Chromosome polymorphisms and natural selection in Leptysma argentina (Orthoptera). II. Gametic phase disequilibrium and differential adult male viability
1993
Colombo, P.C.
Despite the wealth of information concerning inversions in natural selection (mainly in Drosophila), cases of centric fusions or supernumerary segments involved in selective processes have been less well documented. In the present study 149 males from a population of the South American grasshopper Leptysma argentina (whose generations are annual and discrete) polymorphic for a centric fusion (F) and a supernumerary segment (s1) were sampled in October and December 1991 (the beginning and the end of the adult life span in this species) with the purpose of: (i) detecting departures from formal null models, such as Hardy-Weinberg and gametic phase equilibria; and (ii) detecting differential adult male viability among karyotypes. Both methods allow the detection of natural selection occurring in the wild. The comparison between both age classes revealed significant differences in karyotypic frequencies, which gives clear proof of ongoing selection. Gametic phase disequilibrium was not detected in young males but it was significant in aged males: favoured gametes are consistent with favoured karyotypes. Furthermore, while the young male sample does not depart significantly from Hardy-Weinberg equilibrium nor the hypothesis of homogeneity, both tests were significant in the aged male sample. Results signify that strong selective forces act in the adult stage, at least in males, while other components of natural selection remain obscure. The possible causes of selection acting on both polymorphisms are analysed and discussed.
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