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Gastropods and Symbiosis

Katharina Händeler (abbreviated henceforth as KH to avoid the special character) et. al. supplied Frontiers in Zoology (FiZ) with the paper that is the subject of my inaugural entry. This paper is full almost to bursting with important information, which may be why it was one of the top-accessed articles from FiZ. Before I delve into the real substance of this article, I will attempt to convey why I feel it is important both in a general sense and to me personally.

KH’s article contains valuable insights not only in the specific realm of malacology (and the even more specific realm of gastropod taxonomy) but it carries larger implications for one of my personal favorite concepts in biology. One of evolutionary biology’s “big questions that we have a very good answer for” involves the transition from prokaryotic life (organisms like bacteria, which lack organelles and contain their DNA in a circular loop) to eukaryotic life (organisms that possess or are cells that contain membrane-bound organelles with specific functions).

Lynn Margulis first proposed the idea that became known as the Serial Endosymbiotic Theory (SET) in the 60s. This theory claimed that some important ultrastructural features of eukaryotic cells arose from a long-held mutual symbiosis in our very ancient ancestors, and was specifically formulated to address the origination of plastids and mitochondria (in plants and animals, respectively).  Evidence for this theory began to pile up (one of the kickers being that both plastids and mitochondria both contain their own DNA plasmids), and it is now generally accepted that plastids and mitochondria arose from an early symbiotic incorporation of smaller prokaryotes being phagocytized but left intact because they transferred more nutrients alive than they would had they been consumed. Over time these symbionts garnered more molecular support from their hosts in the form of protection proteins and other cellular necessities in exchange for energy production, eventually giving rise to the modern organelles we know today. (For those curious, the mitochondrion is believed to have come from an alpha-proteobacteria, and the plastid is believed to have descended from a small cyanobacterial ancestor).

All that being said, the only evidence science really had for this theory was (mostly) molecularly based. It is very difficult to observe the process of endosymbiosis in a laboratory setting without a model organism (as most creatures immediately digest anything they phagocytize). However, marine biologists rushed to save the day with the discovery of Elysia viridis, one of many opisthobranch gastropods (sea slugs) in the taxon Sacoglossa. Sacoglossans are extremely interesting metazoans in that they are possibly the only known instance of functional plastid integration (known as kleptoplasty).  The level of photosynthetic ability of the integrated plastids varies between species, but some are able to survive for months only on the nutrients derived from the photosynthesis of their plastid symbionts (according to KH’s paper). Kleptoplasty in the Sacoglossans has not been examined from an evolutionary biology perspective until now.

KH hypothesized three things in her study: 1. that kleptoplasty is a key character to the Sacoglossans, 2. that there were different stages in the evolution of kleptoplasty and that derived species are more well adapted to photosynthesizing through integrated plastids 3. a correlation between photosynthetic activity and sequestered food items existed.  To accomplish these goals her team constructed a robust molecular phylogeny of approximately 63 nominal species of Sacoglossan as well as conducting experiments to test the photosynthetic rates for 29 species of sacoglossan (with a sample size of 189 specimens). The molecular phylogeny revolved around the mitochondrial 16s rDNA and coxI genes and partial sequences from the nuclear 28S rDNA gene. Photosynthetic activity was measured by calculating the maximum yield of fluorescence from chlorophyll a (which is contained in photosystem II) a day after the animals were removed from their food source (the algae the plastids originated from).

The phylogenetic analysis supported two lineages of Sacoglossans: the shelled Oxynoacea and the shell-less Plakobranchacea (which contains Plakobranchoidea and Limapontiodea).  Deeper familial relastionships were complex and unresolved. The photosynthetic activity data returned three distinct levels of activity: 1.non-functional retention, 2. short-term functional retention (with functional activity up to 14 days, but generally around 7 days) and 3. long-term functional retention (with plastid functionality even after 20 days).  Interestingly, out of the three groups of Sacoglossans (Oxynoacea, Plakobranchoidea, and Limapontiodea) only Plakobranchoidea species retained any plastid functionality. The two other groups DID retain plastids, but exhibited no photosynthetic activity.

Combining these two sets of data revealed very interesting things about the evolution of functional chloroplast retention.  KH concluded after applying a Bayesian estimate of the probability of a particular phylogenetic node having a particular trait (i.e. one of the three levels of photosynthetic activity) that short-term retention evolved once (in the least common ancestor of the Plakobranchoidea) , and that all instances of long-term chloroplast retention evolved independently.  While several Plakobranchoidean species exhibit no functional chloroplast activity, KH determined this was a result of the trait being lost in that evolutionary lineage.

This was a lot, and your head is probably spinning by now, so I will take this opportunity to explain why this paper is so important to the SET. Dr. Händeler’s data is important because she was able to demonstrate strong evidence in support of her second hypothesis, which postulated that chloroplast retention occurred in stages. The first stage is short-term retention of the chloroplasts, which happens by “accident”, according to Dr Händeler. She proposes that the first step in functional retention was the loss of the ability to rapidly digest plastids by the gastropod. This loss of rapid digestion allowed the slug to receive nutrients from the plastid as it was slowly broken down. The second step is long term retention, and in order for this to occur not only must the first step have occurred, but in addition the organism must actively work to inhibit the destruction of incorporated plastids, whether through support via protein synthesis or through protection from overlong exposure to solar radiation. Astute readers will take note that this is one of the “big deals” in evolutionary biology- random mutations creating a trait that natural selection can act on.

Not only do some Sacoglossans actively engage in behaviors designed to (possibly) limit the plastid’s exposure to harmful solar radiation, at least one species (Elysia chlorotica) has incorporated (through horizontal gene transfer) genes into the slug’s DNA that code for essential photosynthetic proteins. Whether or not ALL Sacoglossans that exhibit long-term retention have incorporated similar genes remains to be seen, and research is ongoing to examine this problem.

With this information, biologists not only have an example of symbiotic organelle incorporation coupled with retention of functionality (and host genetic support of the organelle!) but now also have a powerful clade of model organisms that can be used to examine deeper aspects of the serial endosymbiotic theory. And, to top it all off, Dr Händeler resolved some phylogenetic disagreements in the Sacoglossans. All in all, this paper is amazingly important to evolutionary biologists, and once picked apart, to anyone interested in evolutionary theory.

All material used in this article was taken either from Open-Access journals or from the author’s own brain. Credit for the paper goes to Katharina Händeler et. al., who can be reached by following the link to the paper below.

Händeler K. et. al. Functional Chloroplasts in Metazoan Cells- A unique evolutionary strategy in animal life. Frontiers in Zoology 2009, 6:28 doi:10.1186/1742-9994-6-28



Notes
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