Animals that feed on ancient supernovae

Do some animals on earth get their energy from ancient supernovae? It requires energy input to fuse lighter elements into heavy elements such as uranium. In nature this energy input comes from supernovae: heavy elements are formed in supernovae and store a tiny bit of the supernova's energy [1, 2]. As B. Schutz points out in his excellent book [1], this means that nuclear reactors are powered by energy stored from a supernova which most likely occurred around 6 to 7 billion years ago. Even more interestingly, since much of the Earth's internal heat comes from radioactivity, it implies that the Earth is warmed in part by an ancient supernova, so geothermal power also comes from supernovae.

The spectacular deep sea hydrothermal vent animals and bacteria also get energy from the internal heat of the Earth. Earth's heat drives the reactions that form hydrogen sulfide which the bacteria use as an energy source. Hence, some of the energy they use is stored energy from supernovae. The vent animals get their energy from the bacteria. The vent bacteria require both the hydrogen sulfide coming out of the vent and oxygen formed by photosynthesis to get energy by oxidizing the hydrogen sulfide [3]. Therefore, the vent communities do get energy from the sun too, but still, it is remarkable that they get some of their energy from supernovae.

Of course, all of the matter on earth (with the exception of hydrogen) came from ancient stars originally, but it seems even more exciting that the energy that powers the Earth and some Earth life also came from supernovae. And yes, mass and energy are equivalent, but the form of energy that counts here is the free energy: the energy that can drive a system out of equilibrium, and hasn't simply been lost as waste heat or otherwise converted to an unusable form.

I highly recommend two fascinating and very well written books on gravity [1], astronomy [1, 2] and how scientists determine the age of ancient events [2]. These two books are excellent since they delve deeply into how scientists studying these issues have come to their current understanding of them, and what the open questions are, while still being accessible to pretty much anyone with a minimal scientific literacy. I'm only through the first half of Dr. B. Schutz's book on gravity, but so far it's exceptionally good despite a few confusing spots. Dr. M. Hedman's book is one of the most interesting books I've ever read.

(1) B. Schutz (2003) "Gravity: grom the ground up," Cambridge University Press, pp125-126.

(2) M. Hedman (2007) "The Age of Everything: How Science Explores the Past," University of Chicago Press, pp157-158.

(3) C. van Dover (2000) "The Ecology of Deep-Sea Hydrothermal Vents," Princeton University Press.

Re-building a whole new body

Imagine how handy it would be if you could rebuild your whole body from a tiny fragment. Some animals can do this, but most such animals (e.g. sponges, hydra, and planarians) have elegantly simple internal structure. But it turns out that many botryllid ascidians, among our closest living invertebrate relatives, are also able to do this [1-3]. As mentioned in a previous post, these marine animals have the same basic organization that we do: each individual has a heart, central nervous system, and through-gut, as well as a number of other features shared with vertebrates.

Adult botryllid ascidians can reproduce sexually, with eggs that develop in a precisely choreographed manner to form a swimming larva. The larva metamorphoses into the founding member of a colony of interconnected individuals that live attached to surface. Each individual reproduces asexually by forming buds as pouches off the tissues surrounding its heart. But under certain conditions (which vary by species [1-3]) the colony can form a whole new individual from aggregates of blood cells. Circulating cells clump up on the wall of one of the blood vessels that reach to the outer edge of the colony [1]. Then the clump hollows out, and starts to mold itself into a new body from scratch*. Because these organisms are fairly closely related to humans, this process likely holds insights for regenerative medicine [3].

However, what I find most exciting is that it shows that development can tolerate remarkable variation. Developmental biology tends to focus on the complicated interplay of interactions required in "normal" development from the egg. We usually think of development as fragile, easily thrown off by a mutation here, or a sip of wine there. But this example dramatically shows that – at least in a few organisms surprisingly closely related to ourselves – cells can organize themselves into the same body from lots of different starting points. Botryllids can form complete new individuals in at least three very different ways: a precise series of events from egg to larva to juvenile; from pouches of a specific epithelium; or from disorganized aggregates of blood cells. Understanding how they do this could reshape how we think about animal development.

*The outer epithelium apparently comes from the cells lining the outside of the blood vessel. Their blood vessels do not have the inner lining of cells that our vessels have.

1) The clearest images and diagrams of this process come from the first description, which is freely available online: Oka, H., and Watanabe, H. 1957. Vascular budding, a new type of budding in Botryllus. Biological Bulletin 112:225–240.
2) This has some neat videos, and some interesting results about the importance of maintaining blood flow for regeneration. Voskoboynik A, Simon-Blecher N, Soen Y, Rinkevich B, De Tomaso AW, Ishizuka KJ, Weissman IL. 2007. Striving for normality: whole body regeneration through a series of abnormal generations. The FASEB Journal 21:1335–1344.
3) This one also has neat videos and discusses the some of the possible molecular pathways involved. Brown, F.D., Keeling, E.L., Le, A.D., and Swalla, B.J. 2009. Whole Body Regeneration in a Colonial Ascidian, Botrylloides violaceus. Journal of Experimental Zoology (Mol. Dev. Evol.) 312B:885-900

Ascidian colonies

Erica Westerman (first author on the study discussed in my last post) kindly sent me two nice images of ascidian colonies*. The first image shows a pair of small Botrylloides violaceus colonies (orange) growing up to each other at the top of the image. The lower colony is getting overgrown by a larger colony of Botryllus schlosseri at the bottom (brown). The individual zooids within the colony are orange (or brown) oblongs set in rosettes of 6 or more. Each set of individuals sucks in water through an opening at the colony surface, and the water flows to an opening at the center of the rosette, shared by all the zooids in the rosette.

The second image shows several larger colonies, of Botrylloides violaceus growing into each other on a settling plate. You can still see the boundaries between the colonies, which are distinct in coloration, and the arrangement of zooids.

*As always, please do not use images without permission from the contributor.

Are you feeling like yourself today?

What it would be like if the people you lived with could take over your body? This is a (somewhat creepy) reality for a few of our closest invertebrate relatives, the botryllid ascidians.

Adult ascidians look nothing like vertebrates, but developmental and molecular similarities indicate they are close kin to vertebrates [1]. Adult ascidians live attached to a surface and filter food from the surrounding water. Some ascidians are colonial: they bud asexually to form an array of individuals that each have their own organs but share a common blood system.

At least one type of colonial ascidians – the botryllids – takes this a giant step further. When neighboring colonies meet they can fuse together so that both colonies share their blood, transmitting cells among the colonies [2-3]. Colonies only fuse if they share a particular version (allele) of one gene. If they don't share that allele, one colony may grow over the other, smothering it, or they may sit quietly side by side. However, in laboratory studies colony fusion leads to an unpleasant outcome for one of the colonies. One colony can take over the other, replacing the tissues of the second colony with its own cells*. Certainly a creepy way to die!

But what happens in the wild? To address this question, Erica L. Westerman et al. (2009) set out clean plates in Salem Harbor (MA) for larvae of one species, Botrylloides violaceus, to settle on [3]. The researchers monitored colony-colony contacts over several weeks. Surprisingly, almost all of the colonies fused! This is remarkable, given what has been observed under controlled conditions. Westerman et al. (2009) provide a very interesting and accessible discussion of factors that may differ between the lab and field, and of some of the possible advantages to fusing despite the risk of takeover.

Colony-colony fusion gives a fascinating window into the biology of individuality since all organisms, including ourselves, have to deal with the problem of how to tell what's part of themselves and what's not. And it remains an issue even for vertebrates: in a few mammals with low genetic diversity (e.g. dogs [4] and Tasmanian devils [5]), cancers have arisen that can spread among individuals since so many individuals share the same versions of the genes that distinguish self from non-self**. (This is not a problem for humans because our species has a high level of diversity in those genes). Hence, the peculiarities of the botryllids shed light on one of the most fundamental and challenging biological questions: what is an individual?

*It gets yet stranger since the second colony doesn't always lose out entirely. Sometimes the apparent loser takes over all the reproductive cells and becomes the sole parent of the next generation [2].
**Curiously, different animals use different genes to distinguish self from non-self.

1) Pechenik, J.A. (2000) Biology of the Invertebrates, 4th edition.
2) Rinkevich, B. (2005) Natural chimerism in colonial urochordates. Journal of Experimental Marine Biology and Ecology 322:93-109
3) Westerman, E.L., Dijkstra, J.A., and Harris, L.G. ( 2009) High natural fusion rates in a botryllid ascidian. Marine Biology 156:2613-2619
4) Rebbeck, C.A., Thomas, R., Breen, M., Leroi, A.M., and Burt, A. (2009) Origins and evolution of a transmissible cancer. Evolution 63(9): 2340–2349
5) Murchison, E.P. et al. (2010) The tasmanian devil transcriptome reveals schwann cell origins of a clonally transmissible cancer. Science 327:84-87

Plantimals!

E. chlorotica. Image kindly provide by Dr. S.K. Pierce.

We all know what animals are. We all know what makes plants and animals different. These are things we learned in elementary school, right? Wrong. It turns out that these categories, like many things one learns in elementary school, are not so cut-and-dry in reality.

The neatest SICB talk we saw was given by Dr. Sydney Pierce of the University of South Florida. He and his colleagues are researching the transfer of genes from algae¹ to sea slugs² that eat the algae. They found that many algal genes appear in the genome of a sea slug called Elysia chlorotica^3-5. It's important to note that E. chlorotica doesn't gain algal genes every time it feeds; the original acquisition took place some time in evolutionary history. These algal genes are passed down from one sea slug generation to the next; they are even found in sea slug larvae that haven't yet fed on the algae. Yes, that is as strange as it sounds—as though one of your distant ancestors ate broccoli, incorporated some of the broccoli's DNA into his/her reproductive cells, and eventually passed it down to you, thereby making you part broccoli!

There is another cool part to this story: E. chlorotica slurps the contents out of algal cells and then incorporate the chloroplasts from the algal cells into some of their own cells. (Chloroplasts are the parts of plant cells that contain chlorophyll and carry out photosynthesis.) The slugs can then use the chloroplasts for photosynthesis for awhile. So, that strange scenario above just got stranger: not only would you have broccoli DNA passed down to you, but when you eat broccoli, you could capture some of the broccoli's machinery for photosynthesis. That means you could make your own food (like a plant), rather than having to eat other organisms—like an animal! [Note that sea slugs are animals just as unquestionably as we humans are. They even have many of the same parts: brains, muscles, hearts, etc.]

Among closely related sea slug species, there is great variation in how long they can keep the chloroplasts. Most can only maintain the chloroplasts for a short time; however, E. chlorotica appears to be able to photosynthesize for months, so that in the lab they need only one meal in a lifetime. In yet another twist, Pierce et al. found that the slugs had obtained the ability to make chlorophyll^4,5—not just to use the chlorophyll taken from the algae! This shows something astounding: the sea slugs did not acquire a couple of random algal genes in evolutionary history; they actually acquired an entire complex pathway, including several genes for different enzymes.

To demonstrate that it was indeed the slugs who were actively making the chlorophyll, Pierce et al. gave the slugs radioactively-labeled building blocks for chlorophyll synthesis. Sure enough—they found that some of the radioactive material showed up as chlorophyll in the slugs' bodies.

This is a fascinating example of the transfer of genes between very different species—in this case, allowing an animal to take on some of the characteristics and abilities of plants.

Humans love to put things in nice neat bins and categories. One of the most fun parts about biology, though, is finding out how messy those bins really are. We use the metaphor of a branching tree to represent evolution, but this sea slug research shows us that sometimes the branches can come back together in unexpected ways.

1) Technically, algae are not in the same group as plants according to modern taxonomy; however, like plants, they are eukaryotes (their cells contain nuclei) that obtain energy through photosynthesis. Therefore, they are often informally called plants.

2) "Sea slug" is the informal name for members of the group of exceptionally beautiful molluscs called opisthobranchs. Land slugs (which are also cute) are a completely separate group of molluscs.

3) Pierce, S.K., Curtis, N.E., Hanten, J.J., Boerner, S.L., and Schwartz, J.A. (2007) Transfer, integration and expression of functional nuclear genes between multicellular species. Symbiosis. 43, 57–64

4) Pierce, S.K., Curtis, N.E., and Schwartz, J.A. (2009) Chlorophyll a synthesis by an animal using transferred algal nuclear genes. Symbiosis. 49, 121-131

5) Pierce, S.K., Curtis, N.E., and Schwartz, J.A. Chlorophyll synthesis by a sea slug (Elysia chlorotica). Presented at the Society for Integrative and Comparative Biology meeting, Seattle, Jan. 3-7th, 2010

We're back!

We recently returned from Seattle, where the annual meeting of the Society for Integrative and Comparative Biology (SICB) took place. We'd like to summarize a few of our favorite things from the meeting in the next few posts.