Breeding behavior of plants with a ring of ten or twelve chromosomes

The goal of continuing work is to produce a multiple interchange stock which will produce F1's with a ring of 20 chromosomes. Progress continues on the plan using the Inman convergent scheme of making crosses between lines that have interchanges in common, thus avoiding increases in sterility as interchanges are added (see pp. 113-114 in "Discussions in Cytogenetics").

A second plan was to utilize stocks which, when crossed with each other, would produce F1's with two rings with ten chromosomes each. From backcrosses to either parent, followed by selfing and selection, the goal was to establish a stock which combined the two. An interchange needed to produce the ring with 20 chromosomes would be produced by x-rays. In spite of considerable testing, no stock was established that would produce two rings of ten when crossed with normal. The problem appeared to be a high frequency of crossovers which reduced the number of chromosomes in the rings.

Since these multiple interchange stocks may have other uses, information on the kinds of progeny and their frequency are reported here. For this purpose, F1's with the big rings were crossed with normal stocks and the open pollinated progeny classified for ear sterilities similar to the parental types (either fertile or with very, very high sterility) and new combinations with lower sterility roughly corresponding to that for a ring of 8, 6, or 4 chromosomes. The results are:
 
Ear Sterility on Progeny
Heterozygote

Tested

 Parental Lower Sterility v. low %

parental
F v.v.H.S. 0 8 0 6 0 4
1-5-6-7-8 58 69 39 18 20   62
3-2-4-9-10 66 63 23 44 31 2 56
5-7-1-9-10 28 70 33 26 30   52
6-3-2-4-8 81 83 56 53 62 4 49
5-7-1-9-10-8 32 34 11 7 7   75

The frequency of parental types from parents with a ring of ten ranged from 49 to 62%. For all except the 5-7-1-9-10 ring of ten, the two parental classes were roughly 1:1. Plants in the fertile class should have received from the normal parent the normal homologues of the interchange chromosomes, except for crossovers which had occurred in segments distal to the interchange breakpoints, thus contributing segments from the interchange chromosomes.

Seeds of the homozygous multiple interchange stocks and crosses are available.

Charles R. Burnham

Please Note: Notes submitted to the Maize Genetics Cooperation Newsletter may be cited only with consent of the authors.

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