Incomplete chromosome pairing and its relation to chiasma localisation in Stethophyma grossum spermatocytes

Summary Reconstructions of synaptonemal complexes (SC's) from serial sections of Stethophyma grossum spermatocytes demonstrate a clear correlation of chiasma distribution and the extent of pairing among different chromosome pairs. Proximal localisation of chiasmata in the eight longest chromosome pairs is associated with proximally localised incomplete development of the SC in eight bivalents. Up to three bivalents show complete SC development in EM reconstructions which parallels the non-localised chiasma formation shown by the three shortest bivalents at Metaphase I. The unpaired axial cores (AC's) of the incompletely paired bivalents were studied by a technique which revealed the nuclear membrane attachments of most AC's and allowed an assessment of the relative amounts of SC and AC. The extent of the correlation between chiasma localisation and incomplete pairing is examined and discussed.