Heteromorphic incompatibility system under disruptive selection

Forty populations of Pemphis acidula sampled from Africa to Taiwan have been analysed for heterostylic incompatibility. All populations except one are distylic. The exceptional population comes from southern Madagascar. It consists of 100% long homostyled plants that are self-compatible. A semi-homostyle which is self-incompatible in some and self-compatible in others is found in nine populations at 10% frequency. Measurements of anthers, styles and pollen grains reveal that all the distylic populations are similar. Both the long- and short-styled plants have two levels of anthers with two sizes of pollen grains. In a more extreme form this is a feature usually found only in tristylic species. If distylic Pemphis has been derived from tristyly by the loss of the ‘mid’ type, then the end point of selection would be equal anthers and pollen within each type. The widespread distribution of the forty populations and their similarity indicate that this final equilibrium is unattainable. The explanation of this paradox comes from the homostyle population. The two rows of anthers are at equal heights and the two classes of pollen grains are equal in size. Thus in a self-compatible, single type population, selection has gone to completion. In the distylic populations the selection within the two types, for equality of anthers and pollen grains, is in opposite directions in each type. The 'mid’ anthers have to be raised in the short-styled and lowered in the long-styled plants. Similarly the selection of ‘mid’ pollen is in opposite directions. This disruptive selection keeps the equilibrium at an intermediate point.