Posted on 02/07/2005 1:15:22 PM PST by js1138
Giardia Bares All: Parasite genes reveal long sexual history
Christen Brownlee
While it hasn't yet been caught in the act, a single-celled parasite has been ready for sex for billions of years. A new research finding provides evidence that sexual reproduction started as soon as life forms that have nuclei and organelles within their cells branched off from their structurally simpler ancestors.
The parasite Giardia intestinalis is well known for causing a diarrheal disease that animals and people contract after drinking contaminated water. Many researchers consider this species to be one of the most ancient living members of the eukaryote, or true nucleus, lineage. However, unlike most eukaryotes, G. intestinalis and its relatives have been long considered to reproduce only asexually—by division into two identical cells.
To determine when reproduction via sperm and eggs originated, John Logsdon of the University of Iowa in Iowa City and his colleagues took a close look at G. intestinalis' mysterious reproductive life. They focused on the hallmark of sexual reproduction known as meiosis, the process that halves the number of an organism's chromosomes to make gametes such as sperm and eggs. Among available data on the G. intestinalis genome, the researchers searched for genes similar to those that control meiosis in other eukaryotes, including plants, animals, and fungi.
The researchers' analysis revealed that G. intestinalis possesses genes similar to those used for meiosis by other eukaryotes. At least 5 of those genes function only in meiosis, and 10 others have roles both in meiosis and other functions, Logsdon's team noted in the Jan. 26 Current Biology.
Although the researchers didn't establish that G. intestinalis reproduces sexually, Logsdon notes that a discreet sex life might turn up after further study. "Lack of evidence is not evidence of lack," he says.
On the other hand, the findings suggest that meiosis was established early in eukaryotic evolution, making sexual reproduction "a very central feature of being a eukaryote," says Logsdon. Bacteria and other simple-celled life forms, or prokaryotes, don't make eggs and sperm.
All living eukaryotes, including G. intestinalis, share numerous cellular features and processes that aren't seen in prokaryotes. According to Andrew Roger of Dalhousie University in Halifax, Nova Scotia, establishing that all eukaryotes are capable of meiosis could "make the evolutionary transition from prokaryote to eukaryote even more difficult to sort out.
"A lot had to happen when eukaryotes evolved. Why aren't there any intermediate stages of this process alive today? Did all the intermediate forms go extinct, and why?" Roger asks.
Logsdon says that he and his team plan to continue their research by looking for meiosis genes in other eukaryotes thought to be asexual.
References:
Ramesh, M.A., S.-B. Malik, and J.M. Logsdon Jr. 2005. A phylogenomic inventory of meiotic genes: Evidence for sex in Giardia and an early eukaryotic origin of meiosis. Current Biology 15(Jan. 26):185-191. Abstract available at
http://dx.doi.org/10.1016/j.cub.2005.01.003.
Sources:
John M. Logsdon Jr. University of Iowa Department of Biological Sciences 310 Biology Building Iowa City, IA 52242-1324
Andrew Roger Department of Biochemistry and Molecular Biology Dalhousie University Halifax, NS B3H 1X5 Canada
http://www.sciencenews.org/articles/20050129/fob1.asp
From Science News, Vol. 167, No. 5, Jan. 29, 2005, p. 67.
Unfortunately, my terseness is usually indicative of my impatience, and I'd be far more effective (and a bit less of a jerk sometimes) if I were a more patient debater. =)
I haven't read the entire thread so if you did qualify that statement in an earlier post then I apologize for my misunderstanding.
It was perhaps easy to miss, but the IF was there (really, I promise)!
So...technically, an organism would be genetically superior if it had all of its functionality without also having unused genetic code as baggage. Natural Selection *should* favor those without the unused code...
...and even "zero-to-one", as in the case of pseudogenes.
What is an "older species?"
If all species descended from the same cell, all would be equally "old," in some sense. When a new species branches off from an old one, it does not do so in a pristine state, it carries over most of the mutations the last one had.
So, the basic point is that as I said before: It's an open question which configuration, if either, would be favored by natural selection because it depends on the functional advantages of the difference. Natural selection would favor whichever one were most suited to whatever circumstances later arose, and would favor neither so long as there were no survival advantage between them.
If I actually had to venture a guess, I would guess that the G. intestinalis version with "junk" DNA would ultimately be favored, for the reason Ichneumon described:
Furthermore, in evolution "yesterday's junk" can become tomorrow's genetic innovation, as various bits of DNA get reshuffled, recombined, and mutated. There may be a short-term (and again, *tiny*) energy benefit in taking out the genetic trash, but keeping it around for future adaptive "spare parts" is probably well worth it in the long run.
But that can't be anything but conjectural on my part. It would depend on what circumstances the parasites were confronted with. Period. In their current niche, it'd almost certainly be a neutral distinction.
Besides the 4 unique "root" DNA lines, different branches of descent should have *some* different junk DNA, yes?! New species from a newer branch should have predictably different levels (given orders of magnitude) of junk DNA if Common Descent is actually in play, correct?
We wouldn't expect the levels to junk DNA to jump *both* up and down wildly if Evolutionary Theory was responsible, yes?!
I missed your post among the fast and furious debate. I'm no expert. I know that the evolution of sex has been a source of debate, so it seemed likely that this finding is important. The experts are being cautious about what it means.
Yes, we would expect that.
Looking at the "junk DNA" in different species, we should be able to create a phylogenic tree that resembles phylogenic trees created on the basis of other measures of genetic similarity on and observed characteristics. These trees might not match up perfectly, that's okay, but they should be very, very similar.
New species from a newer branch should have predictably different levels (given orders of magnitude) of junk DNA if Common Descent is actually in play, correct?
I don't understand this. A "newer" species (one that branched off more recently) would still have most of the junk DNA from the older species. We might not expect it to have less new junk DNA than a species that branched off earlier, because their common ancestor would be accumulating junk DNA between the time both species branched off.
It's possible that junk DNA would accumulate more quickly during periods of rapid mutation/evolution, but I don't know if that's the case.
That presumes that most junk DNA was accumulated prior to the branching off into newer species. Do we know that to be the case, or do species accumulate junk DNA on a more regular basis?
Reminds me of a science fiction story I read long ago, and I can't remember the title or author. It's the future, of course, and man has gone extinct -- as he so often does in these tales. What remains are androids which we had created. They reproduce and all that stuff, and they know all about man, and they have a cloning program to try to re-create man from old samples.
The androids had been careful to keep all the material and circuitry in their own bodies, and especially their brains, even though they couldn't understand all of it. They figured if man had put it there, it should stay. (And now you see what it was in your post that triggered this response.) Large sections of their brain circuits were a mystery to them, but they seemed harmless and non-functional.
Then they create a man. They had a woman too. (I don't recall why, but these humans were adults, and could speak. Maybe they had been frozen away somewhere.) One of the androids comes into the humans' room, and the man says: "Oh, here comes another one. What do you want?"
The old, mystery circuits in the android's brain click in. He bows his head, low, and responds: "I want only to serve you, master."
I'm just *dying* to see exactly how he calculated that alleged "probability"...
It's highly suspicious that Spetner just tossed the number out without even a hint of what sort of math he used.
And any number that freakin' low for "six mutations in 30 years" in bacteria (which mutate at the drop of a hat, and have mind-bogglingly large populations and reproductive rates) seems patently bogus on its face.
Jwalsh07, would you care to email Spetner and ask to see his calculations? I can't find a recent reference to an email address for Spetner (he doesn't even seem to have a homepage or a faculty page anywhere), but lspetner@alum.mit.edu worked as of several years ago (no clue if still does).
Interesting. Please elaborate. I remember reading something about this a year or so ago, but I've looked and there's not much information available.
Thanks, MM
"[Dembski writes:] "Far from saying that all 47 have to be precisely specified, I said, “…if only 6 of these 47 mutations were essential for the evolution, the probability of achieving it in 30 years is about 3 x 10^-35.”"
I'm just *dying* to see exactly how he calculated that alleged "probability"... "
Actually I mentioned this in an earlier post. If you assume a 10-6 mutation rate for most mutations (a reasonable guess in bacteria) and you posit 6 mutations occuring at the same time in one bug then you get 10-6x10-6x10-6x10-6x10-6x10-6 = 10-36 probability. Just shows you how people will twist stuff to make it fit their wishes.
Of course, it could take as little as 6 days to get those 6 mutations, one at a time.
You mean like PCB's?
Or fluorinert, the coolant in old Cray supercomputers?
Gives a whole new meaning to computer bugs, that does! /grin
If the cost of keeping the code around is extremely low, then it probably won't get selected out any time soon.
Just like real code...there's always time to do it over, but there's never time to do it right in the first place!
You mean like PCB's?
Or fluorinert, the coolant in old Cray supercomputers?
Gives a whole new meaning to computer bugs, that does! /grin
PCB's you bet. Fluorinert is a new on for me. What's its chemical make up? (It's not in my Merck Index either).
'Twas a trade name for a fluoro- or chlorofluoro- carbon
which circulated within (say) the Cray-2 in direct contact with the circuit boards to dissipate heat.
I'll poke around my old files this evening or so and see
if I can find more for you.
PS Two questions.
a) what the hell do those PCB-eating bugs excrete?
b) I imagine metabolizing conjugated systems leads to
all kinds of free radical production; how do they
deal with it?
and a third:
c) Has anyone thought of getting "two for the price of
one" for the PCB-eating bugs and engineering further
enzymes so they excrete industrially useful
feedstock or intermediates?
Cheers!
Wouldn't you have to account for the ratio of "favorable mutations" to all mutations? Any idea what that ratio is in bacteria?
I would love to but I can't find him. More to the point, I think it more likely that he would answer a felow scientist rather than a blue collar small businessman. :-}
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