Re: Primary endosymbiosis caught in the act

On 18/04/2024 19:21, Arkalen wrote:

On 18/04/2024 19:04, Chris Thompson wrote:

Is it true that primary endosymbiosis is thought to have happened only twice? I'm a little dubious about that. But this is still cool.
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"Scientists have caught a once-in-a-billion-years evolutionary event in progress, as two lifeforms have merged into one organism that boasts abilities its peers would envy. Last time this happened, Earth got plants."
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https://newatlas.com/biology/life-merger-evolution-symbiosis-organelle/
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 One bit I'd found interesting that I didn't know when this article (or another one about the same paper) was posted earlier was that apparently symbioses of plants with N2-fixing bacteria date back to the Cretaceous - both for algae and land plants even though they do it completely differently!
 I'm also dubious about it having happened twice, I thought plants had several plastids corresponding to more than one endosymbiotic event. But maybe they simplified things for effect, mitochondria and chloroplasts are the big ones everyone knows about. Of course (of course).

Above endosymbiosis is qualified as primary endosymbiosis. There are quite a few instances of secondary and tertiary plastids, but the consensus is that primary plastids only originated once. (In some analyses Archaeplastida is not monophyletic, with Cryptista, or even (IIRC) Hacrobia nested within it, thought Wikipedia currently has Cryptista as the sister group to Archaeplastida. If it turns out to be paraphyletic I can see three alternative hypotheses: 1. early loss of plastids in Cryptista, followed by acquisition of secondary plastids in some or all of Cryptista - I haven't checked the plastid status of basal cryptists. 2. independent endosymbiosis of closely related cyanobacteria. 3. an ancient secondary endosymbiosis of which no traces have been identified.)

 Checking that famous drawing of the tree of life with different endosymbiosis events that I thought included several, it does seem to only have the mitochondria and chloroplast one after all:
 https://www.researchgate.net/figure/Schematic-Diagram-of-Symbiosis-in-the-Tree-of-Life-and-the-Union-of-the-Bacterial-and_fig1_332956942
 

On the other hand the cyanelles of some Paulinella species are considered organelles by at least some people. The cyanelle has lost 2/3rds of its genome, with some genes transferred to the nucleus.
I think it possible that other cases of organelles originating by endosymbiosis have occurred over the history of life, and the lineages involved lost to extinction. Not that I can see any way to test this hypothesis.
--
alias Ernest Major