Differential chromosome contraction at the pachytene stage of meiosis in alfalfa (Medicago sativa L.)

Using the length of the total chromosome complement as a measure of the pachytene stage of a cell, most of the variation from cell to cell in chromosome lengths can be accounted for. Significant regression equations were obtained for chromosome and arm lengths upon the cell stage and these provide estimates of the relative contraction rates of the chromosomes. The regression lines for chromosomes 1 and 2 were quadratic whereas they were linear for the remaining six chromosomes. The contraction rates were different for each chromosome as well as for the short and long arms of chromosomes 1 to 5. The relative contraction rates for the heterochromatic short arms of chromosomes 3, 4 and 5 were very low and therefore arm ratios as well as relative lengths of chromosomes could vary with the cell stage examined. The differences in chromosome numbering systems reported by alfalfa researchers are mainly attributed to the pachytene stages observed and the small numbers of observations per study.