Heterostyly, what do we currently know?

Pollen and anther development malfunction in distylous flowers of Palicourea padifolia (Rubiaceae)

“…However, there is evidence that under some ecological scenarios, heterostyly evolves into other derived reproductive systems (Ganders 1979, Bawa & Beach 1981, Lloyd & Webb 1992b, Castro et al 2004, Sakai & Wright 2008, Consolaro et al 2011. For example, with inbreeding advantage (i.e., pollinator shortage, population bottlenecks, and/or colonizing events), natural selection would favor recombination within the distyly supergene and the consequent evolution of homostyly (Barrett et al 1989, Barrett 1990, Carlson et al 2008, Barrett et al 2009, de Vos et al 2012, 2014. In contrast, on the basis of sexual selection and sex allocation theory, floral morphs may exhibit gender specialization and gain differential reproductive success through male versus female function (reviewed in Casper 1992) and, eventually, heterostyly or reciprocal herkogamy evolves into dioecy (Muenchow & Grebus 1989, Barrett 2002, Rosas & Domínguez 2009, Li et al 2010.…”

“…Heterostyly is a genetically controlled floral polymorphism characterized by the presence within populations of two (distyly) or three (tristyly) morphs that differ reciprocally in the position of the stigmas and anthers, functioning to promote cross-pollination (Barrett 2002, Ferrero 2014. The heterostylous syndrome often consists of three associated sets of traits enforcing disassortative mating between floral morphs: reciprocal herkogamy, self-and intramorph incompatibility, and an array of ancillary floral polymorphisms such as pollen grain size and number, stigmatic surface and shape, anther color, corolla length and the timing and amount of nectar reward (Ganders 1979, Barrett 1990, Lloyd & Webb 1992a, Dulberger & Ornduff 2000, Ornelas et al 2004a.…”

Evaluation of the symmetric effect of antagonists in the floral morphs of the distylic shrub <em>Bouvardia ternifolia</em>: a field study

“…In plants with herkogamous flowers is possible to find three types of spatial separation of sexual organs, which include (1) approach herkogamy, where the stamens are short so that the anthers are located below the stigma; (2) reverse herkogamy, where the flowers have long stamens that cause the anthers to overpass the height of the stigma; and (3) heterostyly, which is the reciprocal positioning of anthers with styles of either two (distyly) or three (tristyly) floral morphs (Ferrero 2014, Weber et al 2013.…”

“…If heterostyly is recognized as a mechanism evolved to promote cross pollination (Barrett et al 2000, Webb & Lloyd 1986, theoretically, the pollinator visitation patterns (and consequently pollen flow patterns) for both morphs must be symmetric to fulfill the functioning of distyly and cross pollination. It has been proposed that if this assumption is not met, then the promotion of gender specialization is possible (Beach & Bawa 1980, Ferrero 2014, in which the efficiency of the pollen flow is favored usually in the thrum morph (male function), and its reception in the pin morph (female function), thus allowing the evolution of dioecy (Lloyd 1979, Beach & Bawa 1980, Ferrero 2014. Furthermore, most distylous plants have morphologically analogous tubular flowers that enclose the sexual structures, which restricts pollination to a particular behavior of a specialized pollinator (Beach & Bawa 1980).…”