A key competence of the Institute for Breeding Research on Horticultural Crops
Interspecific and intergeneric hybridization is an important research approach creating usable variability for breeding. Desirable traits like resistance against pathogens or abiotic stress within varying environments, composition of metabolites or morphological traits and their responsible genes are often found only within wild species, related species and genera of the cultivars.
It is possible to use the existing variability of the primary and secondary gene pool by interspecific and intergeneric hybridization. Thereby, an important precondition and basis for the coverage of the further breeding progress is accomplished.
Interspecific and intergeneric hybridization can be carried out both-way, generatively by crossing or somatically by protoplast fusion. The Institute for Breeding Research on Horticultural Crops has acquired wide experience in the development and adaption of the generative as well as somatic hybridization of vegetables, ornamentals and medical and aromatic plants for many years. Both methods are very labour-intensive, long-lasting and fraught with risk. Therefore, the breeding lines are chosen very carefully and are proved for defined valuable traits in advance.
As a first step of successful hybridization by crossing comprehensive studies are carried out to overcome pro- and postgamic crossing barriers. Besides the transfer of defined genes into the genome of the nucleus of the cultivar new combinations with the cytoplasm of the donor leading to alloplasmic lines are of special interest. In both cases, the definition of the direction of the crossing has strategic importance within the primary combination. For instance, the cytoplasm of most cultivars is maternally inherited. Consequently, in this case the maternal parent should be chosen as donor.
After successful fertilization and development of embryogenic tissue embryo rescue is often necessary because of disturbed further development of the embryo and the seeds on the mother plant. By in vitro conditions and embryo rescue the development of the embryo on culture medium is provided up to a complete hybrid plant. Vitality and fertility of the hybrid plants decide on further breeding strategies. Frequently, the primary hybrids can be stabilized and their fertility can be restored by in vitro or in vivo polyploidization and the increase of the ploidy level. Further back crossings allow the transfer of a single or more genes from the donor species into the genome of the recipient species.
The method of somatic hybridization is based on the production of protoplasts (cells without wall), their fusion and regeneration using the inducible totipotence of plant cells. Our institute has one of the few competence laboratories for somatic hybridization, which is not a routine technique until today.
Combinations of complete nuclear genomes (allopolyploidy) as well as different new combinations of cell organelles (alloplasmic hybrids) are possible within primary hybrids. The genetic diversity of the regenerants is high. Different portions of the genomes are combined by the symmetric (protoplasts of both parents are intact) or asymmetric fusion (partial inhibition of one parent e.g. by irradiation). Consequently, the regenerants are completely new genotypes. For that reason, the somatic hybridization can also create new genetic variability, where intergeneric or interspecific hybrids can be reached by crossing. For example, abnormity during plant development (lack of chlorophyll) caused by incompatibility between genes of the nucleus and cytoplasm could be overcome by somatic hybridization. In our institute X-ray equipment and UV-emitter as radiation sources are available for the asymmetric fusion.
After regeneration the DNA content of the new plant material is determined by flow cytometry. Subsequently, plants with a modified DNA content are characterized by phenotype as well as by molecular markers similar to sexual hybrids.
During following pre-breeding programs somatic and sexual hybrids must be backcrossed and selected concerning desirable traits using appropriate bioassays, molecular and analytical methods. The success of interspecific or intergeneric hybrids depends on the expression of the trait or gene as well as the integration into the genome, independently of the used hybridization method. Integration as a stable introgression or translocation implies a pairing of chromosomes during the meiosis and recombination. If this prerequisite does not exist, it is possible to fractionalize chromosomes by irradiation of pollen grains, which may be integrated into the chromosomes of the recipient.
All projects are focussed on developing plant lines which express the desirable traits of the donor and which are suitable for further breeding.