POSTZYGOTIC SEED ABORTION IN THE GENETIC SYSTEM OF STYLIDIUM (ANGIOSPERMAE, STYLIDIACEAE)

Reduction in seed set after self-pollination compared to that after cross-pollination is widespread in Stylidium. No evidence was found for the existence of any kind of prezygotic incompatibility. Evidence is presented to support the suggestion that recessive lethal equivalents act to discriminate against the products of self-pollination. These recessive lethal factors induce ovule or seed abortion after entry of the pollen tube into the micropyle of the ovule, and they are abundant in natural populations of most perennial Stylidium species. Patterns of seed set suggest that an average Stylidium perennial plant may be heterozygous for up to about 20 seed-aborting lethal elements, and in populations of some species, seed-aborting lethal elements may occur at several hundred loci.