Tomato Telotrisomics: Origin, Identification, and use in Linkage Mapping

Abstract

Telotrisomics—sporophytes possessing a telocentric chromosome in addition to the normal complement—were obtained and studied for six different chromosome arms of the tomato. They originated in the progeny of the following kinds of chromosomal deviants: terminal deficiency, tertiary monosomic, and compensating trisomic. Cytological studies at meiotic pachytene established that the centromere of three telocentrics was flanked by a heterochromatic chromomere opposite the intact arm and three appeared to have terminal centromeres. Of the latter group the centromere of two appeared to be of normal size, while one was about half normal size, its chromosome being the only one of the six telocentrics to be mitotically unstable. The longer the telocentric arm, the greater the tendency to form trivalents; when trivalents are not formed, the telocentric is invariably univalent, the preferential pairing between the two normal homologues leading exclusively to 12: 12+telocentric anaphasic separations. Consistent with this cytological behavior, only one in several hundred trisomic progeny was of primary type, the rest telosomic. —The effects of the extra telocentric chromosome on gross phenotype of the plant are much greater for the long arms, and telocentrics for long arms closely resemble the corresponding primary trisomics. As a result of gametophytic elimination, the telocentric is never substituted for the corresponding normal chromosome, but can be transmitted as an extra element for all studied telocentrics on the female side and for the short arms also on the male side. The rate of female transmission generally exceeds that of the corresponding primary trisomic. —Inheritance studies of three telotriosmics permitted assignment of marker genes to their proper chromosome arms. Since efficiency of the trisomic ratio method is enhanced if the telotrisomic is phenotypically distinguishable from the diploid, trisomics for the long arms are better suited for genetic mapping purposes than those for short arms. In this fashion r and wf were proven to lie on 3S, rv and sf on 3L, var on 7S, and cpt on 8L, with correspondingly closer approximations of centromere positions. The incompletely dominant gene La was assigned to 7L by virtue of dosage effects in heterozygous triplo-7L. Considering their higher rates of transmission and less depressing effects on vigor and fertility, extra telocentrics render telotrisomics more useful for determining arm locations than either secondary or tertiary trisomics. © 1968, Japan Mendel Society, International Society of Cytology. All rights reserved.