Invited review: Somatic hybridization in citrus: An effective tool to facilitate variety improvement

Citrus somatic hybridization and cybridization via protoplast fusion has become an integral part of citrus variety improvement programs worldwide. Citrus somatic hybrid plants have been regenerated from more than 200 parental combinations, and several cybrid combinations have also been produced. Applications of somatic hybridization to citrus scion improvement include the production of quality tetraploid breeding parents that can be used in interploid crosses to generate seedless triploids, and the direct production of triploids by haploid + diploid fusion. Applications of somatic hybridization to citrus rootstock improvement include the production of allotetraploid hybrids that combine complementary diploid rootstocks, and to combine citrus with sexually incompatible or difficult to hybridize genera that possess traits of interest for germplasm expansion. A few somatic hybrid tetraploid breeding parents have flowered, are fertile, and are being used as pollen parents to generate triploids. Several allotetraploid somatic hybrid rootstocks are performing well in commercial field trials, and show great promise for tree size control. Seed trees of most of these somatic hybrid rootstocks are producing adequate nucellar seed for standard propagation. Somatic hybridization is expected to have a positive impact on citrus cultivar improvement efforts.