MEIOTIC STUDIES IN LARGUS-RUFIPENNIS (CASTELNAU) (LARGIDAE, HETEROPTERA) - FREQUENCY AND BEHAVIOR OF RING BIVALENTS, UNIVALENTS AND B-CHROMOSOMES

Cytogenetic analysis of an Argentine population of Largus rufipennis h as revealed the presence of autosomal univalents and a supernumerary c hromosome. All individuals present univalents at a variable frequency (0.31-21.83 per cent), and half of them carry a B chromosome at low fr equency. Both the B chromosome and univalents divide equationally at a naphase I and reductionally at the second division. The present popula tion is very heterogeneous with respect to chiasma frequency and distr ibution: desynapsis is responsible for the appearance of univalents, w hile the presence of the B chromosome is associated with an increase i n chiasma frequency. However, two individuals that lack the supernumer ary chromosome show a strikingly high chiasma frequency. This variatio n in chiasma frequency should be the result of the interaction between genes controlling chiasmata and the environmental conditions (both in ternal and external) together with the genetic background of each indi vidual.