J. AMER. Soc. HORT. SCI. 116(6):1082-1088. 1991. Embryo Culture of Lycopersicon esculentum × L. peruvianum Hybrid Genotypes Possessing Heat- stable Resistance to Meloidogyne incognita G.B. Cap 1 , P.A. Roberts 2 , I.J. Thomason 3 , and T. Murashige 4 Department of Hematology, University of California, Riverside, CA 92521 Additional index words. tomato, root-knot nematodes, Lycopersicon peruvianum var. glandulosum, tissue culture, PAGE Abstract. Genotypes of Lycopersicon peruvianum (L.) Mill. and L. peruvianum var. glandulosum (Rick), selected from accessions that possess resistance to Meloidogyne incognita [(Kofoid and White) Chitwood] at high soil temper- ature (30C), were used as male parents in crosses with L. esculentum (Mill.) susceptible cultivars UC82, Lukullus, Tropic, and male-sterile line ms-31, respectively. The incongruity barrier between the two plant species was overcome by embryo callus and embryo cloning techniques. Hybridity of the F, progeny obtained from each cross was confirmed by differences in leaf and flower morphology, plant growth habits, and by acid phosphatase isozyme phenotypes using polyacrylamide gel electrophoresis. In greenhouse inoculation experiments, F 1 plants were highly resistant to M. incognita in soil at 25 and 30C. These results confirmed the successful transfer and expression of heat-stable resistance to M. incognita from L. peruvianum to hybrids with L. esculentum as a preliminary step to introgressing additional root-knot nematode resistance into tomato. The resistance to root-knot nematodes (Meloidogyne spp.), present in all resistant commercial cultivars of tomato (Lyco- persicon esculentum Mill.), is generally considered to be con- ferred by a single dominant gene, designated Mi (Ammati, 1985; Medina Filho and Stevens, 1980; Roberts and Thomason, 1989; Sidhu and Webster, 1981). This resistance was identified in L. peruvianum PI 128657. Using embryo rescue, Smith (1944) obtained one resistant F 1 plant from the cross L. esculentum Michigan State Forcing x PI 128657. Watts (1947) cloned this unique F 1 plant and obtained the first two backcrosses to L. esculentum. Additional backcrosses to the cultivated tomato were obtained by Frazier and Dennett (1949). This material was dis- tributed to tomato breeders and led to the release of the first tomato cultivars resistant to Meloidogyne incognita. All culti- vars with gene Mi have been derived from this one F 1 resistant plant obtained by Smith (1944). Plants possessing gene Mi are resistant to three of the four economically important species of root-knot nematode (RKN): M. incognita, M. arenaria (Neal) Chitwood, and M. javanica (Treub) Chitwood (Ammati, 1985; Fatunla and Salu, 1977; Medina Filho and Stevens, 1980; Roberts and Thomason, 1989; Sidhu and Webster, 1981). Gene Mi does not confer resistance to M. hapla Chitwood. In addition, the resistance conferred by Mi is not effective in soils above 28C (Ammati, 1985; Ammati et al., 1986; Dropkin, 1969). Nonselected virulent populations Received for publication 8 Aug. 1990. Portion of a dissertation to be submitted by G.B.C. in partial fulfillment of the requirements for the PhD in Plant Path- ology, Univ. of California, Riverside. This research was supported in part by the California Fresh Market Tomato Advisory Board. We acknowledge C.M. Rick, Dept. of Vegetable Crops, U.C. Davis, for the L. esculentum male-sterile lines, and V.M. Williamson for the purified Aps 1 1/1 protein. G.B.C. acknowl- edges the Instituto National de Tecnologia Agropecuaria of Argentina and the Dept. of Hematology, U.C. Riverside, for their support. The cost of publishing this paper was defrayed in part by the payment of page charges. Under postal regulations, this paper therefore must be hereby marked advertisement solely to indicate this fact. l Graduate Student. Current address: Departamento de Patologia Vegetal, Castro National de Investigaciones Agropecuarias, INTA Castelar, CC 25, (1712) Castelar, Buenos Aires, Argentina. 2 Associate Professor of Hematology, to whom reprint requests should be ad- dressed. 3 Emeritus Professor of Hematology and Plant Pathology. 4 Professor of Horticultural Science, Dept. of Botany and Plant Science. of RKN, as well as virulent populations of RKN selected for several generations on plants bearing the Mi gene, can overeome the resistance conferred by gene Mi (Roberts and Thomason, 1986, 1989; Roberts et al., 1990; Triantaphyllou, 1987). In view of the narrow genetic basis of resistance to RKN, Ammati et al. (1985, 1986) found additional sources of resis- tance within some accessions of L. peruvianum and L. peru- vianum var. glandulosum, resistance which was expressed at soil temperatures of 28C or above. Additionally some acces- sions, such as L. peruvianum PI 270435 and L. peruvianum var. glandulosum PI 126440 and PI 126443, were resistant to M. hapla. Tests with a cross between two of these accessions showed that the resistance was effective against M. incognita isolates selected for virulence on plants that contain gene Mi (Roberts et al., 1990). Resistance to many other pests and diseases have been found in wild tomato genotypes, especially in the peru- vianum group (Hogenboom, 1979; Medina Filho and Stevens, 1980; Rick, 1976; Rick and Yoder, 1988), but incompatibility barriers between plants of the peruvianum and esculentum com- plexes (Ammati, 1985; Hogenboom, 1979; Medina Filho and Stevens, 1980; Poysa, 1990; Rick, 1963; Rick and Yoder, 1986) have made the transfer of these desirable traits to edible tomato difficult. Several tissue culture techniques have been useful in over- coming the unilateral incompatibility. Examples include somatic hybridization (Adams and Quiros, 1985; Glimelius, 1987; Han- dley et al., 1986; Jain et al., 1987; O’Connell and Hanson, 1986; Wijbrandi et al., 1987; Zelcer et al., 1987), leaf disk transformation mediated by Agrobacterium tumefasciens (McCormick et al., 1986), ovule culture (Imanishi, 1988), and embryo culture (Ammati, 1985; Leshem et al., 1989; Neal and Topoleski, 1985; Rick, 1963; Smith, 1944; Thomas and Pratt, 1981; Young et al., 1987). However, none of these techniques has provided an efficient method to transfer genes routinely from the peruvianum to the esculentum complex. The devel- opment of bridging lines may prove useful in the future for introgression of desirable traits into L. esculentum (Poysa, 1990). The objective of this work was to obtain hybrids with L. esculentum that expressed the heat-stable nematode resistance Abbreviations: IR, index of resistance; RKN, root-knot nematode. J. Amer. Soc. Hort. Sci. 116(6):1082-1088. 1991. 1082