Citrus Hybridization

Our citrus improvement program has relied heavily on interspecific hybridization of complementary parents as needed to package appropriate traits.  Seedlessness is also highly desirable in new citrus fruits, thus we are using ploidy manipulation to generate seedless triploids.   The most efficient method for generating unique triploids is interploid crossing of teteraploids with diploids, using monoembryonic citrus as females.  The availability of quality tetraploids is essential to the success of this approach, and we have invested a great effort to build our tetraploid germplasm.  We have produced quality autotetraploids by several methods, including the screening of seedling populations, in vitro colchicine treatments, and as byproducts from somatic fusion experiments. Crosses of these with selected diploid females of different citrus species generate triploid interspecific progeny. We have also produced numerous allotetraploid somatic hybrids via protoplast fusion, using our model system of fusing embryogenic cell culture protoplasts with leaf protoplasts of a second complementary parent; many of these are interspecific hybrids.  We now have many quality tetraploids that are flowering, and we have an evolving program of interploid hybridization that has generated more than 16,000 triploids to date, mostly interspecific. Significant progress has been made in three primary areas: hybrid mandarin improvement by combining mandarin germplasm with sweet orange and/or grapefruit germplasm with emphasis on flavor, shelf-life and convenience to eat; hybrid pummelo/grapefruit improvement with emphasis on flavor, red pigmentation, disease resistance and reduced furanocoumarins; and acid-citrus fruit improvement (lemon and lime types) with emphasis on disease resistance, cold-hardiness and oil quality. Examples of unique new seedless hybrids in each category will be provided.