Biology Reference
In-Depth Information
17- and 13-year brood emerged together in 1868 in Illinois and “hybridized,”
which resulted in a population that subsequently emerged every 13 years.
Were the cicadas newly emerging after 13 years derived by hybridization or
by loss of a 4-year diapause? To resolve this intriguing evolutionary and ecolog-
ical question,
Martin and Simon (1988)
analyzed the abdominal sternite color,
the frequency of allozyme polymorphisms, and mt DNA of the “hybrid” Illinois
population. The data indicated that mt DNA in the 13-year Illinois brood is like
that in the adjacent 17-year brood and distinct from that of the neighboring
13-year brood. The new brood is like the 17-year brood in abdominal color and
frequency of PGM (protein) polymorphism. The results are consistent with the
hypothesis that the two populations hybridized and that the 13-year life-cycle
trait is dominant.
Nevertheless,
Martin and Simon (1988)
rejected the hybridization hypoth-
esis. They pointed out that most cases of hybridization involve narrow zones
in which the species come into contact, yet this new periodical cicada popula-
tion occurs over a large area. They also noted that the complete elimination
of one of the mt DNA genotypes throughout the entire region would require
extremely
strong selection because there have been only eight generations
since 1868 upon which selection could have occurred. Furthermore, if hybrid-
ization occurred, intermediate phenotypes should have occurred, but did not.
As a result, the findings were interpreted as evidence for a widespread life-
cycle switch,
without hybridization
, in which a large number of 17-year cicadas
(
M. septemdecim
) underwent a 4-year acceleration in development to become
13-year cicadas in northern Arkansas, Missouri, Illinois, and southeastern Iowa
(
Martin and Simon 1988
). After 1868, a portion of the progeny of brood X in
these areas had a permanent 4-year acceleration in development rate. Some
cicadas in the region did not switch life-cycle length after 1868 and continued to
emerge after 17 years.
Martin and Simon (1988)
suggest the switch in life-cycle
length could have been triggered by environmental causes, perhaps by high-
density populations. This change in life-cycle length would disrupt gene flow
and initiate genetic divergence in the formerly unified 17-year cicada popula-
tion. Furthermore, the individuals that switched their life cycle from 17 to 13
years now emerge synchronously with a previously isolated brood that emerges
every 13 years (a
M. tredecim
brood). Gene flow between these previously iso-
lated broods could occur if they can mate and produce viable progeny.
Further analyses of the populations by
Marshall and Cooley (2000)
found that
the two 13-year sympatric cicada populations in the midwestern United States
exhibit song differences and are unlikely to interbreed in the field. As a result,
