Synaptic patterns of rye B chromosomes. IV. The B isochromosomes.

Pairing of B isochromosomes in rye was studied by surface spreading of synaptonemal complexes in pollen mother cells from plants containing one small iso-B (iso-BS), one large iso-B (iso-BL), two iso-BL, one iso-BS plus one standard B (iso-BS+stB), one iso-BS plus two standard Bs (iso-BS + 2 stB) and one iso-BL plus two standard Bs (iso-BL + 2 stB). The main characteristics of iso-Bs were as follows. (1) Their location in the surface-spread nuclei was peripheral. (2) Their period of pairing relative to that of the A-set was delayed. (3) The commonest pachytene configuration was a symmetrical hairpin loop, although they could also undergo nonhomologous synapsis. (4) Synapsis could start at the chromosome ends, at the putative centromeric region or at both regions more or less simultaneously. (5) The frequency of iso-BL bivalents was only 46 per cent rather than the 66 per cent predicted by the random-end-pairing model. Interchromosomal pairing was frequent in these configurations. (6) In pairing competition situations involving three and four doses of the short arm of the standard B (iso-BS+stB, iso-BS + 2 stB), a large predominance of self-synapsed iso-BS univalents (63 per cent and 80 per cent respectively) was found. On the other hand, there was no apparent pairing preference among four doses of the long arm of the standard B in the iso-BL + 2 stB combination. Similarities and differences between the synaptic behaviours of iso-Bs of Crepis capillaris and Secale cereale are discussed.