Molecular Genetics Protocol

Molecular genetics is used as a term in the broad sense to include all activities related to citrus genetic improvement that are based on use of molecular techniques to manipulate genes at the level of nucleic acids (DNA or RNA). These activities also include fundamental genetic studies, as they provide a basis for molecular dissec- tion of key traits based on plant perform- ance (Fig. 2.4). Understanding the inheritance of important traits is fundamen- tal to all genetic improvement strategies whether the traits sought are under qualita- tive or quantitative genetic control. Molecular genetics, likewise, includes all

Fig. 2.4. Protocol for molecular genetics and its relationship with other protocols.

efforts linked to genomic sciences relating genetic information on important pheno- typic traits to a molecular-based under- standing of the structure and function of genes, to provide the basis for more precise and specifi c manipulation of tree and fruit characteristics. Genomic science also includes the development of DNA marker-based maps, as well as integrated physical and genetic maps, and ultimately whole-genome sequence information, for greater understanding and manipulation opportunities in the future. Genetic maps, in turn, by directly linking molecular mark- ers to genes controlling economically important traits, make it possible for selec-

tion targeting certain goals of the cultivar development process to be accomplished more quickly and effi ciently than by actual evaluation of plant traits. Because more effi - cient methods result in the capacity to eval- uate many more plants, the likelihood of identifying plants recombinant for multiple valuable and desirable characteristics is greatly increased.

One outcome of genomic research is the ability to modify very specifi c individ- ual characteristics in powerful ways: by iso- lating genes from other sources within the citrus gene pool for incorporation directly into commercial cultivars; by modifi cation of existing genes to produce specific

changes in plant characteristics; or by uti- lizing genes from any other organism that potentially could have benefi cial pheno- typic effects on citrus tree performance or fruit characteristics. Any genes that are iso- lated and/or modifi ed can be incorporated into existing cultivars or breeding lines using genetic transformation technologies. The programme also includes a Citrus Core Transformation Facility, supported jointly by industry grants and university funds; this service is available to researchers within and outside of the citrus genetic improvement community, to increase the opportunities for testing potentially useful genes from outside the citrus gene pool. Ultimately, transformation of citrus germplasm with new and/or modified genes feeds into the scion and rootstock cultivar development protocol as another source of genetic variation for evaluation and selection. Research is conducted by some of the team members to improve the effi ciency of the existing Agrobacterium -, protoplast- or biolistic-based transforma- tion systems. A major goal is to develop an effi cient system for all commercially impor- tant cultivars and elite breeding parents. Other objectives include minimizing the impact of citrus juvenility on the time from transformation to fruiting, minimizing tech- nology fees associated with certain compo- nent elements of the transformation process, and developing more environ- ment- and consumer-friendly transforma- tion systems.