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.