'Heterostyly' refers to those situations when a plant species has two or more different positional arrangements of its anthers and stigma(s), a genetic polymorphism providing a physical mechanism to promote outbreeding - which natural selection has determined is generally a 'good thing'.
This page currently deals with one such species, the Cowslip (Primula veris), a plant of pastures and short turf on well-drained, often lime-rich soils, still locally common in England but rarer and mainly coastal here in Scotland, though sometimes supplemented by misguided local planting or inappropriate use of 'wildflower' seed. It has suffered local declines and extinctions due to habitat loss and we have just one surviving population here in the Paisley area, native or long-established on a railway embankment.
The Cowslip has two basic arrangements of anthers and stigma:
(It might be noted that the pollen of a thrum plant could drop down the corolla-tube onto its own stigma. In fact the stigma is mature and receptive to pollen some 2-3 days before its own anthers dehisce (i.e. the flowers are protogynous) and as described below, thrum pollen is mechanically unable to fertilise a thrum flower.)
It can be seen in each case that the dominant allele codes for 'thrum' characters and the recessive allele for 'pin' characters. Since thrum × thrum crosses cannot occur (or rarely so in the Cowslip, not at all in the related Primrose), thrums are heterozygous GPA/gpa, whereas pins are homozygous recessives, gpa/gpa.
Excellent scanning electron photomicrographs showing stylar papillae, pollen (much more surface detail) and successful and unsuccessful pollinations are on the University of Hamburg 'Botany online' site here.
So successful pollinations are thrum × pin and pin × thrum (i.e. the reciprocal crosses with either thrums or pins as the pollen parent). Pin plants will produce gpa gametes only, while thrum plants will produce equal numbers of GPA and gpa gametes after meiosis. This is the equivalent of a Mendelian backcross, producing heterozygotes (thrums) and recessive homozygotes (pins) in equal numbers. However, as a small number of pin × pin crosses are successful, there is normally a small excess of pins over thrums in a population (certainly so in Primrose (P. vulgaris) populations, presumably also in Cowslip populations).
The purpose of this page is to provide illustrations of heterostyly, not to duplicate accounts in text-books. Consequently there is no account here of the advantages of outbreeding, beyond mentioning the generally greater 'fitness' of heterozygotes (avoiding inbreeding depression, the possibilities of dominance modification, etc.). It should be noted that natural selection is reactive and can never plan for the future, so while maintenance of variability in a population may enhance its long-term chances of survival, this normally has little directly to do with the chances of a given individual passing on its genes to the next generation.
Rare crossings-over within the super-gene can produce other combinations of characters, notably in the case of the 'long-homostyle' recognised and studied in the Primrose (P. vulgaris), by J.L Crosby. In this variant, the 'pin' style and stigma are combined with the 'thrum' anther position and 'thrum' pollen. Consequently the long-homostyle can self-fertilise and may locally have a selective advantage over pins and thrums where inbreeding depression has not become a factor, where the genetically impoverished population is suited to its environment (as in two woods in S.W. England) and where the exposure of anthers and stigma together to slug damage is not a major issue. Understanding of the situation has been complicated by misidentifications and incorrect claims of homostyles; a useful and reliable account was given by Sheppard (1975).
Our British native species of primrose, i.e. P. veris, P. vulgaris, P. elatior and P. farinosa, are heterostylous, as are other members of the family Primulaceae here. It can be concluded that heterostyly is a clear advantage in these plants. The exception is the Scots Primrose, P. scotica, which is always homostylous and is self-fertile. Although regarded as an endemic species on our north coast, it is closely related to other sub-arctic plants undoubtedly derived from a common ancestor, in which guaranteed seed production in northern latitudes (despite unreliability of insect pollinators) has proved more important than loss of heterozygosity. I'll add a picture sometime.
All photographs: Sedgefield, Co. Durham, May 2001