Electron microscopy of t-allele synaptonemal complexes discloses no inversions

Abstract

The t complex of mice has been an enigma for over half a century¹. The recessive mutations (tⁿ) on chromosome 17 of the mouse have been characterized by four properties: interaction with the dominant mutation T (Brachyury) to cause tail-lessness, transmission ratio distortion in males, a series of homozygous lethal mutations, and cross-over suppression^2–4. The last property has led to many studies of meiosis in t-allele-bearing males. Forejt found fewer chiasmata, the cytological correlate of crossing-over, in the region of t haplotypes^5,6. Lyon et al.⁷ confirmed these results and suggested that chiasma suppression determined by t alleles is probably desynaptic but that electron microscopy would probably be needed to settle this point. We have now extended these observations by using a centromeric translocation and analysed synaptonemal complexes by electron microscopy to study t-haplotype pairing. We have found normal synaptonemal complexes without any evidence of inversions but detected early disjunction of the chromosome 17 homologues, supporting the idea of desynaptic chiasma suppression.