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The Puzzle of Plastid Evolution
October 20, 2012 4:13 AM   Subscribe

The Puzzle of Plastid Evolution: A comprehensive understanding of the origin and spread of plastids remains an important yet elusive goal in the field of eukaryotic evolution. Combined with the discovery of new photosynthetic and non-photosynthetic protist lineages, the results of recent taxonomically broad phylogenomic studies suggest that a re-shuffling of higher-level eukaryote systematics is in order. Consequently, new models of plastid evolution involving ancient secondary and tertiary endosymbioses are needed to explain the full spectrum of photosynthetic eukaryotes. [Full Text HTML] [Full Text PDF]

Our planet is teeming with photosynthetic life. The textbook version of how this came to be is relatively straightforward: oxygenic photosynthesis first evolved in the ancestors of modern cyanobacteria more than two billion years ago [1] and their light-harvesting capabilities were subsequently exploited by eukaryotic (nucleus-containing) cells through the process of endosymbiosis [2 PDF] and [3 PDF]. Co-evolving with their opportunistic hosts, these intracellular cyanobacteria were eventually transformed into bona fide organelles — plastids — ultimately giving rise to the plants and algae that surround us today. Easy, right?

The basic outline of this evolutionary scenario is correct, but the reality is much, much more complicated. Photosynthetic eukaryotes are astonishingly diverse in form and function, a fact that complicates efforts to discern their evolutionary history. Eukaryotic phototrophs can be macroscopic (e.g., land plants, seaweed) or microscopic (e.g., the unicellular green alga Chlamydomonas), sessile or motile (or both), and given a bit of sunlight, they thrive in virtually any habitat imaginable, terrestrial and aquatic, from the equator to the poles. This vast diversity actually makes sense when one considers that the term ‘algae’ can be applied to organisms that are not specifically related to one another. In addition to simple vertical inheritance, plastids have on multiple occasions spread laterally between distantly related groups of eukaryotes. Having evolved ∼one billion years in the past [4], today's plastids weave a tangled web across a very large fraction of the eukaryotic tree. Consequently, large sections of the puzzle of plastid evolution remain unassembled.

This article focuses on the latest advances in our understanding of the origin and spread of plastids. In particular, the merits and shortcomings of competing hypotheses about the evolution of plastids are discussed in light of a flood of new molecular, biochemical, genomic and phylogenomic data. Progress has been swift, but there are still many questions that need to be answered, and many newly discovered protist lineages that need to be investigated, before it can be said that the evolution of eukaryotic photosynthesis is understood with confidence.


Don't miss the very pretty trees.
posted by Blasdelb (8 comments total) 28 users marked this as a favorite

 
Blasdelb, you're my new favourite poster! I have no idea how you manage to find and read so many interesting articles, and still have time for food and sleeping. One small request: Can we have less italics? It's a bit hard to read four straight paragraphs. Quote characters or blockquoting would work as well, I think.

What are the current ideas about the sequence of gene transfer from nucleomorph to host nucleus vs. evolution of a protein import mechanism? It seems hard to imagine how either of these would be an evolutionary advantage independently, without the other.
posted by vasi at 9:04 AM on October 20, 2012 [1 favorite]


Thanks for the feedback, I've been trying to take up less space on the front page by abandoning my blockquoting habit. I'll go towards quotes. Really its that I love reading papers, I've only really started posting the really good ones after I realized that there was a lot of interest when I posted a curated collection of my favorite papers in microbial evolution in a comment.

"What are the current ideas about the sequence of gene transfer from nucleomorph to host nucleus vs. evolution of a protein import mechanism? It seems hard to imagine how either of these would be an evolutionary advantage independently, without the other."

This is really not my field, but for two years most of my drinking buddies were folks who specialized in answering exactly these sorts of questions about mitochondria and Trypanosomes. I'd bet RNA editing has a lot to do with it but here is the most recent review (PDF). I'm having a hard time figuring out if it is actually open access or if my university connection of just making things easy for me, but if it isn't just memail me with an email address.
posted by Blasdelb at 9:52 AM on October 20, 2012


Maybe you could use two blockquotes, one for the above-the-fold para and one for the rest.

As for the paper you linked, I'm supposed to have access to these sorts of things, but I still just get a 403 Forbidden if I follow the link. What's the title of the paper, I'll try looking it up.
posted by vasi at 10:35 AM on October 20, 2012


John M. Archibald. 2007. Nucleomorph genomes: structure, function, origin and evolution. BioEssays 29(4); 392–402.

and apparently there is a new one,

Moore CE, Archibald JM. 2009 Nucleomorph genomes. Annu Rev Genet. 43; 251-64.
posted by Blasdelb at 10:49 AM on October 20, 2012


It seems hard to imagine how either of these would be an evolutionary advantage independently, without the other

IANABiologist, but wouldn't it be likely for the host cell to natively have some proteins of use to the new endosymbiont, just because of shared ancestry and whatever? In which case protein import (and loss of genes from the plastid-to-be) could happen without any gene transfer.
posted by hattifattener at 11:27 AM on October 20, 2012


Short, you be killin' em. Don't quit.
posted by Buckt at 12:02 PM on October 20, 2012 [2 favorites]


As a former Biology major who never finished a degree but shoulda, I thank you for keeping me interested and inspired about cellular evolution. I'm going to go back to my obsessive nerd reading now...
posted by k8oglyph at 3:31 PM on October 20, 2012 [2 favorites]


Blasdelb: "John M. Archibald. 2007. Nucleomorph genomes: structure, function, origin and evolution."

Thanks, that link worked fine.

hattifattener: "wouldn't it be likely for the host cell to natively have some proteins of use to the new endosymbiont, just because of shared ancestry and whatever"

Good point, but it's my impression that the import mechanism can't just grab random useful proteins. Rather, they have to be mostly-unfolded proteins in order to fit through. They also usually need some sort of tag or marker to allow the endosymbiont to take them in. IANAB either though, so I may be very confused.
posted by vasi at 11:44 PM on October 20, 2012


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