photosynthetic animal

The second story is about a sea slug, Elysia chlorotica, which has gained the ability to photosynthesize. It did not evolve this trait in the traditional sense, but rather picked it up from another organism. The slug’s green color is not self-made, but is present due to its collection of chloroplasts, the photosynthetic center of a cell, from its prey. Due to an unknown mechanism, the slug is able to hoard only the chloroplasts of its algal food source Vaucheria litoria. Not only that, but it uses these chloroplasts to go through photosynthesis itself, which it can continue to do 5 months after it last ate V. litoria. (And this is a slug that only lives for 10 months total.)

This is not as simple as it sounds, however. You need more than chloroplasts to photosynthesize; you also need genes to encode all the specialized proteins needed to make sunlight into energy! The big question regarding these slugs was: where did they get these genes? Scientists working together from Maine, Korea, Iowa and Texas (paper here) compared sections of the nuclear DNA between the slug and its algal food and found identical segments, suggesting that the slug had not evolved these genes on their own, but had acquired them through horizontal gene transfer, or a transfer of DNA from an origin other than one’s own parent. In this case, they suggest, a segment of the algae’s DNA broke off and joined the slug’s own DNA, an incredibly rare event. This gene acquisition was so beneficial that it spread through the population, causing E. chlorotica all over the oceans to hoard the chloroplasts of their prey. And there ya have it, folks: a photosynthetic animal.

Ain’t this a wonderful world we live in?

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