Summary

Incompatibility barriers are major impediments in crop improvement programs. Nonsynchronous flowering and/or geographical isolation of parental species is common, particularly in wide crosses, and is critical in tree species. Postpollination barriers may operate before and/or after fertilization. Prefertilization barriers act either on the surface of the stigma (by inhibiting pollen germination or pollen tube entry into the stigma) or in the transmitting tissue of the stigma and style. Occasionally, pollen tubes may be inhibited in the ovary or in the ovule. More often, prefertilization barriers are not restricted to a particular level but may be active at all levels. The proportion of pollen grains that complete sequential postpollination events is reduced at each level, with the result that very few or no pollen tubes reach the ovule. In most of the interspecific crosses, pollen inhibition is passive (not as a result of active recognition of the pollen) because of the lack of co-adaptation between the pollen and the pistil. The most common postfertilization barrier is the abortion of the hybrid embryo at different developmental stages. In many of the crosses, this is a result of the lack of endosperm development or of its early breakdown. An understanding of the details of barriers at different levels is important for the application of effective techniques to overcome such barriers.

Introduction

Hybridization is one of the most effective methods of crop improvement programs. Most of the hybridization work carried out so far has used genetic variability within the species, and thus crossability barriers were not the main constraints in breeding programs.