Anomalous distribution of nuclear and mitochondrial DNA markers in periodical cicadas

Abstract

Ecological genetics theory identifies the age at first reproduction and life-cycle length as important components of life-history strategies, and predicts that variation in these traits can have significant evolutionary consequences1. Changes in the timing of reproduction can affect the fitness of an organism2, disrupt gene flow3 and lead to speciation4. The existence of two life-cycle lengths differing by four years in periodical cicadas of the genus Magicicada provides an excellent opportunity to investigate both the control and evolutionary consequences of changes in the timing of reproduction. In Magicicada, life-cycle length has been assumed to be a genetically fixed species-specific trait5. Alternatively, however, periodical cicadas could switch between the two life-cycle lengths in response to either genetic6 or environmental7 factors possibly providing a mechanism for the generation of the extant year classes of cicadas8, called broods9. Here we describe a survey of populations of 13- and 17-year periodical cicadas which suggests that 13-year cicadas in a large region of central North America are descendents of 17-year cicadas that switched their life-cycle length to 13 years, providing the first strong genetic evidence that life-cycle length is a plastic trait. The change in maturation time and the resultant anomalous distribution of cytoplasmic and nuclear gene markers implies that these cicadas became temporally isolated from the parent brood, joined an already existing brood, and thus brought together two independently evolving gene pools.