Regulation of R-protein and endonuclease activities in meiocytes by homologous chromosome pairing

Achiasmate meiocytes of Lilium cv. Black Beauty do not undergo DNA nicking at pachytene, nor do they acquire the high level of R-protein during zygotene-pachytene that is characteristic of chiasmate types. Induction of tetraploidy in such meiocytes with its accompanying increase in chiasma frequency results in a pachytene nicking pattern that is indistinguishable from what occurs in chiasmate plants. R-protein also increases to a level that on a per diploid genome basis is at least as high, and probably higher, than the level of chiasmate diploids. The absence of nicking in the achiasmate diploid is not due to a lack of appropriate enzyme activity but seems to result from the fact that unpaired or ineffectively paired chromosome regions are not accessible to the enzyme. The regulation of R-protein level by homologous pairing is examined but remains poorly understood.