TOMATO TERTIARY TRISOMICS: ORIGIN, IDENTIFICATION, MORPHOLOGY AND USE IN DETERMINING POSITION OF CENTROMERES AND ARM LOCATION OF MARKERS

Abstract

Seven tertiary trisomics of the tomato were investigated, 2 synthesized from tertiary monosomics and 5 from reciprocal translocations. A cytological study of the translocation heterozygotes was useful for predicting the kinds of primary and tertiary trisomics that might be yielded by non-disjunction. The predictions were confirmed by phenotypic and cytological examination of the aneuploid progeny, each of the 4 tested translocation heterozygotes yielding tertiary trisomics and various other related and unrelated primary trisomics. For each of the 7 tertiary trisomics, the extra chromosome was structurally identified in pachytene, arms of 9 of the 12 tomato chromosomes being represented in the set. The configurations assumed by the tertiary chromosome and related normal chromosomes were analyzed and found to conform in type and frequency with the pattern expected of tertiary trisomics. The tertiary trisomics tend to resemble the related primaries in respect to gross morphology of the plant. Long arms influence pheno-type more profoundly than short ones to an extent that possibly exceeds that expected according to relative chromosome length. Certain characters, particularly those effected by the short arms, fall outside the range predicted on the basis of primary phenotype, others thus predicted did not appear, suggesting interactions in the determination of phenotype. The salient morphological features of each tertiary trisomic are presented. Five of the tertiary trisomics were utilized effectively in genetic tests to assign marker genes to specific arms of chromosomes 1, 4, 5, 7, 9, and 10. One (2n + 4L [center dot] 10L) served to identify markers with arms of both normal chromosomes represented in the tertiary. For each of the 6 tested chromosomes, useful new data are presented to improve understanding of the arm contents, centromere position, and orientation of the linkage group. For genes segregating in tertiary trisomic fashion, the appearance of exceptional trisomic recessive homozygotes gave evidence of double reduction, thereby further abetting the mapping process by roughly approximating distance from the centromere. A set of 6 selected tertiary trisomics could theoretically suffice to cover the entire tomato genome for purposes of arm allocations. The qualifications of such a set are discussed, and the following are concluded to be optimum requirements: both participating normal chromosomes should be broken and rejoined in the centromere so that only complete arms are interchanged; the total length of interchanged arms should be sufficient to condition an easily identified phenotype, yet not exceed the limits of viability of gametes and/or zygotes; arms should be exchanged in such a fashion that each normal chromosome be represented once in the set.