EXOPHENOTYPIC EFFECTS OF CHROMOSOMAL CHANGE : THE CASE OF LEPTYSMA ARGENTINA (ORTHOPTERA)

Most of the literature about phenotypic effects of chromosomal change has dealt with its effects on chiasma frequency and/or distribution, whereas whether or not they affect the exophenotype has received less attention and has remained controversial. In the present paper morphometrical variables of the species Leptysma argentina were analysed in connection with chromosome polymorphisms for a centric fusion and a supernumerary segment. The centric fusion between pairs 3 and 6 (fusion 3/6) and a supernumerary segment in the smallest pair of the complement (S1) were consistently proved to have effects on chiasma frequency and position; preliminary analyses had shown, however, that fusion 3/6 could be correlated with some effects on body size (Colombo, 1989). In the present paper the analysis of femur, tibia, prothorax and total length as well as prothorax height on larger samples were consistent in showing that both polymorphisms are positively correlated with increasing total length; in the case of the supernumerary segment, however, this effect is not constant over all the populations sampled. An adaptive change in body size may be achieved through selection on: (i) genetic factors that affect this trait; or (ii) chromosome polymorphisms with exophenotypic effects. This latter case would be more effective, because genetic factors are usually polygenic and hence their response to selection would be slow. It is tempting to say that this could be the case for L. argentina, because in grasshoppers increased body size can be related to tolerance to cold or dry environments.