Pollen Cytology and Self Incompatibility Systems in Plants

POLLEN CYTOLOGY AND SELF.. INCOMPATIBILITY SYSTEMS IN PLANTS JAMES I.. BREWBAKER* HERE are two tpes of pollen grains in the angiosperms, cytologi- cally speaking. One is binucleate- the other, trinucleate. Binucleate grains contain a generative cell and a vegetative or tube nucleus, the generative cell di- viding during pollen tube growth to form the two male gametes. A trinucle- ate grain is formed when the division of the generative cell occurs before the pol- len is shed. There are two

types of self-incompati- bility in homomorphic angiosperms, ge- netically speaking. One is the gaineto- phytic type-the other, sporophytic. Both types are based on the action and interac- tion of multiple alleles of a single S locus (rarely, of two loci). In the gameto- phytic system pollen behavior (i.e., ftunc- tion or failure in a particular mating) is determined gametophytically by the S allele in each grain. The incompatibility inhibition regularly occurs at some stage during pollen tube growth in the pistil of gametophytic species. In the sporo- phytic system pollen behavior is slx)ro- phytically determined (imposed by the maternal genotype). and inhibition com- monly occurs at the stigma surface so as to inhibit ollen germination or drastic- ally curtail pollen tube growth. It is observed here that these phle- nomena are quite regularly associated. the gametophytic system with binucleate grains and the sporophytic with trinu- cleate grains. Self-Incompatibility Among the several schemes which promote or enforce outbreeding in plants, that of self-incompatibility ap- pears to have played a major role in the evolution of flowering plants. Self-in- compatibility is the inability of a plant producing functional male and female gametes to set seed when self-pollinated. Special attention is given in...

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