RESUMEN
We examined the effect of aphidicolin, colchicine, demecolcine, fluorouracil, hydroxyurea, and nocodazole, as well as nutrient deprivation on the Giardia intestinalis cell cycle. Aphidicolin was the only drug that was able to block the cell cycle at a specific stage (G1/S), and permit cells to resume growth at a high rate upon its removal. Nutrient deprivation resulted in a portion of G2/M cells completing mitosis and cytokinesis in synchrony during the recovery period, but this synchrony was shortly lost and a sample containing a predominance of G1 cells could not be obtained. Flow cytometry analysis of normal and untreated Giardia cultures showed the occasional appearance of a small percentage of cells with a DNA content of 16C, which is twice the DNA content of G2 cells. However, this 16C peak is larger and more frequently observed in drug-treated Giardia. These 16C are likely produced from endoreplication of 8C/G2 cells, and we propose that they represent a pre-encystation stage that is induced by drug treatments and other stressors.