Distyly

Distyly is a type of heterostyly in which a plant demonstrates reciprocal herkogamy. This breeding system is characterized by two separate flower morphs, where individual plants produce flowers that either have long styles and short stamens (traditionally referred to as “pin”, modern nomenclature refers to them as the “long-morph” or "L-morph" flowers), or that have short styles and long stamens (traditionally referred to as “thrum”, modern nomenclature refers to these as the “short-morph” or "S-morph" flowers). However, distyly can refer to any plant that has two morphs if at least one of the following characteristics between flowers produced by different plants is true; there is a difference in style length, filament length, pollen size or shape, or the surface of the stigma. Most distylous plants are self-incompatible so they cannot fertilize ovules in their own flowers. Specifically these plants exhibit intra-morph self-incompatibility, flowers of the same style morph are incompatible. Distylous species that do not exhibit true self-incompatibility generally show a bias towards inter-morph crosses - meaning they exhibit higher success rates when reproducing with an individual of the opposite morph. 
Background.
In a study of "Primula veris" it was found that pin flowers exhibit higher rates of self-pollination and capture more pollen than the thrum morph. Different pollinators show varying levels of success while pollinating the different "Primula" morphs, the head or proboscis length of a pollinator is positively correlated to the uptake of pollen from long styled flowers and negatively correlated for pollen uptake on short styled flowers. The opposite is true for pollinators with smaller heads, such as bees, they uptake more pollen from short styled morphs than long styled ones. The differentiation in pollinators allows the plants to reduce levels of intra-morph pollination.
Charles Darwin made the first scientific account of distyly in 1877 in his book "The Different Forms of Flowers on Plants of the Same Species".
Models of evolution.
There are two main hypothetical models for the order in which the traits of distyly evolved, the 'selfing avoidance model' and the 'pollen transfer model'. 
Genetic control of distyly.
A supergene, called the self-incompatibility (or S"-") locus, is responsible for the occurrence of distyly. The "S"-locus is composed of three tightly linked genes ("S"-genes) which segregate as a single unit. Traditionally it was hypothesized that one "S-"gene controls all female aspects of distyly, one gene that controls the male morphological aspects, and one gene that determines the male mating type. While this hypothesis appears to be true in "Turnera" , the "S"-locus in "Primula" contains five "S-"genes. The S-morph is hemizygous for the "S-"locus and the L-morph does not have an allelic counterpart "." The hemizygotic nature of the "S-"locus has been shown in "Primula" ", Linum" ", Fagopyrum" "," and "Turnera". The "S-"loci of "Primula" and "Turnera" have been completely described, meaning all "S-"genes have been identified.
The presence of the "S-"locus results in changes to gene expression between the two floral morphs, as has been demonstrated using transcriptomic analyses of "Lithospermum multiflorum" ", Primula veris", "Primula oreodoxa" ", Primula vulgaris" and "Turnera subulata".