Crossability between the diploid species S. circaeifolium subsp. circaeifolium (crc) and other diploid species, primarily diploid S. tuberosum subsp. tuberosum (tbr-2x), was studied. Forty-seven hybrids were obtained from crosses between crc as female parent and tbr-2x and some other species from series Tuberosa as male parents. Of these hybrids 17% were diploids; the other 83% were triploids, probably carrying two genomes of crc. Female fertility was sufficient to obtain offspring from backcrosses with the cultivated parent. Pollen stainability of the f1 varied, and micro-pollen as well as unreduced pollen occurred. During meiosis of the diploids and triploids a rather high proportion of univalents was found, and in the triploids on average two or three trivalents per cell were found. All hybrids were resistant to Globodera pallida pathotypes 2 and 3, and 75% of the tested genotypes were highly resistant to Phytophthora infestans. Solanidine, tomatidine, tomatidenol, and demissidine glycosides were found in tubers of the hybrids. Comparisons with somatic hybrids between crc and tbr-2x are made. It is concluded that crc is a valuable Solanum species that can and should be included in potato breeding programs.

Interspecific hybridization between the cultivated potato Solanum tuberosum subspecies tuberosum L. and the wild species S. circaeifolium subsp. circaeifolium Bitter exhibiting resistance to Phytophthora infestans (Mont.) de Bary and Globodera pallida (Stone) Behrens : 1. Somatic hybrids.

Protoplast fusion allows the transfer of both mono- and polygenic traits between species that are sexually incompatible. This approach has particular relevance for potato, and somatic hybridisation has been used to introduce a range of disease resistance genes from sexually incompatible wild species into the cultivated potato gene pool. In addition, protoplast fusion allows the resynthesis of tetraploid genotypes from pre-selected diploid or dihaploid donor parents. A limiting factor for the efficient exploitation of this technology in potato breeding is the difficulty of unequivocally identifying nuclear hybrids (heterokaryons). In order to facilitate the identification of hybrids at an early stage following fusion, Randomly Amplified Polymorphic DNA markers (RAPDs) have been used to characterise molecularly both inter- and intra-specific somatic hybrids of potato. RAPD markers detect naturally occurring polymorphism in the donor genotypes and utilise short oligonucleotide primers of arbitrary nucleotide sequence in combination with the polymerase chain reaction (PCR). The exploitation of RAPDs in the characterisation of both somatic and sexual hybrids is discussed.

Molecular characterisation of inter- and intra-specific somatic hybrids of potato using randomly amplified polymorphic DNA (RAPD) markers.

In recent years, the rapid development of somatic cell genetics has made possible the transfer of alien genes over wide taxonomic distances by somatic hybridization. In this review, the potential of somatic hybridization in the breeding of crops within the Brassicaceae, Fabaceae, Poaceae and Solanaceae is discussed. It is evident from these studies that many hybrids, either symmetric or asymmetric, which are fertile have the potential to be used as a bridge between the alien species and the crop. Progeny analysis of some hybrid combinations also reveals intergenomic translocations which may lead to the introgression of the alien genes. Furthermore, fusion techniques enable the resynthesis of allopolyploid crops to increase their genetic variability and to restore ploidy level and heterozygosity after breeding at reduced ploidy level in polyploid crops.

The potential of somatic hybridization in crop breeding

Somatic fusion of mesophyll protoplasts was used to produce hybrids between the frost-tolerant species Solanum commersonii (2n=2x=24) and dihaploid S. tuberosum (2n=2x=24). This is a sexually incompatible combination due to the difference in EBN (Endosperm Balance Number, Johnston et al. 1980). Species with different EBNs as a rule are sexually incompatible. Fifty-seven hybrids were analysed for variation in chromosome 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 intermediate 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 produced on the latter clone by selfing and on the male-sterile 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 genetic esearch on frost tolerance and cold-hardening is suggested.

Production of somatic hybrids between frost-tolerant Solanum commersonii and S. tuberosum: characterization of hybrid plants.

Interspecific somatic hybrids were produced between tetraploid Solanum tuberosum and a nematode-resistant accession of the diploid species So/ahum bulbocastanum by protoplast fusion. Hybrid cells were selected using dual fluorescent labeling of protoplast preparations prior to fusion. Hybridity of regenerated plants was confirmed with a combination of morphological assessment, chromosome counting and isozyme analysis. Somatic hybrids had the same level of resistance to infection by race 1 of the nematode Meloidogyr~ chitwoodi as the S. bulbocastanum parent used in the fusion. Some of the somatic hybrids were fertile as females when crossed with tetraploid S. tuberosum breeding lines. Thus, these hybrids can be used in a potato improvement program to incorporate a valuable pest resistance.

Interspecific somatic hybridization between Solanum tuberosum L. and S. bulbocastanum Dun. as a means of transferring nematode resistance.

Tetraploid potato plants were regenerated after polyethylene-glycol-induced protoplast fusion between dihaploids. Hybrid vigour of the regenerated calli was used for preselection of fusion products. Nearly all the selected vigorous clones possessed chromosome counts at the tetraploid level. Fusion products were compared to the parental material to auto-fused plants of and to three protoclones expressing different degrees of somaclonal variation. The selected clones, where grown in vitro in growth rooms and in pots in the glasshouse, showed increased vigour compared to their parents, to auto-fused and to 4x protoclones. Plants of clones from very vigorous calli, when assessed by height, the number of nodes per plant, leaf morphology and tuber production, showed hybrid vigour. The hypothesis that superior clones result from heterokaryons after protoplast fusion or that they arise from other in vitro events such as somaclonal variation is discussed.

S. C. Debnath

Papers

Selection of somatic fusion products in potato by hybrid vigour