A culture protocol has been developed for mesophyll protoplasts isolated from various d¡haploid clones of potato. A large number of call¡ was obtained after serial dilution of the cultures w¡th a suitable amount and type of medium at different stages of cell colony development.

A polyelhylene glycol fusion procedure was developed that yielded up to 12 % heterokaryons. Using this fusion protocol hybrid cells have been manually lsolated, cultured and regenerated into plants. Fus¡on products were identified 2-3 days after fuslon by the dual fluorescence emission from the chloroplasts in mesophyll protoplasts (red) and the fluorescein diacetate stain in l¡ght and norflurazon bleached protoplasts (yellow-green). This fusion and select¡on strategy leads to the recovery of almost 100 % hybr¡d plants as established by isozyme analysls.

Cytolog¡cal analysis of protoplast-derived plants and somatic hybrid plants revealed genetic changes as a consequence of protoplast culture and protoplast fus¡on. Twenty-three tetraplo¡d somatic hybrid plants were obta¡ned from 6 different calli and 9 of these were euploid.

Morphological assessments of somatic hybr¡d plants of different ploidy levels demonstrated that tetraploid as well as several hexaploid somatic hybrids showed an increased vigour as compared to the parental plants. Most tetraploid somat¡c hybrids had a similar appearance although not all euploid plants were identical. Loss of vigour was evident in all mixoploid and octoplo¡d plants. These were stunted, weak and had an abnormal leaf morphology.

The plastid type in hybrid plants was determined by RFLP analysis. All analysed hybrids had a cpDNA restrict¡on fragment pattern ¡dent¡cal to one of the parents wh¡ch contained either S. tuberosum or S. stoloniferum cpDNA. A slight preference for S. tuberosum plastids was observed in hybild plants. No influence on tho assortment of chloroplast by the norflurazon bleaching could be detected.