Biology Reference
In-Depth Information
Another issue of interest to molecular ecologists studying the diet of preda-
tors is the fact that generalist predators may be preying on species that are not
yet characterized and
O'Rorke et al. (2012)
reviewed several methods to enrich
prey DNA to improve detection, especially of short DNA fragments that have
been partially digested in the predator's gut.
13.7.3 Population Isolation and Introgression in Periodical Cicadas
Molecular markers have been used to resolve the evolutionary origins of spe-
cies of periodical cicadas (
Magicicada
). The biology, ecology, and evolution of
periodical cicadas are complex and unusual (for a review, see
Marshall 2001
).
Periodical cicadas feed underground on roots in the deciduous forests of the
eastern United States for either 13 or 17 years and emerge in very large num-
bers as adults to mate and deposit eggs nearly every year in some part of the
range. The immense populations, sometimes as large as 1.5 million individu-
als per acre that emerge in the same year, are called broods. This synchronized
emergence may have evolved because the large numbers allow most of the indi-
viduals to escape predation at a particular location and the long life cycles may
prevent predator populations from synchronizing with the local emergences.
In the Mississippi Valley and southern United States, the life cycle of three
Magicicada
species is 13 years, whereas it is 17 years for three species in the
north and west. Each species appears most closely related to another with the
alternative life cycle, so that there are “species pairs” (13 paired with 17). This
pattern suggests that speciation in
Magicicada
may involve a combination of
geographic isolation and life-cycle changes that create reproductive isolation by
changes in emergence patterns.
Thirteen of the possible 17 broods of a 17-year cicada
M. septemdecim
and
three of the possible 13 broods of a 13-year cicada
M. tredecim
have been iden-
tified and their emergence patterns have been determined and monitored
(
Marshall 2001
). Although most broods emerge as scheduled, small numbers of a
brood may emerge “out of step” with their cohort, which has created problems
in understanding the species status of some broods. Some portions of 17-year
broods appear to have accelerated their emergence by 4 years in certain sites,
and both 13- and 17-year broods may emerge in the same geographic region.
The reason for the 13- and 17-year cycles may be because the life cycle of the
17-year cicada includes a 4-year inhibition (diapause) of early nymphal growth.
It was suggested that if this inhibition were eliminated, the 17-year brood could
emerge after only 13 years. Because it appears that a single gene controls this
aspect of the life cycle, a relatively simple genetic change could have a large
effect. An alternative hypothesis for the change in brood duration is that a
