The use of colchicine and colcemid treatments to produce homozygotes for multiple interchanges involving all corn chromosomes

For this experiment F1 seeds from a cross between two interchange homozygous stocks, T5-7-1-9-10-8 and T6-3-2-4-8 were used. The seeds were placed on filter paper, in Petri dishes, and watered with 0.02% aqueous solution of colcemid or colchicine for 5 days (The colchicine concentration was erroneously reported as 0.2% instead of 0.02% in the 1981 Newsletter). The seedlings were placed in sand in the greenhouse for 10 days before transplanting them in the field. Most of the plants were highly sterile and did not shed pollen. However, there were 5 plants from the colcemid treatment that were partially fertile and shed pollen. No fertile plants were obtained in the colchicine treatment. A higher frequency of fertile plants was obtained in previous experiments when the seeds were presoaked in water 20-24 hours prior to treatment.

Microscope examination of pollen samples from the five plants that shed pollen indicated that all samples had starch-filled grains of large size as well as normal size. Four of these plants had tillers. Fertility of the tassels on the tillers was similar to that of their main stalks. Self seeds were obtained on the ear on the main stalk of the five plants and also on the ear on each of the four tillers. The events that gave rise to the fertility, probably chromosome doubling, must have occurred in the tassel, ear, and tiller primordia in the same plant. This is contrary to expectation, since in the mature seed the tassel and ear are each known to be represented by separate multiple-cell primordia. The probability of the same independent event occurring in the three primordia in the same seedling simultaneously is extremely small. Some other mechanism is more likely to account for these results.

Two possible explanations can be offered:

1. One chromosome doubling event in the meristematic tissue with the resulting cell subsequently taking over the other cell lines to form the vegetative portion of the plant.

2. One event, probably chromosome doubling, in a tiller primordium which is possibly represented by one cell in the germinating seed, with it subsequently forming the main stalk and possibly other tillers.

All five plants were testcrossed as male parents on diploid and tetraploid stocks. Crosses on the tetraploids resulted in only shriveled seeds, probably triploids. Crosses on the diploids resulted in normal ears with fully developed seed, and no shriveled kernels. These results are similar to those obtained for the partially fertile plants in the 1981 experiment (this Newsletter). These partially fertile plants appear to have viable diploid and haploid pollen in addition to the aborted class. The haploid pollen seems to have a competitive advantage over the diploid when applied on either diploid or tetraploid female parents.

The F2 progeny will be planted in the field this spring. Fertile plants with haploid-size pollen will be selfed and testcrossed with normal diploids as well as with the two parental translocations. F2 plants, whose testcross with the normal diploid shows an association of 20 chromosomes at diakinesis, must be homozygous for a multiple translocation involving all 20 chromosomes. This would be T5-7-1-9-10-8-4-2-3-6. Such an F2 plant would be produced by gametes resulting from a crossover in the differential segment of chromosome 8, the chromosome common to the two parental stocks. In T5-7-1-9-10-8 the breakpoint in 8 is in one chromosome arm, that in T6-3-2-4-8 in the opposite arm.

Fertile plants with diploid-size pollen would be either normal tetraploids or tetraploids homozygous for the multiple interchange involving all ten chromosomes. The latter combination would be expected to have similar fertility to that of normal tetraploid since it has ten tetrasomes (each tetrasome having four similar chromosomes). The two types can be separated by testcrossing to a normal tetraploid stock. Plants that give partially sterile testcross progeny would be the desired 4n multiple interchange homozygote.

Helmy Ghobrial

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

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