A photosynthesizing mouse?

Question

N. Shubin's *Your Inner Fish* makes the point several times that there is a lot of functional similarity between some seemingly remote gene cousins. If that needed reinforcing we have the spider-goat, whose milk contains spider's silk, and a knock-in mouse whose vision resembles that of humans. The last two examples are interesting because, as I recall, nothing beyond the gene insertion (already a feat) had to be done to confer the extra/new capability. The machinery at the cellular level (for the mouse to perceive a new color or for the goat to somehow process the milk) already existed. My question is whether it might not be possible in theory to create a mammal with the ability to photosynthesize? If this is a polygenic trait perhaps it would be accomplished in multiple stages. I realize an answer here would be highly speculative but a careful answer might cast some light on the process of conferring new traits/abilities in this way. While I see no obvious benefit of creating a photosynthesizing mouse, at least the food bills for their maintenance might be low. This sounds like a joke but it's not a trivial benefit.Thanks for any insights.

Answer 1

We do know of some animals who already photosynthesize, such as the Eastern Emerald Elysia (a type of sea slug) and many kinds of coral. However, these animals cannot photosynthesize per se, rather they are able to absorb either chlorophyll or other organisms capable of photosynthesis from other sources and then use them for their own needs.

So we know it's possible for some animals at least to photosynthesize. Mammals, however, pose a few problems I can think of. First, the mammalian immune system is far more advanced, so the first worry would be how to get an immune system designed to attack any foreign invader to not attack a chloroplast. Next, while chloroplasts are pretty independent (they have their own DNA and reproduce semi-autonomously), the cells of the mammal would also need modification to be able to control the chloroplast and to take advantage of the sugars produced by it. That may sound simple, but we aren't too far to the point that we can just make up our own DNA sequences to do whatever we want. That's why the examples you read about, like the spider goat, "borrow" sequences from nature for genetic engineering. Unfortunately, we still aren't really sure how photosynthesizing animals do it from a genetic standpoint, so we can't "plug it in" to another organism yet.

Finally, we have to think about the benefits. Photosynthesis works for plants because they don't really expend a ton of energy. Yes, they do a lot of neat stuff, but compare that to humans, that maintain ridiculously high body temperatures (that we have to expend energy to maintain) and amble around all day, and the amount of energy a plant uses each day is really miniscule. Despite this, many plants require 5 to 8 hours a day in full sun to photosynthesize enough energy, and that's with specially adapted leaves and systems to maximize the efficiency of photosynthesis. Some people would love to be out in the sun for 5 to 8 hours a day...others not so much. While putting chloroplasts in mammals may eventually be a possibility, it would never be as efficient as a plant's photosynthesis, and would likely never provide more than a tiny fraction of its energy needs. While some preliminary work has been done at engineering animals with chloroplasts (I've read one about a fish, but it didn't work), I think it probably just isn't high up on the list due to these factors.