Chromosome-specific control of chiasma formation

A desynaptic mutant of Hypochoeris radicata, 2n = 8, has been found in a population from France. The mutant is remarkable in that at metaphase-I over 90% of PMC's have a pair of univalents while two pairs are found in only 1% of cells. Only chromosome IV is affected by the desynapsis which is controlled by a single recessive gene. Bivalent chiasma frequency in cells with a pair of univalents is higher than in cells with complete bivalent formation indicating partial dependence of cell chiasma number on the availability of some factor within the anther. The F₂, produced by full-sib mating, has a raised frequency of univalents in the chromosomes other than pair IV. The higher level results from enforced sib-mating of an obligate outbreeder and is not related to the action of the major gene. It is suggested that chiasma formation is controlled both polygenically and by major genes which operate on a hierarchical system. Some genes affect the behaviour of the entire complement while the action of others is specific to individual chromosomes.