Partial genome transfer through micronuclei in plants

Microprotoplast-mediated transfer of single specific chromosomes between sexually incompatible plants

Microprotoplast-mediated chromosome transfer (MMCT) through fusion of small (subdiploid) microprotoplasts of a transgenic triploid potato (Solanum tuberosum) cell line with leaf protoplasts of tobacco (Nicotiana tabacum) and the wild tomato species Lycopersicon peruvianum is reported. The microprotoplasts contained one or a few chromosomes. Monosomic addition plants were produced from the fusion products. We employed mass-scale induction of micronuclei in donor suspension cells of potato using the microtubule inhibitor Cremart. Protoplasts were isolated from micronucleated cells after incubation in a cell wall digesting enzyme mixture. The microprotoplasts were isolated from the micronucleated protoplasts by high-speed centrifugation. By using sequential filtration, small microprotoplasts containing one or few chromosomes were separated from the bigger subdiploid microprotoplasts. These small microprotoplasts were fused with recipient protoplasts of tobacco or tomato using polyethylene glycol. The selectable marker kanamycin resistance (KanR) and the reporter gene β-glucuronidase (gus), carried by the donor potato chromosome, were used for the selection of fusion products and the isolation of hybrid calli. Several monosomic addition plants were obtained within the short period of 3–4 months after fusion. These contained one potato chromosome carrying a single copy of gus and one or two copies of the neomycin phosphotransferase (nptII) gene conferring KanR, and the complete set of chromosomes of tobacco or tomato, as revealed by genomic in situ hybridization and Southern blot hybridization. The alien genes, gus and nptII, were stably expressed in both the tobacco and tomato backgrounds. They were transmitted to the progeny after backcrossing to tomato. Monosomic and disomic additions, and some introgression plants showing integration of gus and nptII in the tomato genome, were recovered in the first backcross progeny. The potential value of MMCT for the transfer of economically important traits, genome analysis, and gene expression is discussed. Key words : chromosome transfer, microprotoplast fusion, monosomic–disomic additions, sexual transmission, DNA integration, alien gene expression.

Allotriploid somatic hybrids of diploid tomato (Lycopersicon esculentum Mill.) and monoploid potato (Solanum tuberosum L.)

Allotriploid somatic hybrids were obtained from fusions between protoplasts of diploid tomato and monohaploid potato. The selection of fusion products was carried out in two different ways: (1) The fusion of nitrate reductase-deficient tomato with potato gave rise only to hybrid calli if selection was performed on media lacking ammonium. Parental microcalli were rarely obtained and did not regenerate. (2) The fusion of cytoplasmic albino tomato with potato gave rise to albino and green hybrid calli and plants. Allotriploids were identified from the two somatic hybrid populations by counting chloroplast numbers in leaf guard cells and by flow cytometry of leaf tissue. Although some pollen fertility of allotriploids and pollen-tube growth of tomato, potato andLycopersicon pennellii into the allotriploid style were observed, no progeny could be obtained. The relevance of allotriploid somatic hybrids in facilitating limited gene transfer from potato to tomato is discussed.

Somatic hybridization between potato and Nicotiana plumbaginifolia : 1. Spontaneous biparental chromosome elimination and production of asymmetric hybrids

Electrofusion was carried out between mesophyll protoplasts from the transformed diploid S. tuberosum clone 413 (2n=2x=24) which contains various genetic markers (hormone autotrophy, opine synthesis, kanamycin resistance, β-glucuronidase activity) and mesophyll protoplasts of a diploid wild-type clone of N. plumbaginifolia (2n=2x=20). Hybrid calli were obtained after continuous culture on selection medium containing kanamycin. Parental chromosome numbers, determined at 2 months after fusion, revealed hybrid-specific differences between the individual calli. On the basis of these differences three categories of hybrids were distinguished. Category I hybrids contained between 8 and 24 potato chromosomes and more than 20 N. plumbaginifolia chromosomes; category II hybrids had between 1 and 20 N. plumbaginifolia chromosomes and more than 24 potato chromosomes; category III hybrids contained diploid or subdiploid numbers of chromosomes from both parents. The hybrids were evenly distributed over the three categories. After a 1-year culture of 24 representative hybrid callus lines on selection medium the karyotype of 10 hybrids remained stable, whereas 8 hybrids showed polyploidization of the genome of one parent, together with no or minor changes of the chromosome numbers of the other parent. Six hybrids showed slight changes in the hybrid karyotype. The elimination of chromosomes of a particular parent was not correlated to their metaphase location. The processes of spontaneous biparental chromosome elimination leading to the production of asymmetric hybrids of different categories are discussed.