Abstract
SUMMARYThe grasshopper Cylindrotettix obscuras (Leptysminae: Acrididae) is polymorphic for a mitotically stable supernumerary isochromosome (Iso B) and a small, mitotically unstable B-chromosome (BT); both B-chromosomes significantly increase abnormal sperm formation, as determined by CONFALONIERI and BIDAU (1986). In the present work 87 male individuals of C. obscurus from Misiones, Argentina, were analysed for total (T), proximal (P), interstitial (I) and distal (D) chiasma frequency with the aim of detecting effects of those B-chromosomes on recombination in this species. It was evident that the Iso B significantly raise T and P chiasma frequency; this effect was more evident in the carriers of two Iso B's. T and P chiasma frequencies descend in follicles with 1 BT with respect to 0 BT ones, and then increase gradually with 2, 3, 4 and 6 BT's; the regression of T and P on BT number was significant. However, the simultaneous presence of both B-chromosomes produces a marked decrease of T and P frequencies, yielding in some cases a high univalent frequency. The effects of both B-chromosomes are discussed in relation with other similar cases in closely related species. It is suggested that the rise of chiasma frequency in both cases would be due to reduced interference; on the contrary, the interaction effect would be due to asynapsis or desynapsis rather than to increased interference.
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