| Abstract: | The aphid Pemphigus betae typically shows a complex life cycle, with annual alternation between cottonwood trees, where it forms leaf galls, and herbaceous plants, where it lives on roots. Distinct phenotypes are associated with each phase. In a population in Utah, aphid clones vary in their tendencies to undergo the cottonwood phase of the life cycle, with certain clones rarely producing the winged migrants that initiate the cottonwood phase. To examine the extent of genotypic variability in life cycle traits and to determine the consequences of phase deletion for fitness in the remaining phase of the life cycle, I compared performances of sets of clones that differed in their tendencies to delete the cottonwood phase. Performances of 35 aphid clones were compared on root hosts in the laboratory, by quantifying size (a correlate of fecundity), developmental time, and mortality for clonal sublines. Clones that tend to delete the cottonwood phase and remain on roots developed faster, showed lower mortality, and were larger as compared to clones that tend to undergo an annual migration to cottonwood. These laboratory comparisons of individual clones agree with earlier findings from field experiments. Performance also varied significantly among clones within sets, with the set of clones not previously subjected to selection on root hosts showing higher variance than clones isolated following selection in the root-inhabiting phase. These data suggest that selection in the cottonwood phase opposes selection in the root phase. Thus, even in cases of polyphenism, in which divergent developmental pathways confer some independence of phenotypic expression in alternative phases, antagonistic pleiotropy limits adaptation within a phase. These findings on the correlates of intrapopulational variation in frequency of phase expression provide some of the best evidence for character release following reduction of a complex life cycle, and they support the hypothesis that the advantages of polyphenism may be limited by negative genetic correlations among fitness traits of alternative phenotypes. |
