Stealing a ‘superpower’

It could be the plot of a summer sci-fi blockbuster: A creature feeds on its prey and inherits its “superpower.” Only this is real.

A new study led by Harvard biologists describes how some sea slugs consume algae and incorporate their photosynthetic organelles into their own bodies. The organelles continue to perform photosynthesis, providing nutrients and energy to their hosts and serving as emergency rations in times of starvation.

“This is an organism that can steal parts of other organisms, put them in their own cells, and use them,” said Corey Allard, lead author of the new study and a former postdoc in the Department of Molecular and Cellular Biology. “And I thought that was some of the craziest biology I’d ever heard of.”

The study, published in the journal Cell, describes how so-called “solar-powered” sea slugs keep the organelles alive inside “kleptosomes” — specialized membranes that function like biological loot bags. This research may yield insights into the evolution of eukaryotic cells and lead to potential biomedical applications.

“I think the wow factor is that sea slugs can essentially steal ‘superpowers’ — here the ability to make energy from light through algae,” said Amy Si-Ying Lee, an assistant professor of cell biology at Harvard Medical School, researcher at the Dana-Farber Cancer Institute, and a study co-author. “Others steal the ability to attack by stinging or the ability to glow in the dark. And what’s very cool is we figured out how they maintain these stolen superpowers to use for their own survival benefits.”

The study began several years ago when Allard, now an assistant professor at the Medical School, worked in the Bellono Lab, which had been studying endosymbiosis, the process in which one species lives inside the body of another. Unlike corals, which integrate whole algae cells, sea slugs used only parts — tiny organelles within the cells of their prey.

In the new paper, the team reports how the sea slug Elysia crispata, a species native to the tropical waters of the western Atlantic and Caribbean, eat algae but do not fully digest the chloroplasts.

Instead, the slugs divert these organelles into intestinal sacs and encase them inside a special membrane that the scientists termed a “kleptosome.” Within this unique slug structure, the stolen organelles are kept alive to continue photosynthesis.

Apparently, the slugs have evolved an ability to downregulate the lysosomes, the “trash disposal” organelles of the cells that normally degrade such material.

Chemical analysis revealed that the stolen chloroplasts contained slug proteins. This suggests the hosts were keeping the stolen organelles alive. Meanwhile, the organelles continued to produce their own algae proteins, proving they were still functioning inside the slugs.

The slugs kept the stolen organelles in leaf-like structures atop their backs, (“Basically, it is a solar panel,” says Allard) and well-fed slugs took on a greenish color.

Then the researchers noticed another peculiarity: When slugs were starved, their bodies turned orange like leaves in autumn. Apparently, the chlorophyll (the green material within chloroplasts) was degraded when the stolen organelles were digested as a “last resort” form of energy.

Some of the existing scientific literature claimed the slugs entirely lived off solar energy, but Allard believes photosynthesis alone is not sufficient to keep them alive.

“The actual function of these things could be far more complicated than simple solar panels,” he said. “They could be food reserves, camouflage, or making them taste bad to predators. It’s probably all of those things.”

The lowly slugs might provide hints about some grand events in the history of life.

Endosymbiosis has been a major driver of evolutionary novelty. For example, both chloroplasts (which perform photosynthesis in plants and algae) and mitochondria (the energy-producing parts of cells) were originally free-living cells that were incorporated as organelles within host cells.

“In many systems of endosymbiosis, like our mitochondria or plant chloroplasts, this is how it started: An ancient prokaryotic cell was taken in and incorporated into the host,” said Nick Bellono, professor of molecular and cellular biology and senior author of the new paper. “In the case of the slug, it’s doing this in one lifetime. Could this transition to a more long-lasting relationship over some crazy amount of time? Maybe.”

The ancient events of endosymbiosis occurred billions of years ago, so the evidence has been lost to time. In the case of sea slugs, the biologists caught the organelle thieves in the act — enabling them to investigate endosymbiosis in real time.

Elysia are not the only sea slugs known to steal organelles. In his Med School lab, Allard is researching another group of sea slugs from the genus Berghia that consume sea anemones, pass the material through their digestive tracts, and mount the venom-coated barbs on their own backs to defend against predators.

Even more incredibly, the slug hosts can connect these stolen organelles to their own nervous systems to fire what Allard described as a “bag full of spear guns.”

Allard believes the findings may extend far beyond slugs. Insights about the organelle regulation might be applicable to neurodegenerative conditions or to lysosomal storage disorders, a class of metabolic diseases in which the body cannot properly break down waste products.

“Often in these cases, the lysosomes either don’t form properly or don’t work properly,” explained Allard, “and it almost mimics what the slugs have adapted to do in some ways.”