Genome composition of asymmetric hybrids in relation to the phylogenetic distance between the parents. Nucleus-chloroplast interaction.

A series of fusion experiments were performed between protoplasts of a cytoplasmic albino mutant of tomato, Lycopersicon esculentum (ALRC), and gamma-irradiated protoplasts of L. hirsutum and the Solanum species S. commersonii, S. etuberosum and S. nigrum. These species were chosen for their different phylogenetic relationships to tomato. In all fusion combinations except from those between ALRC and S. nigrum, green calli were selected as putative fusion products and shoots regenerated from them. They were subsequently analyzed for their morphology, nuclear DNA composition and chloroplast DNA origin. The hybrids obtained between ALRC and L. hirsutum contained the chloroplasts of L. hirsutum and had the flower and leaf morphology of L. esculentum. After Southern blot analysis, using 13 restriction fragment length polymorphisms (RFLPs) randomly distributed over all chromosomes, all hybrids showed L. esculentum hybridization patterns. No chromosomes of L. hirsutum were found. These results indicate that these hybrids were true cybrids.The putative asymmetric hybrids, obtained with S. commersonii and S. etuberosum, showed phenotypic traits of both parents. After hybridization with species-specific repetitive nuclear DNA probes it was found that nuclear material of both parents was present in all plants. In the case of S. nigrum, which combination has the greatest phylogenetic distance between the fusion parents, no hybrid plants could be obtained. The chloroplast DNA of all hybrid plants was of the donor type suggesting that chloroplast transfer by asymmetric protoplast fusion can overcome problems associated with large phylogenetic distances between parental plants.