Advances on Citrus Somatic Hybrids and Inheritance of Their Nuclear and Cytoplasmic Components

Protoplast fusion can circumvent reproductive barriers facing citrus, such as polyembryony and female and/or male sterility, and paves way for citrus cultivar improvement. Two kinds of fusion patterns were employed for citrus, leaf-derived protoplasts + embryogenic callus protoplasts and embryogenic callus protoplasts + embryogenic callus protoplasts. More than 200 cases of somatic hybrids have been obtained via symmetric and asymmetric fusions, most of which are allotetraploids, followed by leaf-parent-type diploids and plants of other ploidy. The somatic hybrids hold great potential for citrus rootstock improvement and can be used as staminate parents for interploidy crosses aiming to produce seedless triploids. The somatic hybrids can be identified by cytology, morphology and genetic markers. Nuclear and cytoplasmic genomes characterization of the somatic hybrids demonstrated that the tetraploid somatic hybrids possessed additive nuclear components from both fusion parents, whereas only leaf parents\' nuclear patterns were detected in the diploid somatic hybrids with morphology similar to the leaf parent. Unilateral or uniparental segregation of cpDNA was observed in most of the somatic hybrids with the exception of cpDNA co-existence in few cases. As far as mtDNA is mainly concerned the organelle derived from the embryogenic parents which presented in the somatic hybrids, whereas recombination or loss of mtDNA were reported for some fusion combinations.