CYTOGENETICS OF MEIOSIS OF MAIZE

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Our goal is to understand the mechanism of chromosome segregation during meiosis, particularly how homologous chromosomes pair and synapse. The maize male meiocyte is the only system where there is a large collection of mutants that affect meiosis, and it is possible to do superb cytology. We have collected 3-D images on a deconvolution light microscope system using fluorescence in situ hybridization (FISH) probes and antibodies against RAD51 to describe the rearrangements of telomeres, chromosomes and the recombination machinery during the homology search. There is no premeiotic association of homologs; rather, homologous chromosomes approach each other at the end of leptotene as telomeres associate with the nuclear envelope (NE) and cluster (bouqet formation). We will use FISH probes which light up multiple spots on one specific chromosome arm, FISH centromere specific probes, and antibodies against the recombination machinery (RAD51, MSH2/6) to analyze in 3-D homolog alignment as pairing is initiated. Analysis of the pairing behavior of chromosomal derivatives deficient in synapsis such as rings, deficiency heterozygotes, ditelocentrics and reciprocal translocations will allow us to test the requirements for chromosome ends, telomeric sequences, and subtelomeric or internal homology, for successful pairing. We will analyze the pairing behavior of chromosomes in mutants known to be deficient in the early stages of the homology search (afd, dsy1) or in later stages (as1, dy1, dsy2, etc.). We are developing novel screens based on partial pollen abortion or altered recombination rates to find new meiotic mutants. We are using directed transposon tagging to clone new mutants or existing mutants which are defective in pairing. We assume that the homology search is dependent on the active movement of chromosomes, mediated by telomere-NE associations. We will analyze homolog alignment and telomere clustering in living meiocytes using large blocks of heterochromatin (knobs) as markers. Colchicine blocks telomere interactions with the NE in vitro. We will analyze how colchicine disrupts other aspects on nuclear organization such as nuclear pore distribution. 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 inaccurate chromosome segregation (aneuploidy) during meiosis is casual in several congenital malformations, a major cause of premature termination of pregnancy, and of poor gamete production in humans.