Nodules associated with axial cores and synaptonemal complexes during zygotene in Psilotum nudum

Two-dimensional spreads of synaptonemal complexes (SCs) from the lower vascular plant Psilotum nudum were examined after staining with uranyl acetate-lead citrate (UP). Staining with UP allows visualization of lateral elements/axial cores (ACs), central elements, kinetochores, and nodules. Numerous darkly stained nodules were associated with forming SCs. In addition to nodules found on the central element of SC segments, other nodules were found at points of convergence between two adjacent ACs. Of these latter nodules, some were obviously associated with a fiber that connected adjacent ACs. No central element material was visible between the ACs, and the nodule complex appeared to be the only structure holding the ACs together. Although the function(s) of nodules during zygotene is unknown, the presence of a nodule-fiber complex that connects adjacent ACs before central element formation suggests that at least some of the nodules may be involved in synaptic initiation.