Synaptic patterns of rye B chromosomes. III. The deficient B.

Chromosomal pairing of a deficient B chromosome in rye has been studied by surface spreading of synaptonemal complexes in pollen mother cells from plants containing one deficient B (dB), one deficient B plus one standard B (dBstB) and one deficient B plus two standard Bs (dB2stB). Single dBs are often located peripherally in surface spread nuclei, and they undergo extensive non-homologous synapsis at pachytene. In the dBstB bivalents, homologous and non-homologous synapses occur simultaneously throughout pachytene without axial equalization. Competition for pairing between the dB and the two stBs results in a large predominance (80%) of stB bivalents. This finding is explained by the identical origin of the standard Bs and/or by the minor homology between the stB and the dB chromosomes.