heterostyly

Heterostyly is a plant reproductive strategy in which individuals of a species develop two or more floral morphs with reciprocally arranged reproductive organs, typically a combination of long styles and short stamens in one morph and short styles and long stamens in another. The most common form is distyly, which comprises two morphs often referred to as pin (long style, short stamens) and thrum (short style, long stamens). A rarer, more complex form is tristyly, involving three morphs with three distinct style and stamen configurations. The reciprocal organization of stamens and stigmas between morphs is thought to promote cross-pollination.

A key feature of many heterostylous species is a self-incompatibility system that is tightly linked to the morphs. This heteromorphic self-incompatibility discourages self-fertilization and fertilization within the same morph, favoring pollen transfer between morphs via pollinators. Consequently, heterostyly is associated with disassortative mating and enhanced genetic outcrossing, which can increase genetic diversity in populations.

Heterostyly occurs in a range of angiosperm groups, and it has been extensively studied in Primula, where the phenomenon was first described and remains a model system for understanding plant reproductive biology. Other genera, including Linum and several relatives, also exhibit distyly or related polymorphisms. The distribution and maintenance of heterostyly are influenced by pollinator behavior, population structure, and ecological conditions, and breakdown of the morph system can occur in degraded or highly inbred populations.

The evolutionary significance of heterostyly lies in its promotion of outcrossing and pollen efficiency, contributing to the persistence of genetic diversity in plant populations.