Chromosome polymorphisms and natural selection in Leptysma argentina (Orthoptera). IV. Survival selection acts on karyotype polymorphisms at the adult stage and before

Deviation from formal null models and the comparisons between age classes a re amongst the most direct methods of demonstrating ongoing natural selecti on. We have sampled 207 males from a population of the grasshopper Leptysma argentina at the onset and the end of the adult life span for this species ; generations are synchronical and do not overlap. The karyotypic frequenci es for a fusion (F) and a supernumerary segment (s(1)) were examined. The y oung male sample was heterogeneous and did not fit to the Hardy-Weinberg pr oportions, whereas the aged male sample did not deviate from H-W expectatio ns in both analyses. Likewise, the young male sample showed fitness heteros is for the s(1) carriers that are fusion homozygotes. We interpreted the si gnificant results of karyotype frequency in both samples as the result of s urvival selection. Evidently natural selection acts on male adult viability as well as on other fitness components prior to adult survival, thus chang ing karyotype frequencies between generations. Likewise, we also detected a strong interaction between polymorphisms, whereas heterosis for sl heteroz ygotes might be contributing to the maintenance of the s, polymorphism. The actual target of selection remains unidentified.