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

It is almost impossible for following reasons:

1. For photosynthesis you need chloroplasts
2. To maintain chloroplasts you need many genes in the nucleus that will support its endosymbiosis

I said almost impossible because there are some natural examples of what you are asking. A sea slug called Elysia acquires choloplasts from green algae on which it feeds. However, it cannot maintain the chloroplasts and pass them on to the next generation but it acquires enough to appear green and survive on photosynthesis when there is no food.

Another example is that of the protist, Rhopalodia gibba, which acquired a cyanobacteria like organism. This protist already had a red alga derived secondary plastid before it acquired a "green" cyanobacteria. This acquisition gave the host the ability to fix nitrogen in the presence of light.

The case of Paulinella chromatophora is very interesting because the acquisition of an endosymbiont happened very recently. The endosymbiont is close to Synechococcus clade of cyanobacteria.

Intuitively, it can be understood that these kinds of acquisitions would be quite difficult for a multicellular organism.

ADDENDUM

Photosynthesis is a complex reaction which requires a dedicated compartment which not only harvests light and produces ATP but also has enzyme complexes required for anabolism (carbon fixation etc). For now we can accept the hypothesis that it would not be possible for a huge eukaryotic cell to perform these functions in absence of a dedicated organelle. For a discussion on why this is so, you can refer this post. Having said that, there is an easy way to impart partial photosynthetic ability to a eukaryotic cell. Some archaea and bacteria employ rhodopsin to pump out protons against its gradient, in the presence of light; this is coupled with ATP-synthase just like the complexes of ETC in mitochondria. In this study, Hara et al have expressed delta-rhodopsin in mammalian mitochondia which now makes the mitochondria generate ATP in presence of light. Furthermore, these cells were immune to mitochondrial toxins that affect complex-I activity.

PS: thanks biogirl. I almost forgot about this and I just remembered when I was reading about something else