Endosymbiosis - A Tale of Two Relationships
This past weekend, I had the privilege of attending the AAAS Annual Meeting in Boston, Massachusetts. While there, I attended a symposium entitled, “How Symbiosis, Horizontal Gene Transfer, and Evolution Call for an Extended Synthesis”. The point that this title attempts to communicate is this: We’re all familiar with what the endosymbiotic theory tells us about the origin of mitochondria and chloroplasts…
WOAH. Hold on – are we? The endosymbiotic theory was originally put forth by a Russian botanist who was familiar with previous observations that plant chloroplasts resembled free-living cyanobacteria. According to the theory, which is still viewed with skepticism by some, certain cellular organelles (mitochondria and chloroplasts) originated as free-living bacteria that were then engulfed by larger cells. The two organisms then begin to exist in symbiosis – the living together of unlike organisms.
Of course, the human-centered definition of this phenomenon conjures images of oxpeckers and elephants, or parasites that only take from the host, of words like benefits, cost, and free-loading.
Let’s redefine endosymbiosis, shall we? To mean the acquisition of one by another, and the subsequent long-term assimilation of the two, such that new structures and metabolisms emerge, creating, in essence, a new organism. We are all, in fact, composites on non-self – of human genes, and acquired bacterial genes.
Great, now that we’ve cleared that up, I will continue where I left off…
We’re all familiar with what the endosymbiotic theory tells us about the origin of mitochondria and chloroplasts, but horizontal gene transfer studies have demonstrated that the lines we’ve drawn between biological individuals may be incorrect. This has consequences for how we define the units of evolution, how we draw the tree of life, and how we conceptualize evolution.
The symposium argued for a new method of “carving up nature”, defined by our understanding of interactions between unrelated organisms.
This being an unfamiliar topic to me, I wondered, what could these interactions be? There are some that are familiar. Termites eat wood, but cannot digest the cellulose without the help of protozoans living in their gut. The stomach of a cow also contains microbes that do the same.
Looking into this, I discover two bizarre relationships that I thought might interest you all:
Besides masquerading as an underwater leaf, this algae-sucking sea slug uses a horn-like structure to puncture filamentous photosynthetic algae and take in their chloroplasts, which then remain function beneath the skin for almost a year! The slug survives on the sugars produced by the stolen photosynthetic organelles!
How do the organelles continue to produce sugars without the algal genome, you ask? Researchers are finding that many of the algal genes have integrated into the slug’s genome, essentially defining a brand new, solar-powered slug!
Salamanders do it too! The spotted salamander prefers single-celled freshwater algae, but it too commandeers the algae’s photosynthetic products. The algae appears to be digested inside the cells of adult salamanders, but researchers at Dalhousie University in Halifax, Canada were able to amplify algal RNA from adult reproductive tracts, which suggests that the benefits of the endosymbiotic relationship may be passed on to the next generation.
So what are these? Do these relationships represent entirely new organisms that require us to draw another branch on our tree of life?
Either way, it’s bizarre, right? How long do you think it’ll take humans to evolve the ability to injest algae and become solar-powered?
1. S.K. Pierce et al., “Transcriptomic evidence for the expression of horizontally transferred algal nuclear genes in the photosynthetic sea slug, Elysia chlorotica,” Mol Biol Evol, 29:1545-56, 2012.
2. R. Kerney et al., “Intracellular invasion of green algae in a salamander host,” PNAS, 108:6497-502, 2011.