II.3 Producing marker genes on a specific chromosome.
D. R. Clark and R. T. Ramage. Department of Agronomy and Plant Genetics, University of Arizona, Tucson, Arizona 85721, USA.
Chromosomes 6 and 7 carry sub-terminal nucleolar organizing regions. Those portions of the chromosomes that are distal to the nucleolar organizing regions are the satellites. The satellite of chromosome 6 is larger than that of chromosome 7. We have a trisomic that has as an extra chromosome a fragment of chromosome 6 consisting of the long arm and most of the short arm; i.e., in the fragment, the satellite, the nucleolar organizer and a small portion of the short arm are deleted. In the selfed progeny of this trisomic, we have been unable to obtain diploids containing the fragment of chromosome 6 plus a normal chromosome 6 or trisomics containing two fragments plus a normal 6. We interpret this to mean that a gametophyte must possess at least one nucleolar organizer, and perhaps satellite, in order to function or that a zygote must have at least two such regions in order to develop. We would like to know if the satellite contains genes for morphological characters. The fragment trisomic can be used to locate genes carried on the satellite. If genes known to be carried on chromosome 6 are crossed onto the fragment trisomic, those located in the deleted regions will show disomic inheritance while those located elsewhere will show trisomic inheritance. Very few marker genes have been assigned to chromosome 6 and none of them have been located in the satellite. To facilitate the search for genes carried on the satellite of chromosome 6, we intend to induce a large number of mutants that we know will be located on chromosome 6.
To produce mutants on chromosome 6, we treated seed which carried an extra chromosome 6 with a mutagenic agent. If a mutant is induced on chromosome 6, it should show trisomic inheritance in the M2 generation. Mutants induced on the other 6 chromosomes should show disomic inheritance in M2.
The smaller and more shriveled seed produced by a primary trisomic 6 in Betzes were selected to obtain a population containing a high proportion of trisomics. The selected seed were treated with a 0.01 M solution of diethyl sulfate and planted in 3-inch clay pots. At the 2 to 3 leaf stage of growth, root-tips were collected from the seedlings showing the characteristic smaller first leaf of a primary trisomic 6 in Betzes. The root-tips of the selected plants were examined cytologically to verify the presence of an extra chromosome 6. Selection of seed and seedlings proved very effective in isolating trisomic plants. Over 95 percent of the plants examined cytologically carried an extra chromosome 6 whereas the original seed lot contained less than 30 percent trisomics.
Trisomic plants were transplanted into gallon pots and grown to maturity in the greenhouse. The M2 generation was grown at Bozeman, Montana in the summer of 1974. A number of mutants, such as albino seedling, glossy sheath, gigas and male sterile, were observed. M2 ratios of several of these indicate that they may be located on chromosome 6. The M3 generation is presently being grown to confirm the trisomic ratios.