周期的セミの謎
Hello everyone, SCIENSPOT is a podcast that shines a spotlight on the latest scientific technology
from Japan. Your host is REN from SCIEN-TALK. Today I'm gonna talk about a biological mystery,
periodical cicadas. These insects found in eastern North America are famous for their
incredibly precise life cycles, emerging every 13 or 17 years. They are often called
prime number cicadas, because their emergence periods are prime numbers.
And a big question has always been, how do these cicadas control their life cycle so strictly,
and how do they count such long periods, sometimes up to 17 years, while living underground?
Typically, cicadas have a long developmental stage because they feed on nutrient-poor sap
from tree roots, and they usually transform into adults after reaching a certain critical weight,
which is the minimum weight required for a final instar larva to decide to metamorphose.
But individual growth rates can vary, leading to different emergence times. However,
periodical cicadas are different. They seem to synchronize their emergence almost as if
they are counting the years. But how can a tiny insect accurately count 13 or 17 years?
The mechanism for now living organisms precisely measure such long durations has been largely
unknown. Scientists noticed an intriguing pattern. Both 13 and 17 can be expressed as a multiple of
4年ゲート仮説の提案
4 plus 1. 13 is 4 times 3 plus 1, and 17 is 4 times 4 plus 1. This sparked an idea. What if
cicadas aren't counting 13 or 17 years directly, but rather counting in units of 4 years?
Furthermore, it was known that in the autumn of the year before they emerge, for example in their
12th or 16th year, their eyes change from white to the characteristic red of an adult cicada.
This eye color change is believed to be a sign that the decision for adult metamorphosis
has been made. These observations led to the groundbreaking 4-year-gate hypothesis.
A joint research team led by Professor Teji Sota of Kyoto University set out to test this
4-year-gate hypothesis. The hypothesis proposed that periodical cicadas
every 4 years check if they have surpassed a critical weight, threshold for adult metamorphosis.
If they have, they decided to transform, but the actual emergence happens the following spring
after overwintering. This hypothesis also provides a beautiful explanation for
4-year-early emergence, a phenomenon where some 17-year cicadas emerge 4 years early at 13 years
old. The hypothesis suggests that fast-growing individuals simply reach their critical weight
at the 12-year-gate instead of waiting for the 16-year-gate, leading to an earlier emergence at
13 years. Due to the extremely long life cycles and the difficulty of reeling
4年ゲート仮説の検証
periodical cicadas in captivity, the team embarked on an extensive field study. Between 2019 and
2022, they collected 17-year cicadas ranging from 11 to 16 years old from seven sites across
states like Ohio and Illinois. They carefully examined their eye color, weight, developmental
stage, and extracted RNA from their heads to study gene expression. First, regarding eye color and
weight. As mentioned, their eyes turned from white to red in the autumn before emergence,
signifying their metamorphosis decision. The study revealed that among 16-year-old
and an astonishing 97% had red eyes and their average weight was significantly higher than
those of other ages. This strongly suggested they had reached their critical weight and
passed the 16-year-gate, confirming it as a key decision point. Even more compelling is the
data from 12-year-old cicadas and about 12% of them had red eyes, and these
individuals also showed a higher average weight than their white-eyed counterparts.
This is direct evidence supporting the idea that 4-year-old emergence occurs at the 12-year-gate,
and crucially, for all other ages, 11, 13, 14, and 15-year-olds, red-eyed were extremely rare,
accounting for only 0.6%. This pattern perfectly aligns with the 4-year-gate hypothesis,
where emergence decisions are made predominantly at ages that are multiple of 4.
セミの変態と遺伝子発現
The team also conducted gene expression analysis. By comparing white-eyed and red-eyed,
they found that red-eyed showed a significant increase in the expression of genes related to
external stimuli, particularly opsin genes, which are involved in light response. This indicates
that these cicadas are acquiring vision, preparing for their adult life above ground.
However, interestingly, genes directly involved in the actual transformation and
molting into an adult didn't show increased expression until the following spring,
just before emergence. This suggests a clever two-stage process. Imagine a student who decides
in high school exactly which university they want to attend, and they prepare vigorously
but don't actually move to university and start classes until the next academic year begins,
when all the conditions are just right. Similarly, periodical cicadas make their
metamorphosis decision and prepare, then enter a period of overwintering dormancy.
This allows the actual final transformation to be suppressed until the spring,
when factors like ground temperature reach a specific threshold. This ingenious mechanism
is what enabled the spectacular, synchronized mass emergence of millions of cicadas all at once.
This groundbreaking field research and genetic analysis provide strong evidence that 17-year
周期的セミの出現メカニズム
periodical cicadas determine their emergence timing using a 4-year gate mechanism,
checking their critical weight at a specific 4-year interval. By incorporating an overwintering
dormancy period after deciding to transform, this cleverly orchestrates their incredible
synchronized mass emergence and a truly remarkable life history strategy. This study represents a
significant breakthrough in understanding the long-standing mystery of periodical cicada life
cycle control. That's all for today's SciencePod. This podcast is broadcast in both Japanese and
English. I'd love for you to listen to the podcast and post your notes and thoughts with
the hashtag SciencePod. Thank you for listening and see you next time.
