Variation and evolution of the complex life cycle in Adelgidae (Hemiptera)

The family Adelgidae is a small group of insects within Aphidoidea (Hemiptera). Adelgids are typically holocyclic with host‐alternation between the primary and secondary hosts, but some anholocyclic species persist either on the primary or secondary host. Like Aphididae, complexities and variation of adelgid life cycles are good models for understanding the evolution of complex life cycles. In this review, we outline the complex life cycles of adelgids, and current status and recent advances in adelgid life cycle studies. We also discuss the evolution of adelgid life cycles by comparing them to closely related aphid life cycles. A switch from holocycly to anholocycly on the primary host needs evolutionary innovations in gallicola behavior and reproduction. This radical evolution can be explained by mutations in a regulatory system that controls the sequence of gene sets producing phenotypes of one morph. In contrast, anholocycly on the secondary host consists of a series of exulis generations already existing in the holocycle. Thus, it may evolve by loss of primary‐host generations through extinction of the primary host, expansion beyond the geographical range of the primary host, or loss of male‐producing sexuparae that return to the primary host. Although the holocycle and its anholocyclic derivatives have been regarded as different species, morphological, ecological and genetic differences are too subtle to separate them into different species. The holocycle and its anholocyclic derivatives should not be split into different species without clearly identifiable morphological differences.