We investigated floral variation and mode of reproduction to clarify the degree of floral dimorphism and to determine the sexual system. The length of the lower and upper lips, the corolla tube, and the calyx usually showed a monomodal frequency distribution within a population. The length of the outer and inner stamens, however, showed a bimodal distribution. Based on these floral characters the populations were tentatively divided into two discrete morphs: one with long (L) and one with short (S) stamens. The anthers of the L-stamen morph usually produced fertile pollen, while the anthers of the S-stamen morph completely lacked fertile pollen, or poorly developed, abortive pollen grains. Both L- and S-stamen morphs produced fruits and seeds in natural populations. The findings suggest that floral dimorphism in T. quinquecostatus may be an example of gynodioecy, as reported in several other species of Thymus. Reproductively, pistillate plants show significantly higher fruit and seed production than hermaphrodites, as in some gynodioecious species.