Fig. 2.

Asynapsis of homologous chromosomes in sterile F1 males. (A) Asynapsis of pachytene spermatocytes revealed by immunostaining of SYCP3 or HORMAD2. Early (histone H1t-negative) pachynemas show multiple asynapsed autosomes decorated by the phosphorylated form of histone H2AFX (γH2AFX). H1t-positive mid–late pachynemas show one or two pairs of asynaptic autosomes engulfed in the sex body. (B) The exceptional multivalents and ring-like chromosomes indicate partial and/or nonhomologous synapsis. (C) Asynapsis was rare in B6 mice and was absent in intraspecific hybrids (B6 × BALB/c)F1 (abbreviated B6CF1) and (PWD × PWK)F1 (abbreviated PKF1). PB6F1 meiocytes with more than four univalents disappear in mid-pachynema and diplonema. (D) Distribution of pachynemas according to the number of asynapsed autosomes estimated by counting the CREST-stained centromeres on SYCP3-labeled synaptonemal complexes. Individual pachytene stages could not be identified in this experiment. In total 111 aberrant pachynemas were counted. (E) Frequency of mid–late pachynemas with autosomal univalents within the sex body in sterile PB6F1 and semifertile reciprocal B6PF1 hybrids. (F) X–Y asynapsis correlates with male sterility; however, the sex chromosomes could not be reliably identified in the majority of early pachynemas. PAR, pseudoautosomal region.