Meiosis is the process by which sexually reproducing organisms reduce their genomes from diploid (2n) to haploid (n) during the formation of gametes. It requires that homologous chromosomes pair, synapse, recombine, and finally segregate. These widely conserved processes are under genetic control, yet the exact details of many of the underlying molecular mechanisms remain under active investigation. The initial pairing and subsequent synapsis events are immediately preceded by the clustering of telomeres on the nuclear envelope in a widely conserved structure referred to as the bouquet arrangement of meiotic chromosomes. In animals and plants, genes required for genome reduction at meiosis I have been characterized and show a high degree of conservation between kingdoms and species within them. Higher plants (most notably maize) have provided an excellent large-genome model system for the study of the cytology of homologous chromosome behavior and therefore have allowed an in depth dissection of the meiotic process in eukaryotes. At the cellular level, meiotic chromosome behavior is accompanied by changes in the architecture of the cell nucleus, particularly with respect to the interaction of telomeres with the nuclear periphery. This dissertation presents the work involving the analysis of a classic meiotic mutant of maize, desynaptic (dy1). The dy1 mutant is characterized by a precocious telomere-nuclear envelope detachment phenotype at mid prophase, resulting in chromosome breaks, anaphase bridges, micronuclei, and defective pollen development. In this study, we observed new phenotypes as early as the telomere bouquet stage of meiotic prophase in dy1 lines of maize. Using linkage and translocation mapping techniques, the dy1 mutation was mapped to the long arm of chromosome 3, where a candidate gene with homology to a nuclear envelope-associated SUN domain protein gene was identified. SUN (Sad1p/Unc-84) domain proteins function with other proteins to form a physical link between the nucleoskeleton and the cytoskeleton. These bridges transfer forces across the nuclear envelope and are increasingly recognized to play roles in nuclear positioning, nuclear migration, cell cycle-dependent breakdown and reformation of the nuclear envelope, telomere-led nuclear reorganization during meiosis, and karyogamy. Using bioinformatic and molecular approaches, we characterized the family of maize SUN-domain proteins, starting with a screen of maize genomic sequence data. We characterized five different maize ZmSUN genes (ZmSUN1-5), which fell into two structural classes likely of ancient origin. Orthologs of these genes and prevalent in the plant kingdom as they are also found in other monocots, eudicots, and even mosses. The first class described here designated canonical C-terminal SUN-domain (CCSD, ZmSUN1 and ZmSUN2), includes structural homologs of the animal and fungal SUN-domain protein genes. The second class, the plant-prevalent mid-SUN 3 transmembrane (PM3, ZmSUN3-5), includes a novel but conserved structural variant SUN-domain protein gene class. Analysis of the expression levels for these genes revealed very low expression in multiple tissue types, with the exception of ZmSUN5 which showed a pollen=preferred expression profile. Cloning and Peptide antibodies specific for ZmSUN3, and ZmSUN4 were used in western-blot and cell-staining assays to show that they are expressed and show concentrated staining at the nuclear periphery. In order to characterize the CCSD class of SUNproteins, we obtained new reagents and performed immunolocalization analyses coupled with high resolution 3D deconvolution microscopy. We identified a novel structure at the maize nuclear periphery we refer to as the "Nuclear SUN Belt", NSB, which was present in multiple somatic cell types as well as meiotic nuclei. During meiosis, the NSB was present at the onset and well into the leptotene stage of meiotic prophase. Surprisingly at the bouquet stage the NSB appeared to be localized opposite of the nucleolus in a crescent shape, occupying a small region (<1/3) of the surface of the nuclear periphery, often co-localizing with meiotic telomeres. During late prophase, the NSB returned temporarily until the release of the telomeres from the NE and subsequent NE breakdown prior to metaphase. The NSB later returned in post-meiotic nuclei including uninucleate cells, and prophase II nuclei. Using peptide antibodies specific for the CCSD class, we detected a severe disruption of SUN proteins at the nuclear envelope in a line of maize defective in meiotic telomere tethering and chromosome synapsis (desynaptic1, dy1) as well as a line defective in the transition from a prophase microtubule array to a metaphase spindle (divergent1, dv1) (SHAMINA et al. 2000b; STAIGER and CANDE 1990a). The findings presented in this dissertation provide valuable new information regarding the spatial distribution and dynamics of maize SUN proteins at the NE and for an initial interpretation of the phenotypes of historical meiotic mutants of maize.
