THE 3-DIMENSIONAL STRUCTURE OF THE CENTRAL REGION IN A SYNAPTONEMAL COMPLEX - A COMPARISON BETWEEN RAT AND 2 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 tomogr aphy (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 t wo lateral elements (LEs). The central region between the two LEs is c rossed by transverse filaments (TFs). The Blaps CE element is the most ordered one with a well-defined ladder-like structure with two longit udinal components bridged by a number of regularly spaced transverse c omponents, the rungs of the ladder. At the junctions between the longi tudinal and transverse components there are prominent dense structures . The CE is multi-layered with the ladders of the separate layers in a pproximate register. In Drosophila the transverse CE components are as distinct and well organized as in Blaps, while in rat they are presen t but are less frequent and less well ordered. The longitudinal CE com ponents in Drosophila are often fragmented and even more so in rat. Th e 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 str uctures. A fibrous lattice connects the two pillars/globules on the sa me TF forming the transverse CE component and those on adjacent TFs fo rming the longitudinal CE component; fibers between pillars/globules a lso 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. W e conclude that the central region in rat, Drosophila and Blaps contai ns the same basic structural unit but the degree of order and concentr ation of the units differ: a higher density seems to be accompanied by a higher order within the CE.