Certain patterns in aphid life cycles and host relationships suggest hypotheses to be tested in a phylogenetic framework. One major hypothesis proposes an evolutionary trend in aphid-plant relationships from conifers to woody dicotyledon angiosperms, to herbaceous dicotyledons and monocotyledons, and (rarely) secondarily to conifers. The major direction in life cycle evolution is thought to be from simple cycles on woody hosts, to host-alternation, to simple cycles on herbaceous hosts (often accompanied by lineage diversification). The evolution of host-alternation, although usually considered rare, may have been a transitional step in the major host shifts of some lineages. A recent molecular phylogenetic study of Aphidoidea and aphid fossil evidence supported the idea that several specialized aphid lineages in the Cretaceous were probably living on woody conifers. The ancestral Aphididae lineage apparently underwent a rapid radiation in the late Cretaceous/early Tertiary that gave rise to most modern tribes. We have asked, how many extant aphid lineages associated with conifers have retained this condition from the common aphid ancestor, and how many origins of host alternation occurred during the Cretaceous radiation. We have tested the hypothesis that conifer-feeding Prociphilina form an ancient lineage and played a key role in transitions between gymnosperms and angiosperms at a deep level in aphid phylogeny. Results suggest that the association with conifers is ancestral and further suggests a different mechanism of evolution, and possibly an independent acquisition, of host alternation. Preliminary studies of the phylogeny of Aphidinae also suggest several origins of host alternation in that subfamily.
Keywords: aphids
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