Gene Flow in Periodical Cicadas
What
are Periodical Cicadas?
One
very interesting North American insect is the periodical cicada (genus
Magicicada).
These insects live underground, sucking juice from roots for many years,
and then they all emerge at the same time from the ground to mate and lay
eggs. The year that they come out varies from place to place, and
there are several "broods" identified according to what year they come
out and whether it takes them 13 or 17 years to come out the next time.
The 13-year broods are mostly southern, and the 17 year broods are mostly
northern, but they do overlap. Other than the periodical nature of the
emergence, there is little to separate these populations. In most
cases, different broods come out in different places so you can usually
find them someplace just by going to a location where the brood of that
year is ready to emerge (maybe last year a 17-year brood in Ohio, this
year a 17-year brood in Georgia and North Carolina, after that a 17-year
brood in Pennsylvania and a 13-year brood in Louisiana, etc.)
 |
17-year cicada nymphs (Brood X) emerging to becomes adults. Brood X emerged 4 years early than calculated in southern Ohio. This photo was taken on May 27th, 2000. |
Are
13-yr and 17-yr Cicadas Different Species?
There can be different broods in the same place (say, some came out
last year, and a different
brood will come out four years from now). Because the periods of
emergence differ, and populations would have little chance to
interbreed, some people
say that the 13-year and 17-year cicadas should be
called different species.
Thus, all 17-year broods would be one species, and
all 13-year broods would
be a second species.
Is
there gene flow between 13-yr and 17-yr Cicadas?
Here's the problem: If they occur in the same place (say, Kentucky
or
southern Illinois), a 13-year
brood and a 17 year brood may come out at the
same time (but not more
often than every 221 years!) and they may be able to mate with each other
under that circumstance. If so, genes can go between 13-year and
17-year
broods when they occur together,
even if only infrequently. We do not know what the“hybrids” do, because
they come out in 13 years or 17 years and they hit an emergence of one
of the parental populations and we don't see anything special. So,
should these be considered different species? They differ in period,
so maybe yes, but they can trade genes, so maybe no, but then they trade
only infrequently, so maybe yes. What about different broods of either
form? After all, they could never trade genes because they would never
be out at the same time (say, one comes out in 1900, 1917, 1934, and the
other in 1908, 1925, 1942)? Maybe different broods of each
form should be considered different species?
The map shown below indicates
populations of 17- (blue) and 13- (red)
year cicadas. Of course, they are not all out at once. To find
out how different broods overlap, you need to:
- First, find out where there are different broods of the two kinds, and when it would happen that a 17- and a 13-year brood come out in the same year. Click on a state below to see the maps for the different broods of that state.
- Then you need to find a different year when one of the first two broods comes out with a third brood. You can use the brood maps and brood emergence table to determine when different broods would overlap.
NOTE: Unlike the map shown
here, the maps for each separate brood are plotted in red, regardless of
whether the brood is 17- or 13-year period.
Blue = 17-year
broods, Red = 13-year broods
Research ideas presented on this web site are based on information from:
Kritsky, Gene. 1999. In Ohio's Backyard: Periodical Cicadas. Ohio Biological Survey Backyard Series No. 2. Columbus, Ohio. vi+83 pp.
Backyard Series Distributed by:
Ohio Biological Survey, 1315 Kinnear Road,
Columbus, Ohio 43202-1192 U.S.A., and can be purchased for $10.00.
Map images used on this page and related
"Gene Flow in Periodical Cicadas" project pages are copyrighted by Ohio
Biological Survey.
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