"Long before it's in the papers"
February 04, 2015


Sea slug found to turn itself into plant-like creature by eating algae

Feb. 4, 2015
Courtesy of the Marine Biological Laboratory
and World Science staff

A bright green sea slug man­ages to live like a plant, “feed­ing” on sun­light—thanks to genes it steals from the al­gae it eats, sci­en­tists say.

The trick en­ables the slug, which even looks like a plant, to skip meals for months at a time, as well as pass some of the stol­en genes on to its off­spring, ac­cord­ing to the re­search­ers.

The sea slug E. chlorotica. (Credit: Patrick Krug)

A study on the find­ings was pub­lished Dec. 1 in the jour­nal Bi­o­log­i­cal Bul­le­tin.

“There is no way on earth that genes from an al­ga should work in­side an an­i­mal cel­l,” said study co-author Sid­ney K. Pierce, an emer­i­tus pro­fes­sor at Uni­vers­ity of South Flor­i­da and at Uni­vers­ity of Mar­y­land, Col­lege Park. 

“And yet he­re, they do. They al­low the an­i­mal to rely on sun­shine for its nu­tri­tion. So if some­thing hap­pens to their food source, they have a way of not starv­ing to death un­til they find more al­gae to eat.”

Al­gae are very sim­ple forms of plants. By gath­er­ing their genes, Pierce and col­leagues said, the slug helps to sus­tain pho­to­syn­the­sis in­side it­self—the pro­cess of gath­er­ing en­er­gy from sun­light. Im­por­tant­ly, they added, this is one of the only known ex­am­ples of an an­i­mal us­ing genes bor­rowed from an­oth­er spe­cies. That’s basically the goal of gene ther­a­py to cor­rect ge­net­ic dis­eases in hu­mans.

“Is a sea slug a good [bi­o­log­i­cal mod­el] for a hu­man ther­a­py? Probably not. But fig­ur­ing out the mech­an­ism... could be ex­tremely in­struc­tive for fu­ture med­i­cal ap­plica­t­ions,” Pierce said.

The sci­en­tists used an ad­vanced im­ag­ing tech­nique to con­firm that a gene from an al­ga called V. litorea gets into the E. chlorot­ica slug’s chro­mo­some. This gene makes an en­zyme, or a type of mol­e­cule, needed by mi­cro­scopic pho­to­syn­thetic “machi­nes” called chloro­plasts typ­ic­ally found in plants and al­gae.

It has been known since the 1970s that E. chlo­rit­ica “steals” chloro­plasts from V. litorea (a process called “k­lep­to­plas­ty”) and em­beds them in­to its own di­ges­tive cells. Once in­side the slug cells, the chloro­plasts con­tin­ue to pho­to­syn­the­sis for up to nine month­s—much long­er than they would per­form in the al­gae. The pho­to­syn­the­sis pro­cess pro­duces car­bo­hy­drates and lipids, nu­tri­ents that feed the slug.

How the slug man­ages to main­tain these pho­to­syn­the­tic struc­tures for so long has been a top­ic of con­tro­ver­sy, ac­cord­ing to Pierce. “This pa­per con­firms that one of sev­er­al al­gal genes needed to re­pair dam­age to chloro­plasts, and keep them func­tioning, is pre­s­ent on the slug chro­mo­some,” he said. 

“The gene is in­cor­po­rat­ed in­to the slug chro­mo­some and trans­mit­ted to the next genera­t­ion of slugs.” While the next genera­t­ion must take up chloro­plasts anew from al­gae, the genes to main­tain the chloro­plasts are al­ready pre­s­ent in the slug ge­nome, he added.

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A bright green sea slug manages to live like a plant, “feeding” on sunlight—thanks to genes it steals from the algae it eats, scientists say. The trick enables the slug, which even looks like a plant, to skip meals for months at a time, as well as pass some of the stolen genes on to its offspring, according to the researchers. A study on the findings was published Dec. 1 in the journal Biological Bulletin. “There is no way on earth that genes from an alga should work inside an animal cell,” said study co-author Sidney K. Pierce, an emeritus professor at University of South Florida and at University of Maryland, College Park. “And yet here, they do. They allow the animal to rely on sunshine for its nutrition. So if something happens to their food source, they have a way of not starving to death until they find more algae to eat.” Algae are very simple forms of plants. By gathering their genes, Pierce and colleagues said, the slug helps to sustain photosynthesis inside itself—the process of gathering energy from sunlight. Importantly, they added, this is one of the only known examples of an animal using genes borrowed from another species. That is essentially the goal of gene therapy to correct genetic based diseases in humans. “Is a sea slug a good [biological model] for a human therapy? Probably not. But figuring out the mechanism of this naturally occurring gene transfer could be extremely instructive for future medical applications,” he said. The scientists used an advanced imaging technique to confirm that a gene from an alga called V. litorea is present on the E. chlorotica slug’s chromosome. This gene makes an enzyme, a type of molecule, that is critical to the function of photosynthetic “machines” called chloroplasts, which are typically found in plants and algae. It has been known since the 1970s that E. chloritica “steals” chloroplasts from V. litorea (called “kleptoplasty”) and embeds them into its own digestive cells. Once inside the slug cells, the chloroplasts continue to photosynthesize for up to nine months—much longer than they would perform in the algae. The photosynthesis process produces carbohydrates and lipids, nutrients that feed the slug. How the slug manages to maintain these photosynthesizing structures for so long has been a topic of controversy, according to Pierce. “This paper confirms that one of several algal genes needed to repair damage to chloroplasts, and keep them functioning, is present on the slug chromosome,” he said. “The gene is incorporated into the slug chromosome and transmitted to the next generation of slugs.” While the next generation must take up chloroplasts anew from algae, the genes to maintain the chloroplasts are already present in the slug genome, he added.