CHIASMA REDISTRIBUTION IN THE PRESENCE OF DIFFERENT-SIZED SUPERNUMERARY SEGMENTS IN A GRASSHOPPER - DEPENDENCE ON NONHOMOLOGOUS SYNAPSIS

Eight different sized supernumerary segments located at distal ends of the long arms of chromosomes M4, M5, M6, and S8 of the grasshopper St enobothrus festivus were studied in males with regard to the synaptic process and chiasma distribution in the bivalents that carry them. The M4, M5, and M6 bivalents heterozygous for extra segments were always monochiasmate, in contrast to their bichiasmate condition observed in basic homozygotes. Furthermore, the presence of any of these extra seg ments led to chiasma redistribution in the carrier bivalents, so that such chiasmata were formed preferentially further away from the extra segment. The intensity of this effect is dependent on the size of the segment. Not all heteromorphic bivalents exhibited synaptonemal comple xes with equalized axes at pachytene, but there was always a variable proportion of heterosynapsis around the distal ends of the long arms t hat was dependent on both the size of the segment and the size of the carrier chromosome. It is proposed that the absence of chiasmata in no nhomologous synapsed regions is responsible for the results obtained. Length measurements of the different extra segments and their carrier chromosomes between pachytene and diplotene indicated that synaptonema l complex is underrepresented in supernumerary heterochromatin.