Chromosome pairing in autotetraploid Bombyx females. Mechanism for exclusive bivalent formation

Abstract

Meiosis in tetraploid Bombyx mori oocytes has been analyzed by three dimensional reconstructions of 1 mid zygotene, 7 early pachytene and 11 mid-late pachytene nuclei. The general pattern of chromosome pairing was found to be essentially similar to that of diploid oocytes. The zygoene stage is recognizable by incomplete pairing and a distinct chromosome bouquet, early pachytene by complete or almost complete pairing and a distinct bouquet and mid-late pachytene by dissolution of the bouquet followed by release of chromosome ends from their attachment sites on the nuclear envelope. At early pachytene the nuclei contained 5–12 quadrivalents, 0–7 univalents, 0–3 trivalents and 27–44 bivalents. The corresponding figures for mid-late pachytene were 0–2 quadrivalents, 0–3 univalents, 0–2 trivalents and 48–56 bivalents. The remarkable decrease in the number of quadrivalents and the corresponding increase in the number of bivalents between early and mid-late pachytene is the result of a correction of the zygotene pairing. In the absence of crossing over and chiasma formation, the pairing of the randomly located leptotene chromosomes into the maximal number of bivalents consists thus of two phases: 1) A specific zygotene pairing with synaptonemal complex formation restricted to homologous chromosome regions and 2) a correction of irregularities in this pairing by partial dissolution of the central region of the complex succeeded by or coinciding with a second round of synaptonemal complex formation, permitting also nonhomologous pairing. It is suggested that in organisms with crossing over and chiasma formation the correction process is impeded by the occurrence of crossovers.