The biology of heterostyly

Abstract Heterostyly is a genetically controlled floral polymorphism that is known to occur in 24 families of flowering plants. The floral morphs differ reciprocally in stamen and style lengths. They often differ in pollen grain size and production, and may differ in pollen exine sculpturing, pollen colour, presence of starch in pollen, stigmatic papillae, or corolla size or morphology. There are two morphs in distylous plants and three morphs in tristylous plants. Distyly is much more common than tristyly. Tristyly is known to occur only in the Lythraceae, Oxalidaceae, and Pontederiaceae, although there are unconfirmed reports in the Connaraceae and in Hugonia of the Linaceae. In distylous plants the supergene determining floral morphology also controls a diallelic sporophytic self‐incompatibility system, so that only pollinations between morphs are compatible. Tristylous plants usually possess a two‐locus diallelic sporophytic self‐incompati‐bility system associated with the floral trimorphism, but whether or not a supergene is involved has not been demonstrated genetically. The functions of the various morphological components of the heterostylous syndrome are not well understood. The difference in style and stamen lengths in the morphs results in asymmetric pollen flow in heterostylous populations. In distylous species, the long‐styled (pin) form has a higher rate of self pollination and captures more total pollen than the short‐styled (thrum) form. Heterostyly does appear to increase disassortative pollination, as Darwin suggested, and may be selectively advantageous because it increases seed set in plants with diallelic self‐incompatibility, where many of the plants in a population are cross‐incompatible. Differences in pollen size, pollen production, and pollen storage products may have been selected to offset the effects of asymmetric pollen flow among forms, and/or to compensate for differences in style length. Current models suggest that diallelic self‐incompatibility systems evolved before the morphological differences, and that the evolution of diallelic self‐incompatibility may involve a functionally gynodioecious stage. Both distylous and tristylous systems have evolved into systems with greater inbreeding, by the loss of self‐incompatibility, the production of homostyles, or the loss of one or more morphs from self‐compatible populations. In addition, distylous systems have occasionally evolved into dioecism.