Fig. 1

a Electron micro spread of a pachytene-spermatocyte nucleus of Mus domesticus 2n = 38, double heterozygote for the Rb11.16 and Rb16.17. The synaptonemal complexes of 16 autosomal bivalents, the quadrivalent (Q) and the XY bivalent (XY) are clearly seen. The nucleolar material is also indicated (Nu). Bar = 1 µm. b Quadrivalent synaptic configuration. The synaptonemal complex is formed in the whole extension of the four chromosomes, similar to a crosshead. Four synapsed telomeres and 4 points of the quadrivalent’s attachment to the nuclear envelope are also seen. The fourth synapsed telomere also reveals that between the short arms and centromere regions of the non-homologous chromosomes 11 and 17 a synaptonemal complex is formed, where a recombination bar might be present. Telomeres 1, 2, 3 and 4 are indicated. Bar = 1 µm. c Unsynapsed quadrivalent associated with the XY bivalent. Electron micro spread showing a thickened single unsynapsed axis of chromosome 11 or 17 connected to the single axis of the X chromosome. Double arrow: well synapsed chromosomes of the quadrivalent; single arrows: single unsynapsed axes of the short arms of chromosomes 11 and 17; XY: sex chromosomes. A dense body of presumed nucleolar material is also present (Nu)