Abstract
Periodical cicadas (Magicicada spp.) in the USA are famous for their unique prime-numbered life cycles of 13 and 17 years and their nearly perfectly synchronized mass emergences. Because almost all known species of cicada are non-periodical, periodicity is assumed to be a derived state. A leading hypothesis for the evolution of periodicity in Magicicada implicates the decline in average temperature during glacial periods. During the evolution of periodicity, the determinant of maturation in ancestral cicadas is hypothesized to have switched from size dependence to time (period) dependence. The selection for the prime-numbered cycles should have taken place only after the fixation of periodicity. Here, we build an individual-based model of cicadas under conditions of climatic cooling to explore the fixation of periodicity. In our model, under cold environments, extremely long juvenile stages lead to extremely low adult densities, limiting mating opportunities and favouring the evolution of synchronized emergence. Our results indicate that these changes, which were triggered by glacial cooling, could have led to the fixation of periodicity in the non-periodical ancestors.
Highlights
Cicadas face predation pressure, but only a small number of species are known to be periodical, e.g., one in India[16,17] and another in Fiji[18]
Our model suggests that size-dependent cicadas could be replaced by periodical cicadas with life cycles of various year lengths
In a cooler climate (ART = 0 ), temporal dynamics indicate that when the 10-year periodicity allele mutation is introduced at t = 1,000 under the adult extinction threshold Nc = 1 00, the fixation of periodicity occurs at t = 2,000 (Fig. 2a)
Summary
The simulation results show that fixation of the periodicity allele is frequently observed some time after the introduction of the mutation (Fig. 2). In a cooler climate (ART = 0 ), temporal dynamics indicate that when the 10-year periodicity allele mutation is introduced at t = 1 ,000 under the adult extinction threshold Nc = 1 00, the fixation of periodicity occurs at t = 2 ,000 (Fig. 2a). If the climate is warmer than this threshold, cicadas survive in almost all simulation runs. A warm environment has a high k (= 300) In this case, extinction occurred under conditions of much higher ART, resulting in the extinction of longer cycles (16–20 years). We found almost no qualitative changes in the outcomes (Fig. S7)
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