Mutualist-induced transgenerational polyphenisms in cotton aphid populations

1. In defensive mutualistic associations, reduced risk of predation should permit defended organisms to produce phenotypes with higher offspring production than non-mutualistic, unprotected conspecifics which require costly defensive traits. 2. Here, we show that cotton aphids, Aphis gossypii, which produce any combination of dwarf apterae (low intrinsic rate of increase), light green apterae (medium intrinsic rate of increase), dark green apterae (high intrinsic rate of increase) and alatae (winged dispersal morphs), alter offspring phenotypes when tended by predatory ants. 3. Aphids tended by the Argentine ant, Linepithema humile, have similar numbers of dwarf, dark green and alate offspring, but greater numbers of light green offspring, compared to untended colonies. 4. Because light green morphs have a higher intrinsic rate of increase than dwarf morphs but a decreased risk of parasitism compared to dark green morphs, increased production of the light green phenotype may optimize offspring production in order to maximize clone fitness. 5. Since many organisms have high levels of plasticity and mutualistic interactions are ubiquitous, mutualist-induced polyphenisms may be pervasive.