An apparently balanced reciprocal translocation 46,X,t(Y;6) (q11.23 approximately q12;p11.1) was observed in an infertile man with severe oligozooteratozoospermia. Different mitotic chromosome banding patterns were performed and fluorescence in situ hybridization indicated a breakpoint in the fluorescent Yq heterochromatin. Molecular genetic deletion experiments for the azoospermia factor region in distal Yq11 showed the retention of the DAZ gene and meiotic pairing configurations suggested that the man's infertility could be due to the pairing behaviour of the Y;6 translocation chromosome with the X chromosome visualised by synaptonemal complex analysis at the electron microscopy level. The morphological appearance of the normal chromosome 6 and the Y;6 translocated chromosome included in the compartment of the sex vesicle may allow an explanation of the degeneration of most spermatocytes after the pachytene stage.

Two males with a 46, Y, der(X), t(X;Y)(p22.3;q11) complement were referred independently for evaluation of sterility with azoospermia. Both patients exhibited minimal symptomatology, characterized only by psychological disturbances. Study of X-chromosome breakpoints with pseudoautosomal probes 68B (DXYZ2 elements), 113D (locus DXYS15), and 19B (locus MIC2) indicated in both patients that at least 97% of the X pseudoautosomal sequences are lost. Hybridization with Xp22.3-specific probes DXS283, DXS284, and DXS31 shows that these loci are retained on the rearranged chromosome. Thus, the X-chromosome breakpoints are located close to the proximal boundary of the pseudoautosomal region, between MIC2 and DXS284.

In an attempt to elucidate the mechanism of sterility of X-autosome translocations in the mouse, we studied the distribution of [3H]-uridine incorporation in sterile males carrying the balanced X-16 reciprocal translocation. The results failed to show an overall reactivation of the X as has been postulated by Lifschytz and Lindsley (1972) but there was some spreading of X inactivation along the translocated and normal chromosome 16 in those regions that were close to the X breakpoint. We feel that this process could be responsible for metabolic disturbances leading to degeneration of primary spermatocytes and, therefore, to sterility.

Silver-stained synaptonemal complexes (SCs) in surface-spread pachytene nuclei from a boar, heterozygous for a reciprocal translocation, were analysed by electron microscopy. In such heterozygotes, cross-shaped quadrivalent configurations are expected to form in order to maximize homologous pairing. Contrary to the classical, expected cross-shaped configuration, heterosynapsis was often observed, with asymmetrical association in the lateral elements of the non-homologous partners of the quadrivalents. This heterosynapsis is assumed to be a mechanism preventing spermatocyte loss, but inducing a secondary segregational type of impairment of fertility due to foetal wastage leading to reduced prolificacy.

Morphological analysis of pachytene spermatocytes obtained from male mice carrying three chromosomal rearrangements – a Robertsonian translocation, Rb(X-2)2Ad; an autosomal reciprocal translocation, T(16;17)43H; and a tertiary trisomic, Ts(113)70H – demonstrated frequent association between the XY bivalent and the T43H and T70H translocation chromosomes. Quantitative autoradiographic data revealed that the normal transcriptional inactivity of the XY bivalent was not significantly disturbed, in contrast to that of the 16;17 quadrivalent and the extra 1¹³ marker chromosome. These results are interpreted as an extension of the XY inactivation process to the associated autosomes and discussed in relation to male sterility.

An electron microscopy study of synaptonemal complexes in two men carrying reciprocal translocations, a t(19;22) and a t(17;21), is reported. It is shown that a delay in synapsis affects the segments corresponding to the short arms of the acrocentrics involved in the formation of quadrivalents. This appears to provoke an interaction with the sex bivalent which could lead to a failure of spermatogenesis. A study of the literature comparing reciprocal translocations that do and do not involve acrocentrics in sterile and fertile men shows the existence of a significant association between the presence of an acrocentric in the rearrangement and sterility. These results on reciprocal translocations involving at least one acrocentric chromosome correspond to those obtained in cases of Robertsonian translocations.

An electron microscopic study of synaptonemal complexes in two heterozygous fertile boars, one a carrier of a 4; 14 reciprocal translocation and the second a carrier of this translocation associated with a 3;7 reciprocal translocation, is reported. The results showed heterologous pairing in almost all quadrivalents, as well as a lack of XY-quadrivalent association. This seemed to be a common feature of translocations in pigs, even if at least one acrocentric chromosome is involved, and may represent a significant meiotic mechanism that prevents spermatocyte loss, while the production of genetically unbalanced gametes leads to loss of progeny through abortion.

Synapsis and "synaptic adjustment" were analyzed, using electron microscopy in silver stained surface microspreads of inversion-bearing spermatocytes, in an infertile human male with an inherited pericentric inversion in chromosome 1. Possible reasons for his infertility are discussed.