Plants have mechanisms to promote outbreeding and thereby to increase their
genetic diversity. In species that are self-incompatible, self-pollen is r
ejected by the stigma. This mechanism has been the subject of intense study
for many years and, in the past two years, significant progress has been m
ade in identifying the genes involved in Brassica. Self-recognition involve
s two genes, one of which determines the male and the other the female spec
ificity. Considerable progress has also been made on the mechanism by which
self-recognition leads to pollen rejection, although the delineation of al
l the genes involved is still not complete.