Primary homostyly in Primula L. subgenus Sphondylia (Duby) Rupr. and the evolution of distyly in Primula

We have studied eight species classified within Primula L. subgenus Sphondylia (Duby) Rupr. Two have monomorphic flowers and four have typically distylous flowers. P. floribunda Wall, includes both monomorphic and distylous races, while P. boveana Decres. has an incompletely discontinuous dimorphism for style length and pollen size, but is monomorphic for anther position. All the species have been tested and they are fully self-fertile. Evidence suggests that dimorphisms for style length (G/g), pollen size (P/p) and anther position (A/a) are each under independent genetic control. In the monomorphic P. verticillata, variations in stigma papilla length and style cell length appear to result from developmental correlates with style-length variation. Both the monomorphic species show some continuous variation for herkogamy. Several of the species studied show an incomplete development of the full distylous syndrome typical of most Primula species. If it is assumed that these represent evolutionary stages in the development of the full syndrome, and if it is assumed that diallelic incompatibility evolved after the evolution of distyly, then it would appear that the components of the distyly linkage group are likely to have evolved from a non-herkogamous ancestor in the following order: (i) short style mutant G; (ii) small pollen mutant p; (iii) low anther position mutant a.