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This thesis describes the selection and evaluation of diploid potato clones (2n=2x=24) that produce unreduced or 2n-gametes with 24 chromosomes instead of the normal reduced n-gametes with 12 chromosomes. To elucidate the modes of origin of the 2n-gametes, the progenies derived from such gametes were analysed for different marker loci of one chromosome. Besides the already known mechanisms of First and Second Division Restitution (FDR and SDR) in potato, the multilocus analysis showed the additional mechanism of Post-Meiotic Restitution (PMR) resulting in completely homozygous 2n-gametes. The FDR and SDR 2n-gametes were used for gene-centromere mapping. Multilocus analysis of SDR 2n-gametes showed the map position of the centromere of chromosome 8 in relation to the RFLP-loci. These genetic analyses also demonstrated the occurrence of a single crossover per chromosome arm (high level of chiasma interference). In the light of these findings, it is possible to construct more critical genetic maps of potato.

In this thesis research is presented on all steps in a potato breeding program via the diploid level: dihaploid induction, selection at the diploid level and sexual polyploidization. In spite of the significant seed parent x pollinator interaction estimated, IVP 101 was found to have a significant higher dihaploid induction ability than the widely used pollinators IVP 35 and IVP 48. Significant differences in dihaploid production ability between seed parents were also found. Dihaploid populations from 31 varieties or breeding lines were evaluated for occurrence and frequency of mutant phenotypes, for tuberization, flowering, pollen stainability, 2n-pollen production and resistance to Ro-1. The expression of yield, yield components, vine maturity, under water weight and chip colour at the diploid and tetraploid level and the parental effects on these characters of 4x . 2x progenies were investigated to find an answer to the question whether at the diploid level other selection criteria should be employed than at the tetraploid level when selecting tetraploid breeding lines. For yield direct selection at the diploid level had no effect on the performance of tetraploid progeny. For under water weight more stringent and for vine maturity less stringent selection criteria were required at the diploid level than at the tetraploid level. The frequently reported superiority of FDR 2n-gametes versus SDR 2n-gametes could be confirmed for yield only. For vine maturity, under water weight and chip colour no considerable differences were found between means of FDR and SDR progenies from reciprocal 4x-2x crosses.

The use of monohaploid potato clones in research in theory has many potential advantages in comparison with the use of heterozygous di- and tetraploids. Because in monohaploids each chromosome is single, recessive as well as dominant alleles are detectable and can be selected for. In addition, monohaploids offer a unique opportunity to produce homozygous clones. The aim of the study presented in this thesis was to determine which are the pros and cons of using monohaploids in practical breeding and fundamental research, and which are the consequences of monoploidy and homozygosity in a normally highly heterozygous crop such as potato.Large-scale production of monohaploids appeared to be possible by prickle pollination, but turned out to be genotype-specific. It was shown that the ability for monohaploid production can be transferred to the progeny by crossing. During the investigation reported herein more than 500 monohaploids have been produced that could be used for further research.In order to combine different selected monohaploid genotypes that will warrant high-level heterozygosity, methods for protoplast isolation, culture and fusion have been adjusted to the material used. Several monohaploid genotypes could be regenerated from protoplasts to plants but always the ploidy level raised to 2x or even 4x. After protoplast fusion, hybrid fusion products could readily be selected because of a differential regeneration capacity. By using isozyme markers for detection of hybridity the first regenerants after protoplast fusion were shown to be hybrids. A difference of at least four weeks in regeneration time was found between the first heterozygous (hybrids) and the first homozygous (parental) genotypes. Some regenerants were triploid. This indicates that at least one monohaploid protoplast was involved in fusion. The majority however was tetraploid and this was probably the result of somatically doubling x+x fusion products. Only in some genotype combinations also i