EFFECTS OF CENTRIC FUSIONS ON CHIASMA FREQUENCY AND POSITION IN LEPTYSMA-ARGENTINA (ACRIDIDAE, ORTHOPTERA) .2. INTRACHROMOSOME AND INTERCHROMOSOME EFFECTS

Leptysma argentina bears both a polymorphic centric fusion between telocentrics 3 and 6 (fusion 3/6) and a fixed one that produced the large metacentric 1 (fusion 1). A complex of phenomena, all of them leading to a general decrease in total recombination, are associated with fusion 3/6, namely: a) reduction of two independent linkage groups into one; b) intrachromosome adjustement of chiasma frequency to a minimal level, together with a strict distal localisation, and c) interchromosome effects, such as i) decrease in total mean cell chiasma frequency; ii) decrease in interstitial and proximal chiasma frequency, and iii) decrease in between-cell variance. On the other hand, chiasma distribution in the metacentric 1 seems to be irrestricted, et least when compared to the rigid localisation in the 3/6 metacentric of fusion 3/6 carriers. Therefore, it is suggested that the intrachromosome effects of fusion 3/6 on chiasma distribution are a consequence of adaptation to the polymorphic condition, although the possibility of a direct effect of the rearrangement cannot be excluded. Conversely, fusion 1 would not have passed through a long-lasting polymorphic situation (or else, it woud have suffered a second chiasma readjustament after definitive fixation). Finally, a model of chiasma formation in ‘Leptysma argentina’ is proposed, and the possibility is suggested that the interchromosome effects of fusion 3/6 are a result of increased chiasma interference. © 1990 Taylor & Francis Group, LLC.