Molecular Genetics and Biotechnology

Citrus breeding and genetics has been fortu- nate to have an early and continuing history

Apomixis

Improved productivity Tree size reduction

Adaptation to soil conditions (salt, calcareous soils) Improvement in poor nutrition

Resolution of bud union problems

Resistance to diseases (Phytophthora, citrus tristeza virus, citrus blight, etc.) Resistance to soil borne pests (diaprepes, nematodes, etc.)

of meeting the needs of a changing industry through scientifi c research on genetic tech- niques. A new generation of scientific methodologies is now available for the study of genomics. It is possible to locate genes rapidly on chromosomes, to isolate genes from plants to study their function at the molecular level, to modify genes and to reintroduce them into living organisms. This is, indeed, a far cry from Gregor Mendel’s 19th century tenant that plant characters are controlled by discrete hered- itary units – which are now called genes.

Modern genetic engineering evolved from the discovery of DNA as the genetic material by Avery and co-workers followed by Watson and Crick’s discovery in 1953 of the double helix structure of deoxyribonu- cleic acid (DNA). Since then, modern day geneticists have been able to use more pre- cise and sophisticated technology to over- come the barriers and greatly to facilitate genetic procedures.

It is essential that future programmes for citrus cultivar improvement emphasize understanding the inheritance of fundamen- tal qualitative and quantitative traits and also be comprehensive. For example, the study of genomics involves a molecular-based knowl- edge of the structure and function of genes. Such information will afford a precise use of such traits in the tree and fruit. Numerous examples can be cited where genomic tech- nology has benefi ted plant improvement efforts. For example, in citrus, the long dura- tion of fi eld evaluation is a major obstacle for developing specifi c characters such as tree size, yield, disease and insect resistance. The use of marker-assisted selections linking molecular markers to genes, which control important traits, will signifi cantly reduce the time required to screen mature tree charac- ters. Other tools of biotechnology have been employed with equal ease to facilitate the progress of work in citrus breeding pro- grammes. Shoot-tip grafting techniques have been used for sanitation of virus-infected germplasm, which has allowed wide dissem- ination and conservation of parental stocks.