Life is very strange
Sep. 9th, 2007 12:04 am![[personal profile]](https://www.dreamwidth.org/img/silk/identity/user.png){kind=small}
lprovenI don't mean my life. I mean life on Earth.
I was reading today about efforts to create artificial life. This led me to read up on Mycoplasma genitalium, the simplest known free-living organism in the world. It's a parasitic bacterium that lives up people's willies.
But it's not the simplest bacterium. Oh no, that is currently Candidatus Carsonella ruddii. This is on its way to becoming an organelle, like a chloroplast or mitochondrion.
(These were once, a billion years ago or so, free-living bacterium-like organisms, but they got symbiotically involved with bigger cells and now play the role of functional parts of a bigger machine. They still have their own genome, though - their own DNA, quite separate from the main cells. Every cell in your body, every one of the billions of them, contains within it swarms of tiny little cells doing their own independent thing that are not actually related to you - they're an entirely different kind of life form to you, more distantly related to we mammals than, say, a mushroom. Bizarre, huh?)
C. ruddii has lost quite a lot of its genome. It can't survive alone any more, but it's got more of its own machinery left than chloroplasts or mitochondria do. It lives inside a special organ inside certain sap-sucking insects, something called a bacteriome. It's an organ which exists to provide a nice cosy place to live for symbiotic bacteria which live on the same plant sap as the insects. The thing is, plant sap is just sugary water. It's not very nutrious. We multicellular animals, we mammals and insects, need stuff like vitamins and so on to live and we can't make all of them ourselves. But bacteria, living alone as single cells, are made of sterner stuff. They have to be more self-sufficient. So, the sapsuckers keep an internal colony of bacteria going, and the bacteria turn sugar water into the vitamins and so forth that the sap-sucker needs to live on.
So far, so strange. This is a more intimate relationship than that that other multicellular organisms - like termites and cows - which eat things they can't live on have with their intestinal flora. Those lightweights just let the bugs break down stuff the hosts can't digest on their own.
Leafcutter ants, by contrast, invented agriculture millions of years before our first upright and less-hairy ancestors evolved, and farm fungi in their nests, which do the digestion for them.
But the sap-suckers do something cleverer. They use their internal symbionts not to break down but to synthesize new nutrients from simple sugars.
But wait. There's more.
There's a family of sap-sucking insects whose bacteriomes, an organ inside their own body, does not actually share their genetic makeup. An organ inside these things' bodies is actually a sort of relative, an internal non-identical twin which functions as an organ.
This is enough to make a biologist's head spin, but it gets weirder still. This organ has a bizarre genetic makeup all its own, unique in nature, because it's made up of polar bodies.
What's a polar body, you may well ask.
Well, when multicellular organisms have sex, both parents contribute half a set of chromosomes. The two halves make a whole. This means that mum & dad both have to make special sex cells with only half a set of chromosomes each. This happens through a special kind of cell division, called meiosis. The ordinary kind, when a cell just copies itself, is called mitosis.
To make an egg cell, a pre-egg-cell divides in half in a special way to make unequal halves. One part is really big, gets most of the parent and all the machinery, and the other is tiny and contains the leftover 2nd-half-a-set of chromosomes. This is the polar body. Normally, they're destroyed.
But in certain scale insects, the polar bodies combine with some other cells to make a special new kind of cell with, rather than one or two sets of chromosomes, five sets. Five!
And this bizarre thing is what grows into a bacteriome inside a scale insect.
JBS Haldane had it right: "the universe is not only queerer than we suppose, but queerer than we can suppose."
I was reading today about efforts to create artificial life. This led me to read up on Mycoplasma genitalium, the simplest known free-living organism in the world. It's a parasitic bacterium that lives up people's willies.
But it's not the simplest bacterium. Oh no, that is currently Candidatus Carsonella ruddii. This is on its way to becoming an organelle, like a chloroplast or mitochondrion.
(These were once, a billion years ago or so, free-living bacterium-like organisms, but they got symbiotically involved with bigger cells and now play the role of functional parts of a bigger machine. They still have their own genome, though - their own DNA, quite separate from the main cells. Every cell in your body, every one of the billions of them, contains within it swarms of tiny little cells doing their own independent thing that are not actually related to you - they're an entirely different kind of life form to you, more distantly related to we mammals than, say, a mushroom. Bizarre, huh?)
C. ruddii has lost quite a lot of its genome. It can't survive alone any more, but it's got more of its own machinery left than chloroplasts or mitochondria do. It lives inside a special organ inside certain sap-sucking insects, something called a bacteriome. It's an organ which exists to provide a nice cosy place to live for symbiotic bacteria which live on the same plant sap as the insects. The thing is, plant sap is just sugary water. It's not very nutrious. We multicellular animals, we mammals and insects, need stuff like vitamins and so on to live and we can't make all of them ourselves. But bacteria, living alone as single cells, are made of sterner stuff. They have to be more self-sufficient. So, the sapsuckers keep an internal colony of bacteria going, and the bacteria turn sugar water into the vitamins and so forth that the sap-sucker needs to live on.
So far, so strange. This is a more intimate relationship than that that other multicellular organisms - like termites and cows - which eat things they can't live on have with their intestinal flora. Those lightweights just let the bugs break down stuff the hosts can't digest on their own.
Leafcutter ants, by contrast, invented agriculture millions of years before our first upright and less-hairy ancestors evolved, and farm fungi in their nests, which do the digestion for them.
But the sap-suckers do something cleverer. They use their internal symbionts not to break down but to synthesize new nutrients from simple sugars.
But wait. There's more.
There's a family of sap-sucking insects whose bacteriomes, an organ inside their own body, does not actually share their genetic makeup. An organ inside these things' bodies is actually a sort of relative, an internal non-identical twin which functions as an organ.
This is enough to make a biologist's head spin, but it gets weirder still. This organ has a bizarre genetic makeup all its own, unique in nature, because it's made up of polar bodies.
What's a polar body, you may well ask.
Well, when multicellular organisms have sex, both parents contribute half a set of chromosomes. The two halves make a whole. This means that mum & dad both have to make special sex cells with only half a set of chromosomes each. This happens through a special kind of cell division, called meiosis. The ordinary kind, when a cell just copies itself, is called mitosis.
To make an egg cell, a pre-egg-cell divides in half in a special way to make unequal halves. One part is really big, gets most of the parent and all the machinery, and the other is tiny and contains the leftover 2nd-half-a-set of chromosomes. This is the polar body. Normally, they're destroyed.
But in certain scale insects, the polar bodies combine with some other cells to make a special new kind of cell with, rather than one or two sets of chromosomes, five sets. Five!
And this bizarre thing is what grows into a bacteriome inside a scale insect.
JBS Haldane had it right: "the universe is not only queerer than we suppose, but queerer than we can suppose."
