Theor Appl Genet (1993) 87:193-200 Springer-Verlag 1993 Production of somatic hybrids between frost-tolerant Solanum commersonK and S. tuberosum: characterization of hybrid plants T. Cardi 1. E D'Ambrosio 2, D. Consoli 1 K. J. Puite 3, K. S. Ramulu 3 1 Research Centre for Vegetable Breeding - CNR, via Universitfi 133, 80055 Portici, Italy 2 Department of Agronomic Sciences and Plant Genetics, University of Naples, via Universitfi 100, 80055 Portici, Italy 3 Centre for Plant Breeding and Reproduction Research CPRO, Postbox 16, 6700 AA Wageningen, The Netherlands Received: 26 August 1992 / Accepted: 11 February 1993 Abstract. Somatic fusion of mesophyll protoplasts was used to produce hybrids between the frost-tolerant spe- cies Solanum commersonii (2n = 2x = 24) and dihaploid S. tuberosum (2n = 2x = 24). This is a sexually incom- patible combination due to the difference in EBN (En- dosperm Balance Number, Johnston et al. 1980). Species with different EBNs as a rule are sexually incompatible. Fifty-seven hybrids were analysed for variation in chro- mosome number, morphological traits, fertility and frost tolerance. About 70% of the hybrids were tetraploid, and 30% hexaploid. Chloroplast counts in stomatal guard cells revealed a low frequency of cytochimeras. The frequency of aneuploids was relatively higher at the hexaploid level (hypohexaploids) than at the tetraploid level (hypotetraploids). The somatic hybrids were much more vigorous than the parents, and showed an interme- diate phenotype for several morphological traits and moderate to profuse flowering. Hexaploid hybrid clones were less vigorous and had a lower degree of flowering than the tetraploid hybrid clones. All of the hybrids were female fertile but male sterile except for one, which was fully fertile and self-compatible. Many seeds were pro- duced on the latter clone by selfing and on the male-ster- ile clones by crossing. The somatic hybrid plants showed an introgression of genes for frost tolerance and an adaptability to cold from S. commersonii. Therefore, the use of these somatic hybrids in breeding for and in genet- ic research on frost tolerance and cold-hardening is sug- gested. Key words: Frost tolerance - Male sterility - Potato - Solarium commersonii Somatic hybridization Communicated by M. Koorneef Contribution no. 105 from the Research Centre for Vegetable Breeding * Present address: Cornell University, Department of Plant Breeding and Biometry, 252 Emerson Hall, Ithaca, NY 14850, USA Correspondence to: T. CardJ Introduction Solanum commersonii Dun. is a tuber-bearing wild rela- tive of cultivated potato (S. tuberosum L.). Its frost toler- ance and ability to acclimatize to cold is well known. In contrast to S. tuberosum, for which a killing temperature of around - 3 ~ has been reported, S. commersonii has shown a killing temperature of - 11.5 ~ or - 4.5 ~ with or without an appropriate hardening treatment, respec- tively (Chen and Li 1980). The two species also show differences in several morphological and physiological characters related to frost tolerance (Palta and Li 1979; Li 1985). In addition, resistances to biotic stress and other agronomic traits of interest have been detected in various accessions of S. commersonii (Hanneman and Bamberg 1986; Johnston et al. 1986; Chavez et al. 1988; Hawkes 1990). However, its use in potato breeding and genetic studies has been hampered so far by a difficulty to cross diploid S. commersonii (2n= 2x=24, EBN= 1) either with dihaploid (diploid) (2n = 2x = 24, EBN = 2) or tetraploid (2n = 4x = 48, EBN = 4) S. tuberosum (Chavez et al. 1988; Novy and Hanneman 1991). In potato, somatic hybridization has been used between S. tuberosum and various wild species, such as S. chacoense, S. phureja, S. brevidens, S. berthaultii and S. eircaeifolium, and the resulting hybrids were tested generally for resistance to biotic stress (Butenko and Kuchko 1980; Austin et al. 1985; PuRe et al. 1986; Pehu et al. 1990; Serraf et al. 1991; Mattheij et al. 1992). In one study, somatic hybrids (S. tuberosum (+) S. brevidens) were tested for resistance to abiotic stress (Preiszner et al. t991), but the response to hardening was not investigated. Further, the potential value of somatic hybrids for potato breeding may depend on various factors. The combination of parental geno- types, the ploidy level and the genome dosage of hybrids can influence plant morphology and vigour, tuberization, length of stolons, virus resistance, the activity of enzymes related to insect resistance, meiotic behaviour, pollen viability and crossability, as has been shown in somatic hybrids of S. tuberosum (+) S. brevidens or S. tuberosum (+) S. berthaultii (Austin et al. t986: Austin and Helgeson 1987; Ehlenfeldt and Helgeson 1987; Fish et al. 1988a; Pehu et al. 1989, 1990; Serraf et al. 1991).