Tristyly

The three floral morphs of tristylous plants are based on the positioning of the male and female reproductive structures, as either long-, mid-, or short-styled morphs. Often this is shortened to L, M and S morphs. There are two different lengths of stamens in each flower morph that oppose the length of the style. For example, in the short-styled morph, the two sets of stamen are arranged in the mid and long position in order to prevent autogamy. In trimorphic incompatibility system, full seed set is accomplished only with pollination of stigmas by pollen from anthers of the same height. This incompatibility system produces pollen and styles with three different incompatibility phenotypes because of the three style and stamen lengths. Tristylous species have been found in several angiosperm families including the Oxalidaceae, Pontederiaceae, Amaryllidaceae, Connaraceae, Linaceae and Lythraceae, though several others have been proposed. There is not a consistent consensus on the specific criteria defining tristyly. In a 1993 review of tristylous evolutionary biology, Barrett proposes three common features for tristylous plants, 1) three floral morphs with differing style and stamen height, 2) a trimorphic incompatibility system, and 3) additional polymorphisms of the stigmas and pollen. == Heteromorphic Incompatibility System ==

This incompatibility system is a specific mechanism employed by heterostylous species, where incompatibility is based on the positioning of the reproductive structure of the flower. In tristylous species this is based on two loci, S and M with one allele dominant at each loci. In tristylous species, incompatibility varies, with some species showing varying degrees of compatibility outside of the reciprocal herkogamy pattern of pollination. Darwin noted weak incompatibility commonly occurring in the M-morph of Lythrum salicaria. == Evolution ==

Heterostyly has been found in at least 28 families, while tristyly has only been found in six families. The rarity and complexity of tristyly coupled with its development in a variety of unrelated plant families has made its evolution and adaptive significant hard to discern. It would be assumed that distyly would be the intermediate stage to tristyly but it has also been proposed that that distyly originated from tristyly through the loss of one of the floral morphs. == References ==