by Greg Mayer

This was a big year for periodical cicadas, with the emergence of Brood II along the east coast attracting attention in the media, including here at WEIT, and other websites (which I also commented at).  Despite this, I overlooked an important paper on them published by Teiji Sota and colleagues back in April. At the Evolution meetings one of the coauthors, my old colleague and teacher Chris Simon, brought it to my attention.

Periodical cicadas (Magicicada) are fascinating animals. They live underground for 17 years, then emerge in huge numbers, mate, lay eggs, and die, all in a few weeks. The huge numbers overwhelm their predators’ ability to eat them. Each year class is called a “brood”, so Brood II previously emerged in 1996, 1979, and so on; Brood III will emerge next year (in Iowa, Missouri, and Illinois). One species of cicada doing this would be pretty marvelous, but it turns out there are also cicadas that do it every 13 years– so there are both 17- and 13-year cicadas. Thirteen-year broods are numbered XVII to XXXI. To make your mind explode, it turns out that each brood consists of three, separate, reproductively isolated species– known as Decim, Decula, and Cassini. (Go to Chris Simon’s Magiciacada Central for all things cicada.)

Although there are 30 possible broods, only 15 currently exist, with two more historically known broods now extinct. A few broods lack one or two species, but most have all three, so there are close to 45 brood-species combinations. What Sota and colleagues have done is performed a molecular phylogenetic analysis, using mitochondrial and nuclear genes, of all extant brood-species combinations, including multiple geographic samples. They find three real interesting things.

First, the pattern of geographic variation is similar in all 3 species groups (Decim, Decula, and Cassini). Each one has major eastern, middle, and western genetic groups, with Decim also having a southern group.

Second, the 13 year cicadas of each species group are not monophyletic, but rather seem to have evolved repeatedly from the 17 year form, remarkably, in all three species!

Third, these geographic patterns were not established simultaneously, but rather the patterns were converged on, at least in part because synchronization of periodicity in a given area is favored ecologically. I think this third one the weakest of the conclusions, because the dating of the divergences doesn’t rest on well-established assumptions, and resulted in large standard errors relative to the divergence times anyway. But, regardless of this last point, it’s a great advance in our understanding of cicada evolution. Sota et al. quite reasonably conclude that the evolution of time-shifting rests on genetic abilities present in the common ancestor of the entire group in the Pliocene.

In a commentary, Stewart Berlocher has produced a helpful figure simplifying the results, but you really must look at the detailed figures in Sota et al. to appreciate the work fully.

And I need to add one more complication. In the western part of Magicicada‘s distribution, the 13 year broods of Decim are different from the other 13 year Decims, but are extremely similar to the local 17-year Decims, and have been named as a seventh species by Marshall and Cooley (2000). Sota’s data supports this, and strongly suggest that this fourth 13 year species has arisen relatively recently by a 4 year shift in the life cycle from the 17 year Decims, as described by Simon et al. (2000).

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Berlocher, S.H. 2013. Regularities and irregularities in periodical cicada evolution. Proceedings of the National Academy of Science USA 110:6620-6621. extract

Marshall, D. C., and J. R. Cooley.  2000.  Reproductive character displacement and speciation in periodical cicadas, with description of a new species, 13-year Magicicada neotredecim. Evolution 54: 1313-1325. pdf

Simon, C.M., J. Tang, S. Dalwadi, G. Staley, J. Deniega, and T.R. Unnasch.  2000. Genetic evidence for assortative mating between  13-year cicadas and sympatric “17-year cicadas with 13-year life cycles” provides support for allochronic speciation. Evolution 54:1326-1336 . pdf

Sota, T. S. Yamamoto, J.R. Cooley, K.B.R. Hills, C. Simon and J. Yoshimura. 2013. Independent divergence of 13- and 17-year life cycles among three periodical cicada lineages. Proceedings of the National Academy of Science USA 110:6620-6621. pdf