Against the odds, cicada teens meet and mate

Members of the cicada genus Magicicada are known as “periodical”, because of their mathematically precise life history pattern. Depending on their lineage, the insects spend the best art of either 13 or 17 years underground, as larvae, before transforming into adults and emerging, en masse, in their hundreds of thousands, suddenly filling the air with their loud and buzzing mating calls.

Six of the seven species in the genus are classified as “pairs” – closely related but, in any given location, with one operating on a 13-year cycle and the other taking 17. (The seventh species, more distantly related, operates on a 13-year plan.) Members of each cycle are called a cohort, or brood.

In each area of their eastern US distribution, only one cohort emerges at any time.

The next brood scheduled for emergence is the species Magicicada septendecim. Each cycle is numbered and named by avid cicada-watchers, and this one has been dubbed Brood VII, or the Onondaga Brood, due to its location in territory within the Native American Onondaga Nation in upstate New York.

This synchronisation of life cycles is thought to have evolved as a predator avoidance strategy, with a mass emergence of cicadas creating a glut of prey, so ensuring the survival of at least a few to start the next generation.

The other outcome, however, is that members of one brood are extremely unlikely to ever come into contact with members of another. Indeed, brute mathematics predict that a 13-year cohort and a 17-year cohort in the same area will only emerge simultaneously once in more than two centuries.

However, a study published in the journal Communications Biology reports that genetic hybridisation has occurred between two closely related 13 and 17 year cohorts belonging to the same species pair – despite the fact that their paths shouldn’t have crossed.

The surprise finding was part of a genetic analysis, led by Teiji Sota from Kyoto University in Japan, conducted to gain greater understanding of the evolution of this extraordinary genus.

The team sequenced RNA from the seven periodical cicada species, confirming the species pair lineages, along with the seventh outlier.

The seven species form three groups, dubbed Decim, Decula and Cassini, with each group containing a pair of 13 and 17 year life cycle species. For example, the Cassini group has M. tredecassini, which emerges after 13 years, and M. cassini which emerges after 17.

Although the three groups differ widely in appearance and calls, the 13 year and 17 year species within each are virtually indistinguishable. The emergence time is the only differentiating feature.

Despite its statistical unlikeliness, Sota and colleagues found “substantial gene flow” had occurred between the 13 and 17 year broods within each species group. They suggest that this may have occurred when the cycles coincided and two broods emerged at once – an event that happens every 221 years.

Matings could also occur during occasional off-schedule emergences, when disorganised cicadas, known as “stragglers”, pop up late, or early.

Despite the gene flow, however, periodical cicadas have been able to maintain their distinct life cycles for up to 200,000 years.

But how did the parallel divergence, or evolution, of these species pairs with 13 and 17 year life cycles, occur?  The researchers were hoping to find genetic evidence that would explain when and how the species split from each other, but they did not.

“The genetic background of the life cycle divergence in periodical cicadas remains unclear,” they conclude, “and future sequencing of the full genomes of these species is needed to further understand how these different life cycles are maintained over evolutionary time.”

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