Biogeographical and paleobotanical evidence suggests that the aphid subtribe Melaphidina has been associated with its sumac host plant since the earl… - Nancy A. Moran

Heteroecy appears to be a consequence of the seasonal polymorphism of aphids, which causes some morphs (the fundatrices) to be more evolutionarily constrained than others (the summer females) in their abilities to acquire hosts. Some aphid lineages have escaped this constraint by replacing the ancestral fundatrix morph and remaining all year on former secondary hosts, thus becoming secondarily autoecious. Most of the large and species-rich groups of Aphidinae on herbaceous angiosperms probably are derived from ancestors showing such life-cycle reduction.

Among the many early revelations from molecular phylogenetic studies of bacteria (Woese, 1987) was the recognition that the mycoplasmas represented an evolutionarily derived condition rather than a primitive one, as once believed. Now that phylogenetic relationships and genome sizes are determined for a broader array of organisms, it is clear that the mycoplasmas are just one example of genome shrinkage that has occurred in a variety of obligately host-associated bacteria. Other prominent examples are Rickettsia and related pathogens within the α-proteobacteria; insect symbionts within the γ-proteobacteria, as exemplified by Buchnera aphidicola in aphids; the chlamydiae; and the parasitic spirochetes, such as Borrelia burgdorferi (the agent of Lyme disease).

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.