Methods for immunoelectron microscopic and fine structural analysis of synaptonemal complexes and nodules in yeast.

Several gene products involved in meiotic chromosome pairing and recombination in yeast have been identified in recent years. Two nuclear structures play key roles in the meiotic processes: the synaptonemal complex (SC), which is essential for the pairing of the chromosomes, and the recombination nodules (RNs), which mark the sites of recombination. Good morphological representation of the yeast SC and RNs is needed in order to show structural changes caused by specific mutations in protein-coding genes and for fine localization of proteins using immunoelectron microscopy (immuno-EM). This paper presents a newly developed preparation method for EM and immuno-EM that allows analysis of fine structural details and localization of proteins in the SC and RNs in yeast. Structural components of the SC are clearly seen and appear strikingly similar to those in the SC in other organisms. Antibodies against the SC protein Zip1, a transverse filament protein, label the central region of the SC strongly and specifically as expected. The improved method will be an important tool in high-resolution determination of the location of proteins in the meiotic yeast nucleus.

Evidence for close contact between recombination nodules and the central element of the synaptonemal complex.

During the first part of meiosis, homologous chromosomes pair and undergo genetic recombination. Two meiotic structures are involved in these processes: the synaptonemal complex (SC) is essential for synapsis of the chromosomes, and the recombination nodules (RNs) represent the sites for recombination. In the present investigation we have used conventional electron microscopy to study the association between the SCs and the RNs in the beetle Blaps cribrosa. This experimental material was chosen because the spermatocytes in B. cribrosa display both exceptionally well-defined SCs and distinct RNs. We find that the RNs are drop shaped, located on top of the SC and oriented in parallel with the ribbon-like SC. The most striking observation is that the RNs coalesce with the top layer of the central element of the SC. The RNs are also connected via fibres to the lateral elements of the SC. These and other structural observations suggest that the RNs could influence the synapsis of homologous chromosomes by affecting both early and late steps in the assembly of the SCs.

Organization of SCP1 protein molecules within synaptonemal complexes of the rat.

SCP1, a major protein component of synaptonemal complexes (SCs), is probably a constituent of the transverse filaments (TFs). The protein consists of three domains: a short, proline-rich N-terminal part, a stretch of 700 amino acid residues capable of forming an amphipathic alpha-helix, and a C-terminal domain of 240 amino acid residues which is capable of binding to DNA. To analyze the orientation of SCP1 molecules within SCs, we elicited polyclonal antibodies against three non-overlapping fragments of SCP1, which comprise, respectively, the N-terminus, the C-terminus, and a fragment from the middle of the SCP1 molecule. Using these antibodies, we performed immunoelectron microscopy on SCs in two types of preparations, namely, surface-spread spermatocytes and ultrathin sections of Lowicryl-embedded testicular tissue of the rat. For each of the three antibodies used, the distribution of immunogold label on surface-spread spermatocytes differed significantly from the distribution of label on sections. Masking of SCP1 epitopes within the lateral elements (LEs) and the central element (CE) of SCs in surface-spread preparations and the influence of the surface morphology of the spreads on the labeling pattern were considered as possible explanations for these differences. We therefore relied on the results from sections for the localization of epitopes. On the basis of the distributions of immunogold label in Lowicryl sections and the predicted secondary structure and dimensions of SCP1 molecules, we present the following model: the C-terminus of SCP1 molecules lies in the inner half of the LE, the molecules protrude from the LE through the central region into the CE, and end up with their N-terminus between the center of the CE and the opposite LE, so that the N-termini of SCP1 molecules from opposite LEs overlap. The model has several implications for the assembly of SCs and the possible functions of SCP1.

The central region of the synaptonemal complex revealed in three dimensions.

The synaptonemal complex plays a key role in pairing of homologous chromosomes during meiosis. Its gross structure was already known by conventional electron microscopy, but only recently has it been possible to reveal the synaptonemal complex in three dimensions at higher resolution by electron microscope tomography. As the molecular analysis of meiosis is developing rapidly, a more thorough understanding of the principal organization of the synaptonemal complex is essential.

The central region of the synaptonemal complex in Blaps cribrosa studied by electron microscope tomography.

The synaptonemal complex (SC) in the beetle Blaps cribrosa contains a highly organized central element (CE), two flanking lateral elements (LEs), and a number of regularly spaced transverse filaments (TFs) crossing the central region. The CE is built like a ladder with two longitudinal components running in parallel and a number of regularly spaced transverse CE components, bridging the two longitudinal components. The CE is multi-layered with the ladders of the individual layers more or less in register. Essentially every TF originates in one of the LEs, crosses the CE through a transverse CE component and reaches the opposite LE; every transverse CE component in a given layer corresponds to one, and only one, TF. In a CE layer, short irregular pillars form the junctions between the transverse and longitudinal CE components. Adjacent pillars are connected to each other by fine fibrous bridges: the two pillars in the same transverse CE component are linked, and so are the pillars along each longitudinal component, and also more occasionally adjacent pillars in separate CE layers. It is proposed that a TF with the two associated short pillars represents the structural unit in the central region. The ordered structure of the CE is accomplished by linking adjacent pillars to each other into the well-defined three-dimensional organization of the CE.

The three-dimensional structure of the central region in a synaptonemal complex: a comparison between rat and two insect species, Drosophila melanogaster and Blaps cribrosa.

The highly ordered central region of the synaptonemal complex (SC) in Blaps cribrosa has recently been studied by electron microscope tomography (EMT), and a simple three-dimensional model presented. Using the same experimental approach we have now compared the central region in Blaps with the central regions in Drosophila melanogaster and rat. In all three species, the SCs exhibit a central element (CE) flanked by two lateral elements (LEs). The central region between the two LEs is crossed by transverse filaments (TFs). The Blaps CE element is the most ordered one with a well-defined ladder-like structure with two longitudinal components bridged by a number of regularly spaced transverse components, the rungs of the ladder. At the junctions between the longitudinal and transverse components there are prominent dense structures. The CE is multi-layered with the ladders of the separate layers in approximate register. In Drosophila the transverse CE components are as distinct and well organized as in Blaps, while in rat they are present but are less frequent and less well ordered. The longitudinal CE components in Drosophila are often fragmented and even more so in rat. The tomographic analysis revealed that in all three species the central region contains the same structural units: a single TF associated with two short pillars (or globules), which correspond to the junction structures. A fibrous lattice connects the two pillars/globules on the same TF forming the transverse CE component and those on adjacent TFs forming the longitudinal CE component; fibers between pillars/globules also link consecutive CE layers together. In the longitudinal component the number of fibrous bridges between the pillars/globules is related to the conspicuousness of the longitudinal component, i.e. Blaps has most, Drosophila almost as many, and rat considerably fewer bridges. We conclude that the central region in rat, Drosophila and Blaps contains the same basic structural unit but the degree of order and concentration of the units differ: a higher density seems to be accompanied by a higher order within the CE.

Solitary and synaptonemal complex-associated recombination nodules in pro-nurse cells during oogenesis in Drosophila melanogaster.

An oocyte in Drosophila melanogaster originates from 1 of 16 cells comprising an ovarial syncytium. The two pro-oocytes proceed into the pachytene stage of meiosis, but only one develops further into a mature oocyte while the other reverts to a nurse cell. It is known that pro-nurse cells also enter meiosis, as they contain incomplete synaptonemal complexes (SCs). We now show that these cells also harbour recombination nodules (RNs). In cells that only occasionally contain SC segments, the RNs are typically not located close to distinct tripartite SC structures. Instead, these RNs are frequently associated with a spherical body of amorphous material and two to three, more or less parallel fibres, possibly representing SC material. The significance of the solitary RNs is discussed in relation to the present knowledge of the assembly and disassembly of the SC.