GAMETOCIDAL GENES INDUCE CHROMOSOME BREAKAGE IN THE INTERPHASE PRIOR TO THE FIRST MITOTIC CELL-DIVISION OF THE MALE GAMETOPHYTE IN WHEAT

Male gametogenesis was cytologically analyzed in wheat lines homozygou s or hemizygous for gametocidal (Gc) factors with different modes of a ction. The first and second meiotic divisions in all lines were cytolo gically normal. The postmeiotic mitoses were normal in the homozygous lines; however, chromosome fragments and bridges were observed in the mitoses of the hemizygous lines. The morphology of the chromosome frag ments suggests that the Gc genes induce chromosome breaks in the G(1) phase prior to DNA synthesis of the first postmeiotic mitosis. The age of an anther was correlated with the frequency of aberrant second mit osis. Younger anthers contained a higher number of pollen undergoing n ormal second mitosis. This observation suggests that the arresting of the cell cycle occurs as the result of chromosome breaks during the fi rst mitosis. Because chromosome bridges were more frequent than fragme nts in the second mitosis, breakage-fusion-bridge cycles possibly occu rred during gametogenesis, which led to further chromosomal rearrangem ents. The Gc factors located on chromosomes 2S of Aegilops speltoides and 4S(sh) of Ae. sharonensis induce severe chromosome breakage in pol len lacking them. However, the Gc factor on telosome 2C(c)L of Ae. cyl indrica only induced chromosome breaks at a low frequency. The observe d partial fertility of Gc lines is presumably due to cell cycle arrest and the competition among gametes with and without chromosome breakag e.