Meiosis in primary trisomics of rye:: considerations for models of chromosome pairing

Meiotic chromosome pairing of primary trisomics of rye was analysed by electron microscopy in surface-spread prophase I nuclei and compared with light-microscopic observations of metaphase I cells. Despite the large-sized chromosomes of rye, prophase I trivalent frequencies were close to the two thirds expected on a simple model with two terminal independent pairing initiation sites per trisome (set of three homologous chromosomes). Direct observations mostly reveal one pairing partner switch (PPS) in prophase I trivalents, which confirms this supposition. There were no significant differences between the number of trivalent and bivalent plus univalent configurations observed at prophase and metaphase I; therefore, synapsed segments form chiasmata. In all of the trisomics, the three homologues showed variations not only in the number of telomeric C-bands but also in the amount of heterochromatin of these bands, which allowed identification of chromosomes or chromosome arms associated in most metaphase I configurations. In trisomics for chromosomes 2, 3 and 5, some metaphase I chromosome configuration frequencies did not fit those expected under the assumption of random chromosome association among all partners, suggesting the existence of preferences for pairing between two given chromosome arms of the trisome. No preferential associations either at metaphase I or pachytene were observed in the trisomics for chromosome 6. The fit between theoretical pairing models and the experimental data is also discussed.