The grasshopper Dichroplus elongatus Giglio Toss is an example of a species with widespread B-chromosome polymorphism. We detected in males considerable variation in adult size, and part of this variation is associated with the polymorphism for B-chromosomes that has a negative effect on male body size. The analysis of fitness components revealed that B-carriers are negatively selected during mating, whereas B-carrier females exhibited higher fecundity than standard females. We recorded adult survival in relation to karyotype and body size, as measured by total, third tibia, third femur, thorax, and tegmen lengths. Our data showed directional phenotypic selection favoring large males and, as expected from the relationship between karyotype and phenotype, chromosome directional selection was detected favoring males with standard karyotype. In females, the effects of phenotype on survival showed an opposite tendency to that detected in males, i.e., small females outlive large females. Phenotypic selection in females may be explained by a trade-off between early and late fitness components. Our results suggest that B-chromosomes cause detrimental effects on body-size related traits that are selected for adult male survival. All available information on B-chromosomes in D. elongatus supports the hypothesis that their persistence in Argentinean populations depends on trade-offs between opposite selective effects.
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Vol. 101 • No. 5