MEIOTIC CYTOLOGY

Meiosis is a complex series of cellular events limited to specialized tissues involved in gamete/gametophyte production. It is comprised of two consecutive cell divisions known as meiosis I and meiosis II. Just as in the interphase prior to mitosis, DNA replication resulting in a 4C nucleus occurs in premeiotic interphase.

Prophase I, the first stage of meiosis I, is of relatively long duration and is marked by many unique and important events. During the first two substages of prophase I (i.e., leptonema and zygonema), a proteinaceous rod-like axial core is constructed in each chromosome. In diploid organisms, each somatic cell contains a maternally-inherited set of chromosomes and a paternally-inherited set of chromosomes. Chromosomes that share the same genes and order of loci are called homologues (homologous chromosomes). During early prophase I, homologous chromosomes pair and align through a yet unknown mechanism. Axial cores of aligned homologous chromosomes synapse together during zygonema via a central mesh of fibers to form a scaffold called the synaptonemal complex (SC).  By convention, synapsed axial cores are referred to as lateral elements.  Each pair of homologues and its adjoining SC are called a bivalent. Chromatin loops extend from lateral elements to form a chromatin sheath around each SC (FIGURE 1).  Under normal circumstances, homologous loci lie directly across from each other along a bivalent. Once synapsis is complete, the substage of pachynema begins. It is during pachynema that crossing over is finalized. The mechanism by which segments of DNA are exchanged between homologous loci is unknown, but cytologically the location and frequency of crossover events is correlated with the presence of ellipsoidal nodules (recombination nodules) associated with the central regions of SCs.

Much of our research has focused on studying pachytene chromosomes.  In particular, we have focused on the relationship between the synaptonemal complex and DNA/chromatin (Peterson et al. 1994, Peterson et al. 1996, Peterson et al. 1999, Draye et al. 2001).  Dr. Peterson's M.S. research centered on electron microscopic (EM) investigation of SCs (e.g., FIGURE 2, FIGURE 3, and FIGURE 4), and he hopes to continue EM work in the future.  At present MGEL utilizes a combination of cytological, molecular biology, and genomic approaches to study bivalent structure at the light microscopic level (e.g., FIGURE 5 and Cytomolecular Mapping).

FIGURE 1: Model of the synaptonemal complex (SC).  Lateral elements (light blue rods) of homologous chromosomes align and synapse together via a meshwork of transverse filaments (white lines) and longitudinal filaments (dark blue rods).  The longitudinal filaments are collectively referred to as the "central element" of the SC.  Ellipsoidal structures called recombination nodules (e.g., gray ellipsoid) are constructed on the central region of the SC.  As their name implies, recombination nodules are believed to involved in facilitating meiotic recombination (crossing over).  The chromatin (red loops) of each homologue are attached to its corresponding lateral element.  Because there are two "sister chromatids" in each homologue, two loops are shown extending laterally from each point along a lateral element.

FIGURE 2: Tomato SC spread (electron micrograph). There are twelve tomato SCs (i.e., 2n = 2x =24).  Note that a "halo" of chromatin is visible around each SC.

FIGURE 3: Partial tomato SC spread (electron micrograph).  Five Solanum lycopersicum SCs are visible.  The centromere of each bivalent is marked by a prominent kinetochore (arrows).

FIGURE 4: Close-up of a portion of an SC from FIGURE 3.  Both lateral elements and the central element are visible in this preparation.  A recombination nodule lies on the central element of the SC (arrow).