Our goal is to understand the mechanism of chromosome segregation during meiosis, particularly initiation of pairing of homologous chromosomes. The maize male meiocyte is the only cell where there is a large collection of mutants that affect meiosis, and it is possible to do superb cytology. We have shown that pairing is initiated at the transition between leptotene and zygotene and is associated with transitory changes in chromosome morphology. We call this stage pre- zygotene. We will describe the changes in chromosome structure and arrangement in the nucleus that occur during meiotic prophase, using the computerized light microscope workstation developed by John Sedat and David Agard (UCSF) that is capable of recording three dimensional images of multiple cellular components in fixed and living cells. The distinctive chromomeres, chromomeres, knobs and centromeres along with the use of paining probes, telomere probes, probes for individual genes and fluorescence in situ hybridization (FISH) will allow us to identify each individual chromosome arm. We will follow the behavior of one region of the same chromosome as it pairs with its homolog. This data will also be used to develop a cytological map of the maize genome. We will use antibodies against functionally important proteins involved in recombination to describe their location during pairing. We will also develop methods to study pairing in living cells. This analysis will allow us to understand the relevance of nuclear envelope associated telomere movements to initiation of pairing and locate the sites on chromosomes where pairing is initiated. We will culture meiocytes or anthers and determine whether microtubule depolymerizing agents applied during prezygotene interfere with telomere movements and the pairing process. We will analyze mutant meiocytes defective in the initiation of pairing to determine whether any of the have abnormal pre-zygotene chromosome morphology or abnormal telomere clustering. Analysis of mutants will eventually allow us to link changes in chromosome morphology and distribution to the corresponding molecular events required for homology recognition and synapsis. We are continuing a screening program to isolate and cytologically characterize new meiotic mutants induced by Mu or Ac transposon mutagenesis. Our highest priority is to find new mutants deficient in the pairing process. Meiosis is essential for all sexually reproducing organisms and the studies described here will further our understanding of this process not only in maize but in all organisms. The mechanism of meiosis is a topic of major medical interest since inaccurate chromosome segregation (aneuploidy) is causal in several congenital malformations and a major cause of premature termination of pregnancy. An improved understanding of meiosis in maize will allow us to understand the basis of abnormal chromosomal segregation during human gamete production.