GENES PAM1 AND PAM2 CONTROL CYTOKINESIS A T DIFFERENT STAGES OF DEVELOPMENT OF MAIZE SPOROGENOUS CELLS

A comparative study was performed on megasporogenesis in maize plants bearing two recessive nonallelic mutations, pam1 and pam2 (plural abno rmalities of meiosis 1 and 2). The corresponding traits had monogenic inheritance. Both mutants were isolated and identified as meiotic muta nts causing similar abnormalities of meiosis. However, classification of these mutations in terms of key cytogenetic events of meiosis was d ifficult because of complexities in interpretation of phenotype. Analy sis of megasporogenesis in isolated maternal cells of megaspores allow ed determination of megasporogenesis events that were critical for the effect of each of these genes. Both genes participate in regulation o f cytokinesis at different stages of sporogenesis, although their effe cts are opposite. Mutation pam1 blocks cytokinesis in sporogenous cell s during cell cycles preceding meiosis, in both male and female gameto phytes. On the contrary, mutation pam2 removes the block of cytokinesi s at late stages of sporogenesis, namely, after meiosis, at the stage of embryo sac formation. Its effect lasts for three postmeiotic mitose s of a haploid megaspore. As a result, a series of haploid daughter ce lls is formed instead of the eight-nucleus embryo sac. Both mutant gen es are currently excluded from the group of meiotic genes, since they exert only indirect influence on meiosis. The phenomenon found in pam mutants is especially interesting from the evolutionary point of view, because it allows elucidation of genetic mechanisms of heterochrony i n the ontogeny of female and male gametophytes.