Boom Insect
English

Free article

Unravelling the story of Hachijo-jima’s stag beetles

Following differences in their jaws into the history of islands and the sea

Members of the saw-toothed stag beetle group live both on mainland Japan and on Hachijo-jima. Imagine placing a male from each place side by side, with bodies about the same size. The Hachijo-jima male has shorter jaws. Would that alone tell you they belong to different species?

Differences in the jaws are one clue when deciding whether beetles belong to different species. When researchers compared many individuals, Hachijo-jima males did tend to have shorter jaws for their body size than mainland males.

Why did this difference develop on Hachijo-jima? The island formed when volcanoes that began on the seafloor rose above the sea. The beetles’ ancestors are thought to have crossed the water after land had formed there.

Once beetles had settled on Hachijo-jima, did they keep arriving from the mainland or other islands? Kodai Kishino and his colleagues focused on these movements. Differences in the connections with places outside the island might help explain why its beetles evolved shorter jaws. Let’s follow that mystery.

To compare the jaws, measure the chest

Body length means the length of an animal’s body. Some stag beetle guides measure it from the tips of the jaws to the rear end. With that method, longer jaws mean a longer body measurement even if everything behind them is the same size. To find out how long the jaws were in relation to the rest of the body, the team used the width of the thorax—the chest section just behind the head—as their measure of body size instead.

They photographed beetles from Nasu Kogen on the mainland, and from Oshima, Niijima, Kozushima and Hachijo-jima, directly from above. They then measured thorax width and jaw length in the photographs. Relative to thorax width, male jaws tended to become shorter in the order mainland, Oshima, Niijima and Kozushima. The shortest relative jaws among the beetles examined were on Hachijo-jima.

Hachijo-jima’s beetles were first treated as a distinctive local form within the same species as the mainland saw-toothed stag beetle. In 1985, differences in their appearance led to their being treated as a separate species, known in Japanese as the Hachijo saw-toothed stag beetle. Kishino’s team investigated a different question: what connections had these beetles had with beetles on the other islands and the mainland?

When did Hachijo-jima break away from the mainland?

If the mainland and Hachijo-jima beetles share ancestors, was there once a land connection? Hachijo-jima is a volcanic island in the southern part of the Izu Islands, which extend offshore from the Izu Peninsula. Geological research has found no evidence that it formed by breaking away from the mainland. The volcanoes underlying the island began on the seafloor and later emerged above the sea.

One clue to the timing comes from volcanic ash that travelled from Kyushu. Ash about 240,000 years old has been found within layers of sediment formed on land on Hachijo-jima. That suggests land was already above the sea when the ash fell.

Given this history, it makes sense to think that the beetles’ ancestors crossed the sea after land had formed. Drifting there on wood is one possible way. Did they come straight from the mainland or via nearby islands? What carried them, and when did they arrive? The geological layers tell us that land already existed at that time, but they cannot answer those other questions.

Looking for later journeys in DNA

Suppose the beetles’ ancestors crossed the sea and settled on the island. Did further arrivals from the mainland and other islands follow? We cannot watch those ancient journeys directly. But if arriving beetles produced offspring on the island, their DNA may have been passed down to beetles living there today. That is where the team looked for traces.

DNA is a substance that carries information involved in how a body is built and works. It contains four kinds of chemical building blocks called bases. Their order stores information. This sequence passes from parents to offspring, though a small part sometimes changes. Comparing which changes remain in different populations helps us investigate how those populations are connected.

The team first compared the DNA found in mitochondria. Mitochondria are structures inside cells that use nutrients from food to produce energy in a form cells can use. In many animals, their DNA is inherited mainly from the mother. Comparing its sequence can therefore help trace connections along the maternal line, rather like following a family tree from a mother to her mother.

The researchers read a stretch of mitochondrial DNA. It contains information for a component needed in energy production.

They compared the beetles’ DNA sequences to find matching and differing parts. From those comparisons, they estimated the order in which lineages had split from shared ancestors. A diagram showing these relationships like the branches of a tree is called a phylogenetic tree.

They built a tree using beetles from eight Izu islands, the mainland and other locations. Hachijo-jima and southern islands including Miyake-jima fell on one branch, separate from the mainland group. This means the beetles on that branch were estimated to share a more recent common ancestor with one another than with the mainland group. Oshima and Niijima fell on the mainland branch. Comparing more of the mitochondrial DNA did not change this branching pattern.

Next came DNA in the nucleus, which is inherited from both the father and the mother. The nucleus is the part of a cell that contains most of its DNA. The researchers compared 1,317 genes in nuclear DNA. A gene is a section of DNA carrying information involved in how a body is built or works.

In a tree made from nuclear DNA, the four islands of Oshima, Niijima, Miyake-jima and Hachijo-jima formed a branch separate from the mainland group. Within that branch, Hachijo-jima split from the other three islands. Miyake-jima, which had shared a branch with Hachijo-jima in the mitochondrial tree, now shared one with Oshima and Niijima.

Could movements after the beetles settled on the islands help explain this mismatch? If later arrivals bred there, their DNA could have become part of the island population too. The team investigated that possibility more closely.

Which islands share more DNA changes with the mainland?

The researchers treated Oshima, Niijima and Miyake-jima as one group and compared it with the Hachijo-jima population. They asked which group retained more sites with the same DNA changes as the mainland beetles.

If neither group had bred with mainland beetles after the two groups separated, those shared changes were not expected to be strongly concentrated in just one group. Yet the three-island group had more sites with the same changes as the mainland than Hachijo-jima did.

One explanation is that mainland arrivals bred with beetles on the three islands, passing mainland DNA to their offspring. Taking other analyses into account as well, the team concluded that mainland beetles had contributed offspring on those three islands. They did not obtain equivalent evidence of mainland beetles joining the Hachijo-jima population.

Did the Kuroshio current hinder journeys to Hachijo-jima?

The team found traces of connections with the mainland on Oshima, Niijima and Miyake-jima, but not the same traces on Hachijo-jima. All these islands are separated by sea. Why the difference?

To investigate, the team also used DNA to estimate when the mainland and Hachijo-jima populations split. The estimate was about 150,000 years ago, with a range reported in the paper of roughly 120,000–190,000 years ago. The calculation assumed that the two populations had not bred with each other after separating. It does not tell us when the first beetles arrived on the island.

The geological layers show that Hachijo-jima already had land about 240,000 years ago. To understand the beetles’ later connections, Kishino’s team turned to the Kuroshio. This powerful ocean current sometimes passes between Mikura-jima and Hachijo-jima. The researchers considered whether it could have hindered movement to the island.

Studies of past seas, based on mud deposited on the seafloor, suggest that the Kuroshio flowed farther south during a cold period. Kishino and his colleagues proposed that the beetles’ ancestors might have reached Hachijo-jima at that time. Later, as the current shifted north and crossed between the islands, it could have swept floating wood in another direction, making Hachijo-jima harder to reach. That is a hypothesis. The estimated split has a broad time range, so we do not know whether it coincided precisely with the change in the current’s path.

Whenever mainland beetles breed on an island, they add mainland DNA to its population. The team suggests that fewer such additions on Hachijo-jima may have allowed changes suited to island life to persist more easily, helping shorter jaws evolve.

What did they change as the larvae grew?

If jaw development had changed, DNA involved in growth might hold another clue. Larval cells have a component that receives growth signals sent in response to the body’s nutritional condition. In Hachijo-jima beetles, the DNA carrying instructions for this component had differences not seen in the other populations.

The team divided mainland larvae into two groups and made it harder for one group to produce this component. When they measured the adult males, the group with reduced production had shorter jaws for its body size than the comparison group without that reduction. The experiment thus confirmed that this component is involved in jaw development.

What we know, and what remains to be tested

The study confirmed that Hachijo-jima males have shorter jaws even after accounting for body size. DNA comparisons revealed traces of mainland beetles producing offspring on Oshima, Niijima and Miyake-jima. The same traces were not found on Hachijo-jima, showing that the islands differ in their connections with the mainland.

The team suggests that because mainland DNA was less likely to enter Hachijo-jima’s population, changes suited to island life could persist more easily, helping shorter jaws evolve. Both the island environment and its history of receiving fewer newcomers from the mainland may have contributed to those short jaws.

There is still work to do to test that explanation. The experiment showed that reducing production of a growth-related component in mainland larvae produces shorter jaws. But the team has not tested whether the DNA differences found on Hachijo-jima themselves alter how that component works. Finding out when shorter jaws might be useful on the island will also require studying where males meet and what they compete over.

The idea that the Kuroshio hindered travel to Hachijo-jima also remains a hypothesis. More evidence is needed to understand when and by what route beetles first reached the island, and how later movements changed. For example, comparing other insects found on both Mikura-jima and Hachijo-jima might reveal whether DNA differences also become greater across the gap between those islands. That could offer clues to the ocean current’s influence.

Following a difference in jaws has led us to the origins of an island and the movement of the sea. Did weaker connections with the mainland help Hachijo-jima’s beetles evolve in their own way? The next challenge is to build on these findings and connect the workings of DNA with how the beetles live on the island.

References