ABSTRACT
Citrus cultivars and their relatives are a vast but comparatively untapped reservoir, possessing resistance to both biotic and abiotic stresses. Genetic recombination between citrus and its relatives by sexual hybridization is impeded by sexual incompatibility, polyembryony, female and male sterility and high heterozygosity observed in many cultivars (Cameron and Frost 1968). Protoplast fusion can overcome these barriers and many somatic hybrids between citrus and related genera (Grosser et al., 2000), i.e. Severinia (Grosser et al., 1996), Citropsis (Ling and Iwamasa 1994), Atalantia (Louzada et al., 1993) and Clausena (Guo and Deng 1998), have been produced. However, most of these hybrids were allotetraploid plants that incorporated the complete genomes from the fusion partners. Integration of whole genomes from both fusion parents has negative impacts on the growth of the resulting hybrids owing to somatic incompatibility. A case in point is the somatic hybrid plants between S. disticha and citrus cultivars that were killed by a fungal disorder (Grosser et al., 1996). As a consequence, the practical use of these hybrids has been hampered, owing to somatic incompatibility at various levels and to linkage and correlation between the desirable and undesirable traits.
