Spreading synaptonemal complexes of B isochromosomes in the grasshopper Omocestus burri

Meiotic pairing behaviour of one and two B isochromosomes (iso-Bs) in the grasshopper Omocestus burri was analysed by electron microscopy in surface-spread prophase I nuclei and compared with light microscopic observations of metaphase I. Iso-Bs display a peripheral location in the surface-spread nuclei and early pairing relative to that of the long members of the A set. Single iso-Bs undergo foldback pairing to give symmetrical hairpin loops. Two iso-Bs may show interarm pairing, mterchromosome pairing, or combinations of the two. Pericentromeric interarm pairing can be delayed in one or both Bs and this delay is mostly observed in bivalents with pairing partner switches. The iso-B bivalent frequencies observed in the three males analysed were 64, 44, and 41%, respectively; the two latter values were significantly lower than the 66% predicted by the random-end-pairing model. There is a reduction in the frequencies of iso-ring univalents (in 1B males) and bivalents (in 2B males) from pachytene to metaphase I. Similarities and differences between the pairing behaviour of iso-Bs from different species are also discussed.