SELF-INCOMPATIBILITY SYSTEMS IN FLORICULTURE CROPS

Self incompatibility is a genetically based mechanism which controls mating within species of Angiosperms by imposing a physiological barrier between pollination and fertilization. Although early workers spoke of self incompatibility as self sterility (leading to S as the designation for the incompatibility locus), the term sterility is inappropriate. Incompatibility differs from sterility in that gametes, both male and female, are formed and functional. Sterility results from lack of flowering, absence of functional male and/or female gametes, post fertilization endosperm or embryo death.

Nearly half of the major crop and ornamental species of the world occur in genera known to possess self incompatibility (Brewbaker, 1957). In fact, the relationship of self incompatibility to phylogeny suggests that this efficient system for enforcing outbreeding may have been one of the fundamental characteristics allowing Angiosperms to evolve to vegetative dominance (Whitehouse, 1950).

The first operational genetic explanation of self incompatibility, based on more than 10 years of study of the system in the ornamental Nicotiana alata, was published in 1925 by East and Mangelsdorf. In the 51 years since that report, genetic models for two more homomorphic and two heteromorphic self-incompatibility systems with broad application have been described. Novel incompatibility systems apparently restricted to a single species have also been reported. A fundamental assumption common to all of these models is that fertilization is prevented whenever a self-incompatibility identity factor in the pollen grain or pollen tube matches a similar self-incompatibility identity factor in the pistil. The models differ in how the identity factor comes into being.