Distant Hybridization of Crop Plants

Wild taxa are invaluable sources of resistance to diseases, insects/ pests, nematodes, temperature extremes, salinity and alkalinity stresses, and also of nutritional quality; adaptation; genetic diversity and new species. Utilization of wild relatives of a crop depends largely upon its crossability relations with cultivated varieties. Sev­ eral wild species are not crossable with the commercial cultivars due to various isolation barriers. Furthermore, in a few cases, hybridiza­ tion is possible only in one direction and reciprocal crosses are not successful, thus depriving the utilization of desired cytoplasm of many species. However, techniques have been developed to over­ come many barriers and hybrid plants are produced. New crop species have been developed by overcoming the F 1 sterility and producing amphidiploids and such crops are commercially being grown in the field. The segregation pattern ofF 1 hybrids produced by distant hybridization in segregating generations are different from the intervarietal hybrids. In former cases, generally, unidirectional segregation takes place in early generations and accordingly, selec­ tion procedures are adopted. In most of the cases, backcross or modified backcross methods have been followed to utilize wild species, and thus numerous types of resistance and other economical attributes have been transferred in the recurrent parents. Protoplast fusion has been amply demonstrated in a number of cases where sexual hybridization was not possible and, as a result, hybrids have been produced.

1 Introductory Considerations on Distant Hybridization.- 1.1 Introduction.- 1.2 Various Uses of Wild Species.- 1.3 Basic Considerations on Efficient Utilization of Wild Species.- 1.4 Interspecific Barriers.- 1.5 Sexual Versus Somatic Interspecific Hybridization.- 1.6 Asexual Transfer of Alien Genes into Plants Through Genetic Engineering.- 2 Crossability Relations.- 2.1 Introduction.- 2.2 Crossability Relations of Selected Crop Plants.- 2.3 Barriers to Crossability.- 2.4 Genetics of Crossability Relations.- 2.5 Conclusions.- 3 Unilateral Incompatibility.- 3.1 Introduction.- 3.2 Unilateral Interspecific Incompatibility.- 3.3 Incongruity.- 3.4 Genetic Studies.- 3.5 Genetics of Other Reciprocal Cross Differences.- 3.6 Physiology of Unilateral Incompatibility.- 3.7 Ultra Structural Aspects of Unilateral Incompatibility.- 3.8 Biochemical and Molecular Basis of Unilateral Incompatibility.- 3.9 Implications.- 4 Overcoming the Barriers in Hybridization.- 4.1 Introduction.- 4.2 Techniques for Overcoming Pre-Fertilization Barriers.- 4.3 Techniques for Overcoming Post-Fertilization Barriers.- 4.4 Conclusions.- 5 Amphidiploids/Synthetic Crop Species.- 5.1 Introduction.- 5.2 Genetics.- 5.3 Cytology.- 5.4 Evolutionary Advantage.- 5.5 Cytogenetics of Amphidiploid Crop Plants.- 6 Cytogenetics of F1 and Their Progenies.- 6.1 Introduction.- 6.2 Cytology of the F1 Hybrids During the Premeiotic Stages.- 6.3 Meiotic Behavior.- 7 Apomixis in Distant Hybridization.- 7.1 Introduction.- 7.2 Embryo Sac Development in Apomictic Species.- 7.3 Embryo and Endosperm Development in Apomicts.- 7.4 Adventive Embryogenesis.- 7.5 Special Cases.- 7.6 Genetics of Apomixis.- 7.7 Factors Affecting Apomixis.- 7.8 Apomixis in Crop Improvement.- 7.9 Improvement of Apomictic Crops.- 7.10 Seed Production in Apomicts.- 7.11 Concluding Comments.- 8 Selection Theory and Procedure in Progenies of Distant Hybrids.- 8.1 Introduction.- 8.2 Theoretical Considerations.- 8.3 Self-Pollinated Crops.- 8.4 Cross-PollinatedCrops.- 9 Utilization of Wild Species.- 9.1 Introduction.- 9.2 Use of Wild Relatives.- 9.3 Breeding Methods.- 9.4 Enhancement of Recombination.- 9.5 Cytogenetical Technique.- 9.6 Prebreeding.- 9.7 Haploidy.- 10 Somatic Cell Hybridization.- 10.1 Introduction.- 10.2 Methodologies for Protoplast Isolation and Regeneration.- 10.3 Protoplast Fusion.- 10.4 Selection Procedures.- 10.5 Identification and Characterization of Somatic Hybrids.- 10.6 Cytoplasmic Genes.- 10.7 Fusion-Mediated Gene Transfer (FMGT) of Nuclear Traits.- 10.8 Some Examples of Agricultural Applications.- 10.9 Concluding Remarks.- 11 Molecular Biology/Genetic Transformation in Distant Hybridization.- 11.1 Introduction.- 11.2 New Methodologies.- 11.3 Introduction of DNA in Cells.- 11.4 Summary.- References.