Antimetabolites and Chiasma Formation in Delphinium

A number of chemicals including actinomycin D, mitomycin C, cycloheximide, 5-fluorodeoxyuridine (FUDR) and potassium pyrophosphate (K4P2O7; 3H2O) have been tested for their effects on chiasma formation in the pollen mother cells of Delphinium ajacis. These chemicals are known to inhibit the syntheses of macromolecules like DNA, RNA or proteins. In general, it has been found that all the chemicals bring about a significant reduction in chiasma frequency. An exception, however, is provided by mitomycin C which increases chiasma frequency at lower concentrations and decreases it at higher concentrations. Another important finding is that none of the chemical treatments resulted in a failure of chromosome pairing. FUDR has been found to induce chromosomal aberrations in addition to its effect on chiasma frequency. The significance of these findings in relation to the mechanism of crossing over has been discussed.