THE ALLIUM MICRONUCLEUS (MNC) ASSAY MAY BE USED TO DISTINGUISH CLASTOGENS FROM ANEUGENS

Employing Allium MNC assay, standard genotoxic agents namely maleic hy drazide (MH) and paraquat (PQ), both clastogens, colchicine, an aneuge n, and methyl mercuric chloride (MMCl), an aneugen as well as clastoge n were tested and compared for induction of MNC.(1)) Growing root meri stems of Allium cepa were treated with MH 3 x 10(-4) M, PQ 10(-3) M, C olchicine 2.5 x 10(-4) M and MMCl 1.26 x 10(-6) M for 2 h and subseque ntly recovered for 48 h in tap water. During recovery root meristems w ere excised periodically at 3 h intervals and fixed in ethanol:acetic acid (3:1) for cytological analysis. Slides with root squash preparati ons were scored for mitotic index, c-metaphases, polyploid cells and i nterphases with MNC. Diameter of MNC as well as that of nucleus from s ingle micronucleated cells were also measured. The duration of mitotic cell cycle determined by VAN'T HOF'S method following treatments with colchicine and MMCl were approximately 22 and 24 h, respectively. The peak (range) of hours yielding cells with MNC for MH, colchicine and MMCl were 24 (6-48) h, 9 (6-26) h and 21 (6-48) h, respectively. For P Q where no clearout peak hour in recovery was observed, the cells with MNC induced by PQ in the range 2.8-15.4 were significant at all te ho urs of recovery. The nuclear diameter ratio (diameter of MNC/diameter of nucleus) determined for MH, PQ and colchicine were 0.27 +/- 0.01, 0 .1 +/- 0.03 and 0.4 +/- 0.1, respectively. The MNC induced by MMCl, on the other hand, were of two sizes, the larger ones with a diameter ra tio of 0.42 +/- 0.1 comparable to that induced by colchicine and the s maller ones with a diameter ratio of 0.15 +/- 0.06 comparable to that induced by PQ or MH. MMCl induced MNC that shared features of MNC indu ced by the clastogens as well as the aneugens. The study, thus, demons trated that by considering two additional parameters, the time of appe arance and size of MNC, the Allium MNC assay can distinguish clastogen s from aneugens.