II. 17. Transmission of extra chromosomes derived from the translocation T3-6j.
Elaine Hixon and R.T. Ramage. Department of Plant Sciences, University of Arizona, Tucson, Arizona 85721, U.S.A. "R"
The translocation T3-6j was X-ray induced in the cultivar 'Bonus' (Hagberg, 1967). Its break-positions are in the short arm of chromosome 3 and the long arm of chromosome 6. In addition, the translocated T3-6j chromosome was broken in the centromere forming two telocentric chromosomes; one consisting of the long arm of chromosome 3 and the other consisting of the centromere end of the short arm of chromosome 3 and the translocated end of the long arm of chromosome 6. The homozygous translocation has 16 chromosomes (8 pairs) instead of the normal 14 (7 pairs). The two telocentric chromosomes are of different lengths and can be recognized in root-tip squashes. Also, the translocated T63j chromosome can be recognized by its longer long arm. Therefore, a plant heterozygous for the translocation T3-6j has 15 chromosomes and the five chromosomes involved in the translocation complex can all be identified in root-tip squashes; i.e., a normal chromosome 3 (N3), a normal chromosome 6 (N6), a translocated T63j chromosome (T63).
Lehamnn et al. (1975) reported that about 15 percent of the plants obtained from a heterozygous T3-6j F1 had extra chromosomes. They also presented diagrams of the chromosome constitutions of the 15 kinds of plants with extra chromosomes that they found. We wanted to know if additional kinds of plants with extra chromosomes could be obtained from a T3-6j heterozygote.
We made the assumptions: (1) all gametes with deficiencies abort, (2) female gametes with duplications are functional, and (3) the only male gametes with duplications that are functional are those carrying an extra 3L or a T36 telocentric chromosome. Based on these assumptions, nine kinds of female and two kinds of male gametes with extra chromosomes can be expected to be produced by a T3-6j heterozygote. Based on our assumptions, we determined the chromosome constitutions of the gametes that produced the 133 plants with extra chromosomes reported by Lehmann et al. (1975). Also, we isolated 25 additional plants with extra chromosomes from a T3-6j heterozygote and determined the chromosome constitutions of the gametes that produced them. The chromosome constitutions and the number of gametes carrying them are given in Table 1.
Only five out of 158 male gametes analyzed carried an extra chromosome. Of the male gametes with a complete complement of chromosomes, 85 had the two normal chromosomes and 68 had the translocated arrangement. This is probably a reflection of Anaphase I separation of the chain of 5 chromosomes in the T3-6j heterozygote.
Of the nine possible kinds of female gametes with extra chromosomes, seven were found. Fifty-nine of the gametes carried an extra T36 chromosome, 50 of them carried a duplication of the short arm of chromosome 3 and 32 of them carried a duplication of the long arm of chromosome 3. Ten of the female gametes had an extra T63 chromosome while four had an extra normal 6, two had an extra normal 3 and one carried both telocentric chromosomes.
References:
Hagberg, A. 1967. The use of induced mutations in practical barley breeding at Svalöf, Abh. Deutsch. Akas. Wiss. Berlin. Klasse Med. 2:147-154.
Lehmann, L., G. Hagberg, P. Hagberg and B.A. Karlsson. 1975. T3-6j - Cytology can be fun. (A translocation with two telocentric chromosomes). Barley Genetics III: 247-251.
