Life‐cycle control of 13‐ and 17‐year periodical cicadas: A hypothesis and its implication in the evolutionary process

Abstract

AbstractPeriodical cicadas of the genus Magicicada exhibit a spectacular life‐cycle phenomenon, with periodic mass emergence being observed every 13 or 17 years in the eastern United States. It is entirely unclear how their periodical life cycles are controlled. Here, I review previous knowledge and hypotheses about Magicicada life cycles and propose an integrated hypothesis of the life‐cycle control mechanism, which includes critical body weight for adult metamorphosis, an internal 4‐year clock, associated 4‐year gate in 4 × n years of age for the timing of the metamorphosis decision, and genetic differences in growth rates between 13‐ and 17‐year cicada nymphs (the former is faster). If the last (fifth) instar nymphs reach critical body weight in any of the 4‐year gates (8, 12, 16, and 20 years of age, but not other years), they prepare for adult eclosion in the following year. The proposed hypothesis can explain the synchronized emergence of cicadas with varying growth trajectories in scheduled years (13 or 17 years of age) and the off‐schedule emergence of some cicadas in 4 years earlier or later than the scheduled year, owing to their phenotypic plasticity that arises in response to large variations in the realized growth rate. The proposed hypothesis can also explain how evolutionary life‐cycle shifts between 13‐ and 17‐year cycles occur through phenotypic plasticity and selection on growth rate during climatic changes. It also facilitates understanding of the evolutionary mechanism of Maigicicada, which contains three species groups that are parallelly diverged into 13‐ and 17‐year life cycles.