SYNAPSIS AND CHIASMA FORMATION IN A DITELO-SUBSTITUTED HAPLOID OF RYE

Synapsis and chiasma formation were analyzed in a ditelo-substituted haploid of rye (Secale cereale), in which chromosome 1R was replaced by its telosomes. The study was made by comparing synaptonemal complex formation at early meiotic prophase I, chromosome associations at metaphase I, and recombinant chromosomes at anaphase I and prophase II. For the analysis of synaptonemal complexes, 41 nuclei at stages ranging from leptotene to early diplotene were selected. In the leptotene and early zygotene nuclei, numerous alignments of axial cores involving the same or different chromosomes were observed. Pairing initiation sites occurred at both interstitial and distal segments. Throughout zygotene the extent of pairing gradually increases, with values up to 84.8% at a stage that is morphologically comparable with late pachytene. Pairing-partner exchanges were frequently observed in zygotene nuclei, giving rise to multiple associations encompassing all or most of the chromosomes. In cells at metaphase I multivalents were very rare (2%), indicating elimination of most pairing-partner exchanges. In cells at metaphase I, anaphase I, and prophase II chromosome length, centromere position, and C-banding pattern enable the identification of the chromosomes 1RS, 1RL, 4R, 5R, and 6R, and the distinction of three metacentric chromosomes (RM1, RM2, and RM3). Metaphase I bonds were found to be nonrandomly distributed. Associations between the arms 4RL, 5RL, 6RL, and RM1L, all of them without telomeric C-bands, were more frequent than between the remaining arms. The bonds were mainly located at the distal parts of the chromosomes. The frequency of recombinant chromosomes at anaphase I or prophase II suggests that metaphase I bonds were true chiasmata.