Embryo Rescue Techniques for Wide Hybridization

Abstract

In crop improvement it is necessary to expand gene resources by selective introgression of alien genes in a good agronomic base. This is more important if the existing genetic variability in the breeding material is limited, as is the case in oleiferous Brassica. Wide hybridization can be used to transfer the desirable variability. Usually, it is difficult to produce such hybrids due to cross-incompatibility barriers. These may result from either failure to bypass the sexual constraints or hybrid breakdown. Cross-incompatibility is primarily caused by incompatible pollen-pistil interaction which precludes the formation of a viable zygote. This phenomenon, however, has not been investigated extensively (Shivanna 1982). Hybrid breakdown, hybrid inviability and hybrid sterility are some other recognizable post-fertilization barriers (Raghvan 1986). They may result from arrested embryo development, endosperm disintegration, abnormal development of ovular tissue or chromosome and genetic instability. Hybrid inviability may result from the action of specific genes or due to disharmony between the nucleus of one species and the cytoplasm of another, or adverse embryo-endosperm interaction. Recent developments in biotechnology and embryo rescue techniques may provide new genetic variability for the breeding of cruciferous crops (Chiang et al. 1978, 1980; McNaughton et al. 1978; Mohapatra et al. 1987). Various embryo rescue techniques are: ovary, ovule and embryo culture. In case of an incompatible cross, after a certain period of growth of the hybrid embryo the incompatible reaction becomes acute and it is imperative to culture the embryo or ovule before initiation of the incompatibility reaction (Takeshita et al. 1980). For ovule culture, younger ovules (10–21 days old) can be used, but for embryo culture, older ovules should be used, depending upon the size of the embryo. Embryos cultured during the globular stage may not survive in vivo but such embryos can survive during ovule and ovary culture. These techniques will help in broadening the genetic base of crop brassicas through the exploitation of secondary and tertiary gene pools. Various aspects and achievements of embryo rescue techniques are discussed in this chapter.