The tolerance of extra chromosomes by primitive tomatoes.

T H E unexpected composition of the progeny of a triploid plant of the Red Cherry variety of tomato (Lycopersicon esculentum) attracted our attention to the possibilities of greater tolerance of aneuploidy in primitive than in highly selected horticultural varieties. Previous experience with the latter varieties revealed striking morphological manifestations, reduced vigor, and decreased fertility caused by a single extra chromosome and an upper limit of three extra chromosomes tolerated (LESLEY 1928; RICK and BARTON 1954). Progenies in the following studies were derived from open pollination of polyploids or from crosses between the polyploids and diploid staminate parents of the corresponding variety. For the intended comparisons the nature of the pollination is inconsequential since the rate of transmission of extra chromosomes through tomato pollen is negligible. Triploid plants of Red Cherry had been sought as a source of specific primary trisomics of that variety, and one triploid was found as an unfruitful plant in a commercial culture in San Diego County in October, 1958. From 51 fruits that were naturally set on this plant, approximately 200 seeds were harvested, and the progeny were grown in 1959. Following our procedures for raising trisomic stocks and F, hybrids, we sowed the seeds in a greenhouse culture, later transplanted the seedlings to nursery flats, and, before transplanting to the field, discarded 23 plants of greatest vigor. Identification of trisomics of large-fruited tomatoes in the seedling stage, learned from ten years experience, has proven highly effective in greatly reducing the proportion of undesired diploids. Separations of such trisomics as triplo-1,2,8,9, and 10 are often 100 percent accurate as verified by the phenotypes of subsequently developed mature plants. I t was unfortunate, however, for purposes of the present study that any seedlings had been eliminated, for further experience cast doubt on the accuracy of seedling identification in this variety. As the retained members of this family grew in the field, phenotypic identification of trisomic types was attempted, but without success. Previous research (LESLEY 1928; RICK and BARTON 1954) showed that each tomato chromosome in