Abstract. Synaptonemal complexes (SCs) are zipperlike structures that are assembled between homologous chro- mosomes during meiotic prophase. They consist of two axial elements (AEs) (one along each of the two homol- ogous chromosomes), which, in mature SCs, are con- nected by numerous transverse filaments along their length. Several proteins involved in the later steps of meiotic recombination most probably function in close association with the AEs of SCs, because the proteins in- volved in these steps have all been localised along AEs or SCs by immunocytochemical methods. It is not known at which step in meiotic recombination this asso- ciation with the AEs is established. In order to shed some light on this issue, we analysed the localisation of two proteins that are involved in early steps of meiotic recombination, RAD50 and MRE11, relative to AEs and SCs by immunofluorescence labelling of paraffin sec- tions of the mouse testis, using affinity-purified polyclo- nal antibodies against RAD50 and MRE11, and mono- clonal and polyclonal antibodies against SC compo- nents. The localisation patterns of MRE11 and RAD50 within spermatocytes were very similar. MRE11 and RAD50 appeared in high abundance in preleptotene spermatocytes, just before SC components could be de- tected. From preleptotene until early zygotene they were present throughout the nucleus. In mid and late zygo- tene, MRE11 and RAD50 concentrated in distinct areas; in early pachytene the two proteins had almost disap- peared from the nucleus, except from the sex vesicle (the chromatin of the XY bivalent), where they persisted in high abundance until diplotene. We propose that MRE11 and RAD50, together with other proteins, prepare chro- matin throughout the early meiotic prophase nucleus for the initiation of meiotic recombination. Possibly, only a small fraction of the RAD50- and MRE11-containing (pre)recombination complexes associates transiently with AEs, where further steps in meiotic recombination can take place. Introduction Meiosis is responsible for two essential features of the sexual cycle: the transition from the diploid to the hap- loid state, and the generation of new combinations of al- leles. Both effects are accomplished during the first mei- otic division, when homologous chromosomes pair, re- combine and disjoin. At the cytological level, meiotic chromosome behav- iour has been analysed in great detail in many species, particularly in higher plants and animals (reviewed in Loidl 1994). Meiotic homologous chromosome pairing culminates in the formation of the synaptonemal com- plex (SC), a proteinaceous zipper-like structure, which keeps homologous chromosomes in close apposition. Each chromosome develops a single axial element (AE), which supports the chromatin loops of both sister chro- matids. The AEs of homologous chromosomes are then connected by numerous transverse filaments to form the structure of an SC (reviewed by Von Wettstein et al. 1994; Heyting 1996). The cytological studies of meiosis furthermore revealed electron-dense structures, approxi- mately 100 nm in diameter, in close association with (precursors of) SCs: recombination nodules or RNs (re- viewed by Carpenter 1994). In many species, two types of nodules can be distinguished on the basis of shape, size, time of appearance and/or numbers per bivalent, namely early nodules (ENs) and late nodules (LNs). Ear- ly nodules are more numerous than LNs, and occur along AEs and SCs from leptotene until early pachytene. Early nodules possibly mark the sites of early steps in meiotic recombination. Late nodules occur along pachy- tene SCs, and correspond in number and position to crossovers. Late nodules therefore mark the sites of late steps in crossover formation (Carpenter 1994). At the molecular level, meiotic recombination has been analysed primarily in budding yeast (Saccharomy- Edited by: D. Schweizer Correspondence to: C. Heyting e-mail: Christa.Heyting@molcelgen.el.wau.nl, Tel.: +31-317-482150, Fax: +31-317-483146 Chromosoma (2000) 109:123–132 Localisation of RAD50 and MRE11 in spermatocyte nuclei of mouse and rat Maureen Eijpe, Hildo Offenberg, Wolfgang Goedecke, Christa Heyting Laboratory of Genetics, Wageningen University and Research Centre, Dreijenlaan 2, 6703 HA Wageningen, The Netherlands Received: 16 November 1999; in revised form: 29 December 1999 / Accepted: 3 January 2000 © Springer-Verlag 2000