, Volume 42, Issue 2, pp 103-151

Three-dimensional reconstruction of chromosome pairing during the zygotene stage of meiosis in Lilium longiflorum (thunb.)

Abstract

Chromosome pairing in early zygotene in Lilium longiflorum has been studied by three-dimensional reconstructions at the electron microscopical level. One nucleus has been completely reconstructed, and the pairing behaviour and the length of the 12 partially synapsed bivalents was analysed and measured. The total length of the 24 chromosomes comprises 7,4 mm, and pairing is initiated at several sites along the chromosomes. Analysis of the pattern and temporal sequence of pairing suggests that all regions of the lateral component contain the required specificity for site-to-site matching. Measurements of the length of the unpaired homologous regions demonstrate that identical chromosome length is secured for both homologues before pairing is initiated. Pairing is accomplished by the attachment of a piece of central region to one of the lateral components after rotation of the associated chromatin, followed by subsequent binding of the central region to the homologous lateral component. The majority of the telomeres are attached via a special substance to a restricted portion of the nuclear envelope. The occurrence of chromatin associated structures at the nuclear envelope in combination with the highly polarized distribution of nuclear pores suggests that considerable membrane activity of possible significance for chromosome movements takes place during this interval. The observations are discussed in relation to current hypotheses on the structure and biochemistry of early meiotic chromosomes and the processes of alignment and pairing.