This paper discusses a number of experiments performed, involving the fusion by an electric field of mesophyll protoplasts from Solanum tuberosum cv. Bintje, S. tuberosum dihaploid clones 243, 299 and the wild tuberous disease-resistant species S. bulbocastanum and S. pinnatisectum. Three fusion experiments (S. bulbocastanum + S. tuberosum dihaploid 243, S. pinnatisectum + S. tuberosum cv. Bintje and S. pinnatisectum + S. tuberosum dihaploid 299) yielded 542 calli, the 52 ones of which produced shoots. Obtained regenerants were estimated by the flow-cytometry (FC) and RAPD analysis to determine hybrid plants.The utilisation of the FC as a useful method for detecting somatic hybrids is also discussed in this paper. The combination S. bulbocastanum + S. tuberosum dihaploid 243 led to the creation of eight somatic hybrids, the combination S. pinnatisectum + S. tuberosum cv. Bintje yielded four somatic hybrids and the combination S. pinnatisectum + S. tuberosum dihaploid 299 resulted in no hybrid regenerants. Morphology in vitro, growth vigour and production of tuber-like structures were evaluated in hybrid plants. Plants were transferred in vivo for further estimation (acclimatization, habitus evaluation and tuberization ability).

Variability of 31 somatic hybrids of Solanum pinnatisectum Dun. with Solanum tuberosum L. for leaf morphology, plant vigor, resistance to Phytophthora infestans, ploidy level, and cytoplasm type was evaluated in vitro. The composition of these somatic hybrids was as follows: [S. pinnatisectum Dun. (2n = 2x = 24; cytoplasmic type Wc) ? S. tuberosum L. (2n = 4x = 48; cytoplasmic type Tß)]. Based on leaf morphology and plant growth vigor, plants were divided into three groups, including plants close to tbr parent with unlobed leaves, small plants with scarcely dissected leaves, and vigorous plants with asymmetrically and pinnately lobed leaves. Nine of the somatic hybrids were found to be highly resistant to P. infestans. Somatic hybrids were either tetraploid or hexaploid, with hexaploids being predominant. The cytoplasm of somatic hybrids was either Tßc or Wßc, with Tßc being more common. Overall, in contrast to leaf morphology and growth vigor, level of resistance to P. infestans was not related to either ploidy level or type of cytoplasm. These findings demonstrate that early in vitro selection of promising hybrids can be useful in breeding programs.

Mitotic polyploidization in vitro was used in selected wild Solanum species and Solanum tuberosum dihaploids. The efficiency of polyploidization by colchicine was compared with that of oryzalin. Oryzalin was more effective than colchicine (P ¼ 0.1). The rate of non-affected to mixoploid to tetraploid regenerants was 22 : 2.5 : 1 (colchicine) and 14 : 2 : 1 (oryzalin). Optimal concentrations and durations were 3.5 mM/24 h for colchicine and 25 or 30 lM for 24 or 48 h for oryzalin (variations in concentration and duration are necessary owing to possible diversity of responses in selected genotypes). Tetraploids were obtained from S. berthaultii, S. bulbocastanum, S. pinnatisectum, S. verrucosum and eleven S. tuberosum dihaploids. The yield of tetraploids derived from tbr dihaploids was lower than that from the wild species (P ¼ 0.01). Tetraploid regenerants were tested in intra-and inter-specific crosses. Three of 43 intra-specific combinations (298 pollinated flowers) were successful and yielded 440 seeds. Inter-specific crosses (138 combinations, 1672 pollinated flowers) yielded 48 seedless berries.Cultivated potato (Solanum tuberosum L., 2n = 4x = 48) is a vegetatively reproduced species with tetrasomic inheritance and high heterozygosity (Tai and Xiong 2005). Unfortunately, the genetic pool of cultivated potato cultivars is limited owing to its narrow germplasm base. Wild Solanum species on the other hand are sources of allelic diversity and have many valuable traits, including biotic and abiotic stress resistance and nutritive quality of tubers (Cardi et al. 1993, Helgeson andHaberlach 1999). Solanum is a model system with a strong mechanism of sexual isolation due especially to ÔeffectiveÕ ploidy (endosperm balance number = EBN; Carputo et al. 1997). Diploid (2n = 2x = 24) wild Solanum species with 1EBN are sexually isolated from wild Solanum species with higher EBN (i.e. diploid/2EBN, tetraploid/2EBN or 4EBN, hexaploid/4EBN) and from tetraploid (2n = 4x = 48, 4EBN) or dihaploid (2n = 2x = 24, 2EBN) Solanum tuberosum.This breeding barrier can be overcome by increasing or decreasing ploidy and EBN prior to crossing (Bamberg et al.

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