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
To me, the fascinating thing is why they stay in the ground for a prime number of years (13 or 17).
I am guessing 4N + 1
If they stay in the ground a non-prime number of years (say, 12), any predator that hibernates for that number of years or any number that is a prime factor will wake up when the cicadas do. Not good. Better to bump into the ones for which your hibernation time and theirs share no factors.
It is a very nice way of calculating prime numbers!
from Cicada 17 – Numberphile
http://www.youtube.com/watch?v=j7jfHM-mMC4
Bob,
That is fascinating indeed.
I have read somewehere that coming out in prime number intervals throws off the maximal amount of specialized predators which might try to adapt to a rhythm because you have to adapt to exactly the same prime or you wont get food for centuries. But this is very vague
I think the minimal attempt to answer is in the existence of various broods with the same period as described above. They won’t overlap with a shorter period, making it less likely for similarly periodical predators to adapt.
As I heard it this was supposed to be pretty much untestable terrain, for the imagined periodic predators are AFAIU unseen, in the model extinct as a result of cicadas stretching their period to ludicrous lengths. The existence of continued speciation that changes period should make for testability, why else would the new species still latch on to a prime period?
So I suspect the answer is more complicated. Your somewhat more ecological answer (a possible niche, not existing species) sounds more “morally right” (to a layman).
Of course, a “save” of the minimal answer may be a ‘relic’ regulatory mechanism for the period that somehow puts out prime numbers no matter what. Species that evolved other mechanisms happens to be extinct (or it was a fortuitous coincidence).
So maybe one need to know the mechanism before one puts that answer to rest.
I agree, but it is highly amusing to speculate. The “speciation” 17->13 or 13->17 is indeed surprising if no extant periodic predators are present that put a strong penalty on divisors. So yes, knowing the regulating mechanism would maybe be interesting. Somehow I cannot imagine that some kind of chemical biological clock can crank out prime numbers. Neurons on the other hand… Do these guys have a neural digital counter? I bet they don’t, because that would be too cool to be true.
Wondering how the last glaciation would have contributed to this, pushing the distribution away from the main part of the Appalachians…? I will have to read the paper!
I heard only about these periodical cicadas in America. Does something analogue exist in Europe?
Wiki only mentions periodical cicadas as an American species.
Found this: ” Periodical cicadas, ones that appear every 13 or 17 years, are found only in the United States east of the Great Plains.”
(http://www.ask.com/question/where-are-cicadas-found)
…the emergence of Brood II along the east coast attracting attention in the media
Yeah, the New York media, who made it sound like they were coming everywhere. These outfits usually have an MD at least on retainer who can come in and give a commentary on medical issues. They need to do the same with a competent biologist.
Reblogged this on DownHouseSoftware and commented:
A great discussion of cicada broods now attracting attention along the Eastern seaboard of the United States. Of note is how cicada broods are not synonymous with cicada species. In fact each brood consists of a number of reproductively isolated species.
Very interesting. So how do the annual cicadas relate, genetically? Does anyone know?
And are there ever stragglers going 18 or 19 years, or early birds of 15 or 16 years?
Annual cicadas are related, but are further “down the tree” from periodical cicadas. And, yes, there are stragglers; see Chris Simon’s website, especially this page.
GCM
Thanks!
This is fascinating. Down here in the Deep South, we have deafening cicadas every summer, so I’ve never paid much attention to learning about the various species and their cycles. From the tree figure, it looks like we have 13-year cicadas from each of the three species (the blue dot on the map is almost exactly on top of my city), comprising brood 2, but I reckon I should read the paper to find out more. Neat!
Brood 22, that is.
There are also several species on one-year and three-year cycles that can be heard every year, especially in the South and Mid-Atlantic regions; these are usually larger than the 13-year cicadas and have green and/or brown coloring, instead of black and orange. In Virginia the big green and black ones are sometimes called “jarflies”. My grandmother used to say that one of the three-year species was regarded as a bad omen during WWII because of the dark “W” mark (for “war”) on the wings.
Here in the Sonoran Desert we have several small black mesquite-loving species on three-year cycles.
Oh yeah – all the ones I’ve seen are huge and green. They’re pretty awesome.
I remember from a graduate ecology course that during the year a cicada brrod emerges, the predator populations do well—with birds, cats, rodents, etc., taking advantage of the emerging cicadas to increase their populations to peak levels. This is followed by a decline over the next few years in predator levels in the absence of the cicada broods. (I certainly remember seeing this in bird populations during and after the last brood.) By emerging every 13 or 17 years, the broods make it likely that their predators are back to near-baseline levels at next emergence, even if other prey with shorter cycles have become available during the interim.
sub
I just got this on the internet!
It is just being reported that cicada DNA, in a homeopathic solution, is in all christian baptismal waters since 1920. This investigative reporting is a response to several christians seen digging elaborate tunnels extending deep underground. The following interviews are just coming in.
REPORTER: If cicada homeopathy was indeed applied to all baptismal waters since 1920 why is it that just recently christians have begun burrowing?
CHRISTIAN SCIENTIST: It appears, as all christians know, that homeopathic medicine doesn’t always work immediately. Plus, in the case of cicada homeopathy, there is the added time delay needed to synchronize a brood.
REPORTER: Will all christians be confining themselves to burrows? Will you be sealing yourself in? How many broods will there be?
CHRISTIAN SCIENTIST: If they have Faith they will. Only True and Real christians will receive the calling. There will be this year’s brood of the Chosen and maybe a following brood next year. I wasn’t sure at first but now I can feel the need to tunnel, in fact, I need to go right now.
REPORTER: Looks like this christian has completed Its burrow and is about ready to enter. There is a cement slab propped up over the burrow. Are you going in now? Have you stored enough food for sustenance through your grub stag? What’s with the cement slab?
REAL CHRISTIAN: Yes, everything is ready, my burrow is surprisingly warm and snugly, I spent last night there just to check it out. I won’t be bringing extra food, the Lord will provide. I’ve sampled the roots, at first I didn’t like them but now they are the only thing I want to eat. I’m sure the Lord will provide any supplemental food in any form that is needed. The cement slab is propped up by a plastic rod specially made to quickly deteriorate in the solar UV. When the slab drops the entrance will be sealed off preventing kittens, hedgehogs, bunnies, and atheists from getting in. You know, when you think about it, this was all foretold in His book, in the end times the True and Real christians will be lifted up into their burrows. Look, I’d like to stay and chat but there is an overwhelming urge to get into my burrow, see ya in 13 or 17 years. Just think, the cicada was a test case all along so the Lord would be sure we could safely spend a wonderful and short few years in our burrows. As we are the Chosen Ones, we need to go through our grub stage while we are alive so we can develop wings that will carry us to heaven. I bray that you too will receive your tunneling urge soon.
To what precision has the reproductive isolation of these species been established? The insanely large number of pairings in any particular mating swarm would seem to imply that even at a very low rate of successful crossing there might be a mechanism to introduce correlations among them and their fellow broodmates.