Cytogenetics of an Unusual Tertiary Trisomic of Tomato

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Abstract
A tertiary trisomic of the tomato, which originated as a spontaneous aberrant aneuploid in the progeny of triplo-5, was determined through pachytene analysis to involve the extra chromosome 5L·7S with breakpoints in either the centromere or proximal heterochromatin. The genes af and tf on 5S gave normal segregation with 2 n+5L·7S while var on 7S gave trisomic segregation. Genetic confirmation for 5L was not obtained. A male transmission rate of 4.1–4.6% for the extra chromosome was found. The data on transmission rates of tertiary trisomics reveal that in tomato the transmission rate is determined not by meiotic behavior but by viability factors on the gametophyte or the sporophyte. The phenotype of 2n+5L·7S unlike that of corresponding triplo-5 and triplo-7 indicating genes responsible for triplo-5 phenotype are located on 5S arm. The usefulness of various tomato trisomic types exhibiting pronounced morphological alterations, for morphogenetic studies, is suggested.