Introduction. Although somatic hybridization techniques are being ignored by variety improvement programmes for most commodities

Although somatic hybridization techniques are being ignored by variety improvement programmes for most commodities, their contribution to citrus variety improvement continues to expand with increasing com- plexity. During the last decade, a primary application has been for citrus rootstock breeding. Somatic hybridization allows the addition of all dominant traits, irrespective of the heterozygosity level of the breeding material. At the international level, agro- nomic evaluations are ongoing for several interspecifi c and intergeneric hybrids with some very interesting results. Somatic hybrids have been obtained between sexu- ally incompatible species, but their interest for breeding seems to be limited by unfavourable traits and problems of nuclear genome instability for the intersubtribal and intertribal combinations. A second pri- mary application is the exploitation of polyembryonic (apomictic) and sterile cul- tivars for the synthesis of fertile tetraploid

hybrids. The fi nal objective of such ploidy manipulation is the synthesis of seedless triploid cultivars. Somatic hybridization has resulted in a great diversifi cation of the tetraploid gene pool available for sexual hybridization with diploid cultivars, and thousands of triploids have been produced from interploid crosses using somatic hybrid parents. Somatic hybridization has also allowed the direct synthesis of triploid hybrids by protoplast fusion between diploid and haploid lines. Several allo- plasts and cybrids have also been obtained by symmetric somatic hybridization. This material provides an opportunity for nucleo-cytoplasmic interaction studies and should open new avenues for citrus germplasm valorization. An extensive review that describes the fundamental roles of somatic hybridization in broad-based citrus improvement programmes, along with comprehensive lists of somatic hybrids and cybrids produced around the world, was recently provided by Grosser et al. (2000). The primary goal of this chapter

© CAB International 2007. Citrus Genetics, Breeding and Biotechnology (ed. I.A. Khan) 235

is to complement this previous review with evolving molecular techniques that facili- tate somatic hybrid and cybrid genome characterization, and new applications of somatic hybridization and cybridization technologies to facilitate citrus scion and rootstock development. These new applica- tions are possible due to feedback from fi eld trials of scion and rootstock citrus somatic hybrids produced previously, along with molecular marker information that identi- fi es the progenitors of important citrus cul- tivars. These new applications have great potential to facilitate the development of high quality seedless fresh market citrus and superior widely adapted rootstocks. Extensive protocols for the production and characterization of citrus somatic hybrids and cybrids are also provided.