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

Meiotic chromosome pairing of primary trisomics of rye was analysed by elec tron microscopy in surface-spread prophase I nuclei and compared with light -microscopic observations of metaphase I cells. Despite the large-sized chr omosomes of rye, prophase I trivalent frequencies were close to the two thi rds expected on a simple model with two terminal independent pairing initia tion sites per trisome (set of three homologous chromosomes). Direct observ ations mostly reveal one pairing partner switch (PPS) in prophase I trivale nts, 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 ch iasmata. In all of the trisomics, the three homologues showed variations no t only in the number of telomeric C-bands but also in the amount of heteroc hromatin of these bands, which allowed identification of chromosomes or chr omosome arms associated in most metaphase I configurations. In trisomics fo r chromosomes 2, 3 and 5, some metaphase I chromosome configuration frequen cies did not fit those expected under the assumption of random chromosome a ssociation among all partners, suggesting the existence of preferences for pairing between two given chromosome arms of the trisome. No preferential a ssociations either at metaphase I or pachytene were observed in the trisomi cs for chromosome 6. The fit between theoretical pairing models and the exp erimental data is also discussed.