Synaptonemal complexes of chains and rings in mice heterozygous for multiple Robertsonian translocations.

Complex Robertsonian translocation heterozygosities in the mouse have been used to test different hypotheses regarding the correlation between male hybrid sterility and chromosomal abnormality. Synaptonemal complexes of meiotic super-chains and super-rings involving 15 to 18 metacentric chromosomes were studied in relation to spermatogenic histology. Both types of multivalents showed a characteristic pachytene pattern of alternating paired and non-paired segments. The amount of unpaired segments in rings was about 18% and in chains about 23% of the total length of multivalent chromosomes. The meiotic chains were associated with the proximal part of the X chromosomes in more than 60% of pachytene cells; a similar tight proximity of rings with X or Y chromosomes was never found. Complete arrest of germ cell maturation correlated with super-chains and inconspicuous testicular histology with super-rings. This demonstrates that an excessive amount of unpaired chromosomal axes does not lead per se to male infertility through gametogenic breakdown. On the contrary, the results clearly indicate spermatogenic impairment in this system of multimetacentric heterozygosity as a reflection of X chromosome super-chain interference.