Evolution of dimorphism and population polymorphism in Pemphis acidula Forst

Abstract

The Lythraceae is one of the three families that have a tristylic breeding system based upon long, mid, and short styles and corresponding levels of anthers. Pemphis acidula is unusual in this family by being distylic. Three populations in Singapore and Malaysia have equal numbers of long- and short-styled plants but no mid-styled plants. In the long-styled plant the ‘mid'-level anthers are below the mid position and are close to the low-level anthers. In the shortstyled plant the mid-level anthers are close to the high-level anthers. Pollen of the shortstyled plant is characteristically larger than the pollen of the long-styled plant. Pollen of the mid-level anthers is similar but not identical in size to the pollen of the low anther in the long-styled and similar to the pollen of the high anther in the short-styled plant. This rapprochement of anther heights and pollen sizes within the two types is mainly due to changes in the mid anthers and the degree to which it has occurred varies greatly both within and between populations. Pollen tubes of pollen from both levels of anther in the same type are inhibited in their own type style and grow normally on the other style. The measurements support the hypothesis that Pemphis acidula has been tristylic like other Lythraceae, has lost the mid type, become fully distylic and is still in the process of adjusting by mutation and selection to distyly.