March 19, 2013
Prof. Sota
The enigmatic evolutionary history the periodical cicadas (Magicicada) in eastern United States was studied in detail, for the first time, by the research group comprising Prof. Teiji Sota and Dr. Satoshi Yamamoto at Kyoto University, Prof. Jin Yoshimura at Shizuoka University, and Prof. Chris Simon and two colleagues at University of Connecticut. Their article has been published online in Proceedings of the National Academy of Science of the USA released on March 18, 2013.
Periodical cicadas (genus Magicicada; Cicadidae) in the eastern United States represent one of the most spectacular life history and population phenomena in nature. These periodical cicadas spend most of their lives (13-years in the south, 17-years in the north) as underground juveniles except for a brief two- to four-week period when adults emerge simultaneously in massive numbers. With few exceptions, at any given location, all the periodical cicadas share the same life cycle and emerge on the same schedule, forming a single year class referred to as a “brood.” Surprisingly, each brood consists of multiple species from three species groups named Decim, Cassini, and Decula. Each species group is divided into one 13- and one 17-year species with the exception of the Decim group, which contains two 13-year species. Taxonomically, 13-year species are Magicicada tredecim, M. neotredecim, M. tredecassini and M. tredecula; and 17-year species are M. septendecim, M. cassini and M. septendecula.
Periodical cicadas, Magicicada cassini
The three species groups were considered to have diverged from each other allopatrically and to have formed 13- and 17-year life cycles. The prolonged, prime-numbered life cycles were hypothesized to have evolved in response to Pleistocene climatic cooling to avoid the adverse effect of low population density on mating success. Another view hypothesized that the long synchronized life cycles evolved in association with the predator avoidance strategy and that this took place prior to both the glacial periods and the split of the three species groups based on approximate genetic distances among species groups. To test these hypotheses, phylogenetic information about the relationships of species, broods, and populations is essential. However, a comprehensive molecular phylogeny covering all the extant broods, their phylogeography and divergence time has been lacking until now.
The researchers conducted molecular phylogenetic and population genetic analyses by using nuclear and mitochondrial DNA markers for samples collected over a 30-year period (1978-2008), which represent all 15 extant broods and all known species. They show that the divergence leading to the present 13- and 17-year populations differs considerably among the species groups despite the fact that each group exhibits strikingly similar phylogeographic patterning (i.e., divisions among eastern, middle and western parts of the eastern US). The earliest divergence of extant lineages occurred approximately 4 million years ago (mya) with one branch forming the Decim species group and the other subsequently splitting 2.5 mya to form the Cassini and Decula species groups. The earliest split of extant lineages into 13- and 17-year life cycles occurred in the Decim lineage 0.5 mya. All three species groups experienced at least one episode of life-cycle divergence since the last glacial maximum. The researchers hypothesize that despite independent origins, the three species groups achieved their current overlapping distributions because life-cycle synchronization of invading congeners to a dominant resident population enabled escape from predation and population persistence. The repeated life-cycle divergences supported by the data suggest the presence of a common genetic basis for the two life cycles in the three species groups.
Phylogeny of periodical cicadas with divergence time estimation based on nuclear and mitochondrial gene sequences (left). Geographic distribution of mitochondrial haplotype groups in each species group (right). Modified from Sota et al. (2013, PNAS Early Edition).
This research was Supported by Grants-in-Aid from the Japan Society for Promotion of Science (nos. 22255004, 22370010, and 23128507) and Ministry of Education Culture, Sports, Science and Technology (the Global Center of Excellence program A06).
Publication
[DOI] http://dx.doi.org/10.1073/pnas.1220060110
[KURENAI Access URL] http://hdl.handle.net/2433/172230
Independent divergence of 13- and 17-y life cycles among three lineages of periodical cicada lineages
Teiji Sota, Satoshi Yamamoto, John R. Cooley, Kathy B.R. Hill, Chris Simon, and Jin Yoshimura
Proceedings of the National Academy of Sciences of the USA.