Wednesday, August 10, 2011

When Amazing isn’t Enough- Sea Slug Hybrids, part 2

As you undoubtedly remember, last time we talked about a fascinating exception in biology, an animal that can perform photosynthesis. The sea slug, Eylsia chlorotica, eats algae and places the intact, functional chloroplasts in its tissues by a process called kleptoplasty. From that point on, the animal can turn light and CO2 into carbohydrates – it no longer needs to eat. You might also recall that I hinted that the mere ability to perform photosynthesis isn’t the most amazing thing about this animal. So let us jump in right there.

The average life span of our sea slug of interest is ten months. Not enough time to read War and Peace, but forever compared to the mere 24 hours allotted to the mayfly. So E. chlorotica has roughly a year to make hay while the sun shines. However, the life span of the proteins that are needed for photosynthesis is much shorter.

RuBisCO, a complex protein of photosynthesis

The single most abundant protein on earth is called RuBisCO (Ribulose-1,5-bisphosphate carboxylase oxygenase). This protein adds carbon from CO2 to the growing carbohydrate during photosynthesis, and has a turnover rate of about 5 days. This is an abnormally long life time for a protein. Chlorophyll can have a turnover rate of a mere 10 hours in some plants. Proteins get old fast, they start to work poorly or just stop working altogether. This is especially true for proteins that work in photosynthesis, since light can damage the very proteins that harness its energy.

Scientists, under laboratory conditions, have kept E. chlorotica alive for 14 months using just water and sunlight. The take home message is that there is active photosynthesis in sea slugs for months and months, when the proteins that make photosynthesis work may need replacing in just a few hours. It makes one wonder how E. chlorotica maintains active chloroplasts for so long.

Science has considered three main possibilities, but there might be more. First, there is something unique about the V. litorea (the algae E. chlorotica eats) photosynthetic proteins that makes them extremely long-lived. This is a tenable possibility, as a few plants have chlorophyll that might never be replaced. But even with immortal chlorophyll, these plants have hundreds of other photosynthetic proteins that must be constantly replaced. So this idea must take a back seat.

Second, there might be something unique about E. chlorotica that keeps the proteins from degrading. This would be amazing, since the sea slug’s own proteins degrade just as in other animals and are replaced regularly. Again, not the strongest hypothesis. Third, E. chlorotica has managed to find a way to make photosynthetic proteins. Intriguing possibility, isn’t it?

An animal that makes RuBisCO or chlorophyll takes the idea of a plant/animal hybrid to a whole new level. It isn’t just the ability to selectively save chloroplasts from digestion and then make use of them. It would be as if the sea slug bought an old motor (the chloroplasts) and but produces replacement parts by itself. But to make the replacement parts, the instructions must be there, and this means DNA.

For our sea slug to have the proper DNA, the plant genes must be consumed, avoid digestion, and be transported to the animal cell nucleus. What is more, the genes must be incorporated into the animal's chromosomes. This is a tall order.

Chloroplasts do have some of their own DNA, since they used to be their own organism (remember endosymbiosis?), but biologists know that many of the hundreds of photosynthesis genes have been transferred to the plant nucleus and are no longer housed in the chloroplast. Therefore, just maintaining functional chloroplasts is not sufficient to produce the proteins needed to keep them active.

Perhaps the slug retains the algae nucleus after feeding. This would provide all the genes needed to produce the proteins needed for photosynthesis, as long as the animal cell can reach and read the plant DNA. Since the chloroplast is not digested, perhaps neither is the nucleus. This would be a good idea, except that scientists have starved E. chlorotica for months, and then searched the slug for plant nuclei. They haven’t found any, so it is probable that the nuclei aren’t retained.

This leaves us with the possibility that the plant genes needed for photosynthesis have been donated by the algae and added to the animal’s cell chromosomes. Don’t laugh, this happens all the time in bacteria. It is called lateral (or horizontal) gene transfer, and it can account for things like antibiotic resistance and sex change in gut bacteria (yes, bacteria can change sex). Even viruses can help accomplish horizontal gene transfer. Viruses can insert their own DNA into the infected cell’s DNA and when they cut themselves back out, they may bring more than they put in. The next infected cell is then the recipient of DNA it may not have had previously.
In vertical transmission, all DNA in the offspring
comes from the parent. In horizontal gene transfer,
the movement is between two different organisms
of the same generation; the recipient cell now has
DNA it did not have before.

Lateral gene transfer can also occur in eukaryotes, but it is usually at the primitive end of the scale. The transfer of some chloroplast and mitochondrial genes to the nucleus millions of years ago is an example of horizontal gene transfer. Horizontal gene transfer with passage of the new genes to the next generation is easy in bacteria or lower eukaryotes because they don’t reproduce through sex. In fungi, even though some progeny are produced by mating, the DNA transferred to the progeny is still the same DNA that was laterally transferred.

Sex on the other hand, means sex cells. The DNA in sex cells (gametes) is the only DNA that gets passed on to the progeny (you get half your DNA from Mom’s egg and half from Dad’s sperm). For DNA to be passed on through horizontal gene transfer, the new DNA must be transferred into either an egg or sperm, and that has to be the particular egg or sperm that participates in fertilization. This is especially difficult to imagine for E. chlorotica, as the algae is eaten, and the chloroplasts are put into the gut cells. Nothing about this leads to algae nuclear DNA getting anywhere near the sex cells. It doesn’t seem very likely - but this is exactly what happens.

Pea aphids have incorporated fungal genes to
help them blend in to their surroundings.
Scientists have found several photosynthesis-specific genes in both mature E. chlorotica that have been starved for algae for months and in immature veligers that have never fed on algae. This can only mean that the genes have been passed vertically, from parent to child, and this means that the plant DNA has entered the gamete cells. The only similar instance I can think of is the transfer of a fungal carotenoid (pigment) gene to pea aphids (ant cows, a neat story on their own) that changed the aphid’s color to match their environment, giving them a camouflage advantage. This is itself a biological exception, the only instance of an animal that produces carotenoid pigment.

Lets summarize. We have an animal that can do photosynthesis – amazing. This same animal has taken up DNA from algae, and has incorporated the new genes into its germ line cells so that they are passed on to its offspring – more amazing. Next time, we’ll talk about how one of the greatest ideas of science might be run aground by a sea slug. Could it be that a discarded version of evolution might be correct?

For more information on horizontal gene transfer, as well as web-based activities and experiments, go to: