In a preliminary experiment (reported in Maize Genetics Newsletter 42:120, 1968), when germinating corn seeds with two rings of 10 chromosomes each were treated with colchicine, a few plants were found to have fertile pollen of large size. The purpose of this study is to produce more of these fertile plants and to study their chromosomal constitution and cytogenetic behavior. This report presents the data obtained last summer.
Five diploid stocks were used. Each stock is homozygous (designated A) for multiple chromosome interchanges for big rings that involve the chromosomes indicated below:
stock #1: involves chromosomes 3, 2, 4, 9, 10
stock #2: involves chromosomes 1, 5, 6, 7, 8
stock #3: involves chromosomes 6, 3, 2, 4, 8
stock #4: involves chromosomes 5, 7, 1, 9, 10
stock #5: involves chromosomes 5, 7, 1, 9, 10, 8
F1 seeds from the following crosses were used:
cross A: stock #1 (D3-2-4, 9, 10) x stock #2 (D 1-5-6-7-8)
cross B: stock #3 (D 6-3-2-4-8) x stock #4 (D 5-7-1-9-10)
cross C:stock #5 (D 5-7-1-9-10-8) x stock #3 (D 6-3-2-4-8)
The A and B F1's have 2 rings of 10, C has a ring of 20. All three F1 types have 20 different chromosomes. About 200 F1's of each of the three crosses were soaked in water for 20 hours, then placed on filter paper and watered for 5 days with 0.2% aqueous solution of colchicine. Due to the limited quantity of colcemid, a .02% aqueous solution of colcemid was used only on F1 seed of cross A. The solutions were adjusted to pH 7.0. Tap water was used for watering the following 2 days. The seedlings were planted in sand in the greenhouse for 10 days after which they were transplanted in the field. About half the seeds survived the treatment and reached the mature plant stage.
The majority of the plants were, like their untreated siblings, very highly sterile (over 99%) with shriveled anthers that did not open. However, all the tassel branches of 8 plants (1 from cross A, 3 from cross B, and 4 from cross C) had plump anthers that shed pollen. Examination of their pollen samples with a single lens field microscope confirmed that they were partially fertile and had some large pollen similar in size to that from tetraploid plants. Tassel samples from these plants were preserved in alcohol for examination in the laboratory. Also these plants were selfed and their pollen was used to pollinate as many plants from the same cross as possible. Unfortunately it was very late in the season and seed set occurred only on one selfed plant (38 seeds, all plump and well formed).
Pollen from two separate plants from each of crosses B and C and a duplicate sample from the one plant from cross A was stained with a solution of iodine plus potassium iodide. Pollen was classified and counted under a light microscope, using the procedure described elsewhere in this newsletter. The following table lists the number of pollen grains and the percentage in each class. In one F1 tassel from cross B there were a few sterile spikelets. All anthers of these spikelets were shriveled and highly sterile like those in sterile tassels.
As shown in the table, all 8 plants had pollen similar in size to that of tetraploids. The percentages varied somewhat for different plants from the same cross. The percentage of these grains (class a) was higher in both plants from cross B than that from cross A. F1 plants from cross C had a higher percentage than those of crosses A and B. Some of the pollen in classes b and c may be functional. Some of these are possibly 1n pollen. These may have 10 normal chromosomes, one or both parental multiple interchanges. Further studies will be undertaken to ascertain the nature of the observed changes in fertility. More F1 seeds from the three crosses will be treated and planted in the field early in the coming season to obtain more of these partially fertile plants and to obtain F2 seed's from all three crosses. Pollen from fertile plants will be applied to tetraploid plants as well as to the parental homozygous multiple interchange stocks and to the sterile sibs.
Helmy Ghobrial and Charles R. Burnham
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