effectiveness of SI in promoting outbreeding is believed to be one of the most important
factors that ensured the evolutionary success of flowering plants, an idea first promoted
by Darwin. It is a genetically controlled phenomenon, and in many cases the control is
by a single locus known as theS locus. This locus often has up to several hundred
alleles in some species. The SI mechanism promotes outcrossing by arresting “self”
pollen tubes as determined by the genotype at the S locus (Fig. 2.12). SI is based on the
Figure 2.12.Self-incompatibility systems in plants may be gametophytic (a) or sporophytic (b). In
gametophytic self-incompatibility, the pollen grain will not grow and fertilize ovules if the female
plant has the same self-incompatibility (S) alleles. In sporophytic self-incompatibility, the diploid
parent prevents germination of pollen grains that share an allele with the parent. (Adapted from
Briggs and Walters 1997).
2.4. PLANT REPRODUCTIVE BIOLOGY 37