EFFECTS OF SUPERNUMERARY HETEROCHROMATIN ON CHIASMA FORMATION AND CHROMOSOME SEGREGATION IN DOCIOSTAURUS-GENEI (ORTHOPTERA)

Dociostaurus genei is a grasshopper in which most of the autosomes pre sent polymorphisms for distal heterochromatic supernumerary segments. C-banding, DAPI banding and fluorescence in situ hybridization (FISH) of a probe, which is representative of the highly repetitive DNA seque nces comprising the segments, have been used to analyse the effects of the segments on chiasma formation. Results show that the supernumerar y segments are associated with two major effects. First, a higher cont ent of supernumerary heterochromatin is correlated with the formation of a lower number of chiasmata within a given chromosome complement. S econdly, because the methods used strongly suggest that the distal seg ments are truly terminal, they could be considered to be involved in t he maintenance of achiasmate associations that persist up to the first anaphase migration. These terminal associations occurred in about 50 per cent of homozygous and heterozygous bivalents and did not impair t he correct segregation of homologues at anaphase I.