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Periodical cicadas – insects that count

If you live in the eastern United States, chances are your ears have come in contact with some really loud, recklessly theatrical insects that always make their presence known. These raucous insects are periodical cicadas, and they have a strict party schedule -- they develop underground for thirteen or seventeen years and then emerge in the spring for large parties composed of billions of individuals. The intensity of the bedlam is due to the males exercising their powerful tymbals (drum-like membranes that vibrate to produce sound) to best their competitors in hopes of attracting a mate.

Magicicada septendecula. Photo by Chris Simon

While the cacophony may sound like a single unified group to the untrained ear, this tymbalic chorus is often composed of multiple species with distinct songs designed to only attract females of the same species. For example, the most recent large emergence (Brood X -- pronounced Brood Ten) was composed of three species, all in the genus Magicicada: M. septendecim, M. cassinii, and M. septendecula. The naturalist who named them truly believed they were “magic cicadas”!

These insects use accumulated soil temperature to determine the day of the party so that they do not arrive unfashionably early and become the dinner rather than the guests. The trick is to arrive not too early but not too late. Soil temperature determines the day of emergence but not the year. It’s important for researchers to understand how Magicicada keep track of years and how climate change may impact the timing of emergence because the dramatic pulses of insect biomass have profound effects on many members of the ecosystem!

One thing that makes periodical cicadas even more interesting is that the evolutionary history of these boisterous insects remains a mystery. Scientists are really not sure when, where, why, or how these large synchronous emergences started, how the original population was broken up into year-classes (broods) that fit together like a jig-saw puzzle, or why substantial proportions of individual populations sometimes come out in large numbers four-years early or four-years late.

A group of Magicicada (Tymbalic symphony not included), photo courtesy of Chris Simon

We hypothesize that the answer may be found through comparative studies of their genomes or external modifications to their genomes. Here we share the first chromosome-level genome assembly for one of the Brood X species, Magicicada septendecula. The first draft genome was assembled with work from Jonas Bush, Paul Frandsen, Chris Simon, and Ed Wilcox at BYU, using flash-frozen cicadas and four PacBio SMRT II cells. We improved it here using Hi-C data from a single M. septendecula adult. Annotation was carried out by Jill Wegrzyn and Cynthia Webster. We are grateful for funding from the BYU College of Life Sciences Undergraduate Research Award, which made this project possible, as well as several research awards from the National Science Foundation to Frandsen, Simon, and Wegrzyn. A genome report has been submitted for publication.

Check out the interactive contact map of the M. septendecula chromosomes below, and visit the genome assembly page for more details!

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