Polymorphic fundatrices in thimbleberry aphid - Ecology and maintenance

The thimbleberry aphid, Masonaphis maxims (Mason) lives on patches of plant s that support 3, 4 or 5 generations depending on site and weather. The lif e cycle requires sexual females and males to produce overwintering eggs. Th e eggs hatch in the spring to produce the first 'fundatrix' generation; sub sequent generations are produced parthenogenically. Males and other morphs are produced by wingless virginoparae, but sexual females are produced by ' gynoparae', a winged morph that is specialized to produce only sexual femal es. The fundatrices have no indication of the number of generations that th e plants will support in the current year. There are two fundatrix types th at coexist in different ratios depending on the number of generations suppo rted by the patch the previous year. One type produces sexual females in ge nerations 3 and 5, and males in generations 4 and 5; the other type produce s sexual females in generations 4 and 5, and males in generations 3, 4 and 5. The dimorphism adapts the aphid to its heterogeneous and somewhat unpred ictable environment. The role of sex in the maintenance of the dimorphism i s discussed. This is the first report of fundatrix polymorphism and consequ ent differential sex expression in aphids.