Serial-Section Analysis of Clustering within Anthers of Maize Microsporocytes with Specific Crossovers

The distribution within anthers of maize plants of microsporocytes of differing crossover class was studied by application of the serial-sectioning technique to plants heterozygous for a paracentric inversion. In such material, cells with four-strand double crossovers within the inversion are distinguishable from cells with other classes of crossovers within the inverted region and from cells with no crossovers within the inversion. Evidence was found for coarse clustering of cells with four-strand double crossovers within the inversion and for coarse clustering of cells of other crossover classes (with at least one crossover within the inversion). The local frequencies of these two major categories of crossover cells appeared to vary independently of each other, with regional increases in each occurring at the expense of noncrossover cells. There is also some suggestion that cells with four-strand doubles within the inversion may be clustered independently of other double-crossover classes within the inversion, but these other classes are not directly scorable.