EFFECTS OF SEVERAL MEIOTIC MUTATIONS ON FEMALE MEIOSIS IN MAIZE

A modified enzyme digestion technique of ovary isolation followed by staining and squash preparation has allowed us to observe female meiosis in normal maize meiotically dividing megaspore mother cells (MMCs). The first meiotic division in megasporogenesis of maize is not distinguishable from that in microsporogenesis. The second female meiotic division is characterized as follows: (1) the two products of the first meiotic division do not simultaneously enter into the second meiotic division; as a rule, the chalazal-most cell enters division earlier than the micropylar one, (2) often the second of the two products does not proceed with meiosis, but degenerates, and (3) only a single haploid meiotic product of the tetrad remains alive, and this cell proceeds with three rounds of mitoses without any intervening cell wall formation to produce the eight-nucleate embryo sac. This technique has allowed us to study the effects of five meiotic mutations (am1, am1-pra1, afd1, dsy*-9707, and dv7) on female meiosis in maize. The effects of the two alleles of the am1 gene (am1 and am1-pral) and of the afd1 and dsy*-9707 mutations are the same in both male and female meiosis. The am1 allele prevents the entrance of MMCs into meiosis and meiosis is replaced by mitosis; the am1-pral perm its MMCs to enter into meiosis, but their progress is stopped at early prophase I stages. The afd1 gene is responsible for substitution of the first meiotic (reductional) division by an equational division including the segregation of sister chromatid centromeres at anaphase I. The dsy*-9101 gene exhibits abnormal chromosome pairing; paired homologous chromosomes are visible at pachytene, but only univalents are observed at diakinesis and metaphase I stages. These mutation specific patterns of abnormal meiosis are responsible for the bisexual sterility of these meiotic mutants. The abnormal divergent shape of the spindle apparatus and the resulting abnormal segregation of homologous chromosomes observed in microsporogenesis in plants homozygous for the dv7 mutation have not been found in meiosis of megasporogenesis. Only male sterility is induced by the dv1 gene in the homozygous condition.