Primula Fertility and Seed Set

 

A feature of primulas is their heterostyly nature. Almost all Primulas have a pin form (where the level of the stigma is above the anthers) and a thrum form (where the level of the stigma is below the anthers). A few species are homostyle (where the level of the stigma and anthers are equal). Often homostylous species are self-fertile, but for the majority of Primula species, the best seed set is between pins and thrums. When planting out, group at least three or more seedlings to ensure you have both types growing together to encourage seed production . Hand pollination can be used to increase your seed harvest.

Why do you get better seed set between Pins and Thrums?

Primula heterostyly is controlled by a supergene which is a group of neighboring genes on a chromosome which are inherited together. Different loci on the same supergene control style length, anther height, pollen size and papilla size on the stigma. Pin x pin and thrum x thrum mating rarely produce seed due to incompatibility (see also) between the pollen and style which differ in the two morphs. Occasionally cross over occurs between the genes and two complementary kinds of homostyle, long and short, are formed, with the stigma characteristic of one type and the pollen characteristic of the other, allowing self-fertilization to happen. Note that Primula are protogynous, that is the female stigma is receptive well before the male pollen is mature which is another barrier to self-fertilization.

See also Growing Primula From Seed.