Giardia Bares All: Parasite genes reveal long sexual history

Science News Online ^ | Jan. 29, 2005 | Christen Brownlee

[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.

[js1138]

A lot has changed in 40 years, and a lot has remained the same. Creationists don't care a hoot about molecular biology and the infighting that takes place among those at the bleeding edge.

What the creationist/ID crowd cares about is common descent, which has only been strengthened in the last 145 years.

[jwalsh07]

The Nylon Bug

[furball4paws]

"Completely unused code requires more energy for genetic copying, yet delivers, by definition, no benefit to the organism."

Actually, Guv, Southack is right, only very marginally, on this. I can think of some environments where blowing ATP on conserving DNA that is not used would put a real squeeze on an organism (thermophilic organisms growing at the expense of coal gas combustion emmissions, for example). However, this would not be true of Giardia - it's got plenty of plenty when it's inside you. And that goes for the vast majority of organisms. The minor "waste" of energy (since that what it all boils down to) would probably not put any organism out of existence in competition with other organisms in its niche. It would take something more substantial.

[furball4paws]

Guy (and ladies, too, of course):

It's a rare day when I can discuss with friends things that I really know a lot about.

Thank you all; it has been a wonderful afternoon/evening.

[js1138]

A couple of things: one, natural selection alters the frequence of alleles regardless of their origin; The probability estimate at your source is bogus. It relies on the (missing) certainty that all the alleles had to mutate, and no intermediate step confers any advantage.

It seems to me that many of these altered amino acids are essential to the catalytic effect of the enzyme. How many, I don't know.

[AntiGuv]

To be sure, I didn't say Southack wasn't right. I said: "So what?"

Southack was wrong by his insinuation that this undermines or contravenes the premise of natural selection - through his elliptical hinting that natural selection would predict a different outcome. By illustration, that is ultimately the same kind of nonsense as the illogic behind this question: If humans descended from monkeys, why are there still monkeys?

[NRA2BFree]

If you ever get it, YOU will.

I DID get it. LOL I got it from drinking water from a fountain in a 'state' recreation park. It was absolutely horrible. I lost 14 pounds in a week, and that was all water. They had a hard time diagnosing it because it was not common to the area that I lived in.

When they couldn't find the cause they asked me if I had been camping anywhere that I might have got into water that was bad. When I told them where I had been they tested me to see if I did have Giardia, and I did. The medicine cleared it right up. You would think since it's known to be in that area that they wouldn't put water fountains for people to drink from. Needless to say, I NEVER want to have that experience again. :)

[jwalsh07]

"His second point is just wrong. It seems that he did not read what I wrote about the number of mutations that have to occur. He thinks that I wrote that 47 amino-acid differences separate the nylon bug from its evolutionary predecessor. 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.”

He also mistakenly thought that my calculation required all 6 to occur simultaneously. The calculation only assumes that they occurred within 30 years and that there is no selection for any combination other than the correct 6." Spetner

[Ichneumon]

["If you read the page at my source, you will find a discussion of bacteria that eat nothing but Nylon. Find that in your pre-existing genes."]

Pilgrims had nothing about eating corn in their genes either,

Sure they did. They had genes which enabled them to eat and digest a wide range of sugars, proteins, and fats.

Since corn consists of these same materials, the Pilgrims' genes most certainly *did* "have something about eating [lots of stuff, including corn] in their genes". (If you still want to disagree, feel free to point out the "corn-eating gene" which indigenous Americans have which Europeans don't have...)

The same can't be said about genes for eating nylon, however, especially since nylon didn't even *exist* before the 1930's (it's a man-made substance). And that's by no means the only example (e.g. Evolution of Enzymes for the Metabolism of New Chemical Inputs into the Environment).

[js1138]

I just set up the pins. I rely on you to knock 'em down.

[PatrickHenry]

They had genes which enabled them to eat and digest a wide range of sugars, proteins, and fats.

The fact that the pilgrims could live off the land supports the theory of common descent. Otherwise, if we crossed an ocean and encountered a whole new continent of "specially created" life, it should be expected that much of it would be totally inedible. Granted, this isn't "strong" support, but if things had turned out otherwise, it would be strong evidence against common descent.

[Southack]

"Granted, this isn't "strong" support, but if things had turned out otherwise, it would be strong evidence against common descent."

Oh please. Such "evidence" would be widely disputed by today's Darwinists, lets not go there.

Instead, lets go for evidence that *won't* be disputed: code skipping.

Pick 3 species. Find genetic code that exists in what Evolutionary Theory would say have been in both an earlier and a modern species, but that skips entirely a species that Darwinists claim came in between the two.

Evolutionary Theory completely and totally breaks down in the face of genetic code skipping. DNA that skips an entire intermediate species falsifies Evolution...and it can't be argued otherwise by even the most rogue-ish and obstinate of Darwinists.

[furball4paws]

I know, I know, but he's better than most.

[furball4paws]

I take that back.

[Southack]

Pilgrims had nothing about eating corn in their genes either...

"Sure they did. They had genes which enabled them to eat and digest a wide range of sugars, proteins, and fats. Since corn consists of these same materials, the Pilgrims' genes most certainly *did* "have something about eating [lots of stuff, including corn] in their genes" ... The same can't be said about genes for eating nylon, however, especially since nylon didn't even *exist* before the 1930's..."

Oh please. Nylon is just a polyamide, and finding an enzyme that can cleave to a polyamide is not surprising since natural amides are reasonably common.

[Southack]

Completely unused code requires more energy for genetic copying, yet delivers, by definition, no benefit to the organism. - Southack

"So what?" - Antiguv

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...

[Doctor Stochastic]

I taught genetics over 40 years ago. There was no "one gene, one function" perception at that time. The mapping from "function" to "gene" was varied; sometimes one-to-one, sometimes many-to-one, sometimes one-to-many.

[Southack]

"The mapping from "function" to "gene" was varied; sometimes one-to-one, sometimes many-to-one, sometimes one-to-many."

Interestingly enough, we find precisely that same situation when analyzing database keys and computer programming code today...

[midnightson]

Perhaps they'll develop an oral contraceptive to prevent it's replication in your intestines- could prevent the over-prescribing of antibiotics.

[furball4paws]

Depending on the bug, I could do those 47 amino acid changes in about 6 months, so 6 is a piece of cake. This would use modern molecular biology techniques. To use only "natural" mutational events, it might take a year or two.

The point is well taken that you cannot determine probability based on 6 mutations all occurring at one time in one bug. However, given that the average bacterial culture contains 1,000,000,000 bacteria per ml, a liter would have 1,000,000,000,000 bacteria. An average mutational rate of 1/1,000,000, means that there are 1,000,000 mutants in a liter of bacteria. Digest these numbers and you will see how easy it is to get to a nylon eating bug in a short period of time.