Evolutionary reduction of complex life cycles: loss of host- alternation in Pemphigus (Homoptera: Aphididae)

In a Utah canyon, Pemphigus betae, exhibits 2 life cycles: a cycle involving host-alternation between cottonwood Populus trees and roots of herbaceous plants and a secondarily reduced cycle, in which the cottonwood generations are eliminated so that wingless forms live year round on roots. Relative frequencies of the life-cycle types vary along a 30km stretch of the canyon, with the reduced cycle predominating at upper sites. Variation in frequency of life-cycle reduction is dependent on adaptive phenotypic plasticity, microgeographically variable cues affecting mechanisms of morph determination, and genetically based variation in tendency to show reduction versus alternation. Genetic variation between sites corresponds to microgeographic variation in success of life-cycle phases. Where cottonwood hosts are absent (lower elevations) or where the cottonwood phase has low survival (upper elevations), clones tend to produce fewer migrating morphs, compared to clones from middle elevations, where the cottonwood phase is relatively favorable. -from Authors