Genetic evidence of hybridization between 13- and 17-year cicadas is reported in a study published in Communications Biology this week. These findings are unexpected as 13- and 17-year cicadas rarely meet, despite being closely related.
Periodical cicadas spend the majority of their lives underground as larvae feeding on the juices of tree roots. After either 13 or 17 years underground, cicadas from the same brood emerge synchronously and molt into adults. They then spend the rest of their lives - less than one month - focused on reproducing. The seven species of periodical cicadas are split into three groups, with each group consisting of one 17-year species and either one or two 13-year species. Any given 13-17-year paironly emerge at the same time once every 221 years, with few exceptions, and their geographical distributions do not significantly overlap.
Teiji Sota and colleagues sequenced RNA from the seven periodical cicada species to gain a deeper understanding of their relationships to each other. Unexpectedly, the authors found evidence that within each of the groups, hybridization, or intermixing, had occurred between the 17-year species and its closest 13-year relative. Despite this intermixing, the cicadas have maintained their distinct life cycles for up to 200,000 years. The authors were unable, however, to pinpoint any genetic explanations for the parallel divergence. Future sequencing of the full genomes of these species is needed to further understand how these different life cycles are maintained over evolutionary time.