Genetic improvement of the potato through classical breeding has been limited by its tetraploid nature, the narrow genetic variability within cultivars, and interploidy barriers between tetraploid cultivars and diploid germplasm. Breeding at reduced ploidy levels has been proposed as a solution to these problems. Because of sterilities, somatic hybridization via protoplast fusion has been considered an alternative to sexual polyploidization for resynthesizing superior diploids from selected monoploids, and tetraploids from selected diploids and dihaploids.

Successful application of somatic hybridization largely depends upon protoplast culturability and regenerability of a plant. The ability of callus formation and plant regeneration from protoplasts varies among plants. To understand the genetic basis for this variation, the mode of inheritance for protoplast culturability, defined as the ability to develop calli from cultured protoplasts, was studied in the diploid potato species, Solanum phureja. Based upon data from F₂ as well as from F₁ and backcross progenies, it was found that protoplast culturability in this potato species was controlled by two unlinked loci with dominant effect. In addition, there was quantitative variation for protoplast plating efficiency among culturable genotypes.

Male sterility in cultivars of Solanum tuberosum ssp. tuberosum results from nuclear-cytoplasmic interactions. 'Donor-recipient' protoplast fusion and regeneration were conducted between a sterile S. tuberosum ssp. tuberosum cultivar, Russet Burbank, and fertile selections of S. tuberosum ssp. andigena which have a non-sensitive cytoplasm and were used as the cytoplasmic donor. Sixteen regenerated plants possessed nuclear background and chloroplast DNA of Russet Burbank. However, two of these regenerants had improved pollen stainability. The possible causes for the improvement of pollen stainability are discussed.

In the last chapter, allelic polymorphism in a monoploid population derived from anther culture of a clone of S. phureja was assessed by isozyme electrophoresis. Fourteen monoploids and their anther donor were examined for six enzymes. No allozyme variation was detected in these plants. However, genetic variability among these monoploids was manifested by variations in some growth characters and general morphology. The limitation of enzymatic markers in detecting allelic polymorphism in these monoploids is discussed.