Plant Cell, Tissue and Organ Culture 61: 165–168, 2000. © 2000 Kluwer Academic Publishers. Printed in the Netherlands. 165 Research notes Transmission of organelle genomes in citrus somatic hybrids Cristina D. Moreira 1,* , Christine D. Chase 1 , Fred G. Gmitter Jr 2 & Jude W. Grosser 2 University of Florida, Institute of Food and Agricultural Sciences, 1 Horticultural Sciences Dept., P.O. Box 110690,USA; Gainesville, FL 32611-0690, USA; 2 CREC, P.O. Box 1088, Lake Alfred, FL 33850, USA ( * requests for offprints; Fax: 352-392-6479; E-mail: cdmor@ufl.edu) Received 30 November 1999; accepted in revised form 4 May 2000 Key words: chloroplast DNA, citrus, mitochondrial DNA, RFLPs, somatic hybrids Abstract Restriction fragment length polymorphisms (RFLPs), were used to analyze the organelle composition of two-year- old trees, recovered from two different experiments: protoplasts from embryogenic cell suspensions of ‘Succari’ sweet orange (C. sinensis L. Osbeck) were fused with leaf protoplasts of Citropsis gilletiana Swingle & M. Kell or to leaf protoplasts of Atalantia ceylanica (Arn.) Oliv. The somatic hybrids of both fusion combinations had the mitochondrial genome from the embryogenic partner. In some somatic hybrids, non-parental fragments were ob- served among the mitochondrial patterns. Somatic hybrids between ‘Succari’ + Atalantia had plastid DNA from the embryogenic parent, while the somatic hybrids of ‘Succari’ + Citropsis all had both parental chloroplast genomes. The relative abundance of organelle DNAs in the donor embryogenic and leaf cells may explain the consistent transmission of the embryogenic parent mitochondrial DNA and the inheritance of the chloroplast genome from either parent. In somatic hybridization of citrus, protoplasts derived from embryogenic suspension culture are fused with protoplasts derived from leaf or other non-totipotent cell types (Grosser and Gmitter, 1990). Protoplasts isolated from embryogenic suspensions of ‘Succari’ were fused with leaf protoplasts of Citropsis gilletiana or with leaf protoplasts of Atalantia ceylanica by the PEG method (Grosser and Gmitter, 1996a), with the objective of obtaining tetraploid somatic hybrids for variety improvement. Only ‘Succari’ protoplasts have the capacity to undergo somatic embryogenesis. This trait is under the control of dominant genes and can therefore be expressed in fusion products (Grosser and Gmitter, 1990). Protoplasts of the leaf parents (Citrop- sis and Atalantia) lack the capacity for culture and re- generation in growth regulator-free protoplast culture medium. Also, a 0.3 M concentration of sucrose in the protoplast culture medium was used to reduce or elim- inate whole plant recovery from the embryogenic par- ent. There was no selection at the heterokaryon level. However, regenerating colonies maintained physical integrity throughout the regeneration process. Because the percentage of heterokaryons was relatively low in the mix, regenerating somatic hybrids were generally surrounded by regenerating ‘Succari’ calluses that do not undergo somatic embryogenesis in the media se- quence used. Somatic hybrid embryos were recovered from individual regenerating somatic hybrid colonies, and germinated to provide these plants. As part of this work, we analyzed the organelle genome of 14 somatic hybrids from each of these two combinations. At the time of analyses, these plants were juvenile trees less than two years of age, maintained in a greenhouse. Fifteen micrograms of total DNA from leaves of each of the somatic hybrid parents and the resultant so- matic hybrids were digested with restriction enzymes, separated by electrophoresis through a 0.8% agarose gel, blotted and hybridized as described by Moreira et al. (1996). Table 1 describes the probes used for the organelle DNA analyses. The mitochondrial atpA probe was used to eval- uate the mitochondrial DNA (mtDNA) profiles. All