SUPERNUMERARY CHROMOSOME SEGMENTS AND INTRABIVALENT CHIASMA REDISTRIBUTION IN PYRGOMORPHA-CONICA (ORTHOPTERA)

Two kinds of supernumerary chromosome segments have been found on the megameric chromosome of the grasshopper Pyrgomorpha conica. One of the m is euchromatic and distally located while the other one is heterochr omatic and proximally located. Their effects on chiasma distribution i n heterozygous, homozygous and doubly heterozygous bivalents for these segments have been analyzed. Both segments distort chiasma distributi on so that the single chiasma is preferentially formed far from the lo cation of each supernumerary segment. This effect is more pronounced i n bivalents that are homozygous for the presence of the proximal heter ochromatic segment. In doubly heterozygous bivalents that possess both segments on the same chromosome the chiasma distribution is significa ntly different from that observed in heterozygous bivalents for each s egment and a significant frequency of univalents was scored.