Synaptic patterns of rye B chromosomes. I: The standard type.

Chromosome pairing of standard B chromosomes (Bs) in rye has been studied by synaptonemal complex surface spreading of pollen mother cells containing from one to eight Bs. The main characteristics exhibited by the Bs are: (i) They are often located peripherally in surface spread nuclei and do not show presynaptic alignment. (ii) The period of pairing is delayed relative to that of the A set. (iii) Bs undergo extensive non-homol-ogous pairing at pachytene. (iv) The frequency of trivalents and quadrivalents in plants with 3Bs and 4Bs is about 66% following the expectations on the basis of random pairing among the short and the long arms with a single distal pairing initiation site per arm. Some quadrivalents containing two partner switches were observed, but on the same basis of randomness there was a deficiency of pachytene multivalents in plants with 5-8 Bs. The possible causes of the discrepancy between these observations are discussed. (v) There is a reduction in the frequency of multivalents between pachytene and metaphase I.