Right!
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.
Right!
I agree. A more productive use of the time would be trying to find a quick cure for people infected with this bug. I've known some people who were infected with this organism. It's hard to shake off and your miserable while waitng for the drugs to have effect.
See post #67. It takes longer to diagnose than it does to treat it. :)
I suspect Southack may be referring to this:
http://nwcreation.net/articles/recombinationreview.html
"I know there is a mechanism called "back mutations" which can restore lost information such as your hypothetical gene 451. What I don't know is the rate at which such mutations occur. Any idea?"
It depends on what kind of mutation it was. A frame shift or base pair substitution could back mutate at frequencies as high as 10^-4 (more typically 10^-6 - 10^-7). If it's a deletion mutation, it ain't going to happen, ever, without the introduction of extra DNA from an outside source.
I can't think of any kind of mutation that "adds information to the genome". The only way I can see this happening is through genetic recombination (yes, bacteria have sex), via the mechanisms transduction, transformation and conjugation. You just can't stick a whole gene's worth of DNA into a cell without getting it from somewhere.
You are correct, but I've seen many hikers in the Sierras that have no clue about such things and they drink the water in lakes and streams.
A "favorable" mutation is one that increases the chances of having offspring. Unfavorable would mean that the chances of offspring were reduced. Neutral means that no tendence is introduced. These are all ex-post terms. Generally, one cannot give an ex-ante specification of which types of mutations fall into each catgory. A change in environment can change a mutation from favorable to unfavorable or vice versa.
Ding! Ding! Ding! We have a winner!
This "problem" is merely an artifact of your model. You seem to assume that the species you call B at present is the same as it was as the first branching point and that it must have lost gene 451 between the first and the second branching point.
However, the more likely scenario is the one where species B still has gene 451 after branching off from species A and only loses it after splitting off from species C.
This confusion could be avoided if you used different labels after each branching point, i.e.:
Species A branches into species B and C. Later, species C branches into species D and E.
Today we observe that species B and E both have gene 451 but not species D. This may be explained by the fact that species C lost the gene but somehow species E magically regained it or which is more likely, that species C still had gene 451, inherited it to both D and E but later D lost it. (Of course there is also the scenario where species A didn't have this gene and B and E 'evolved' it independently of each other, but this is even more unlikely)
For those creationoids who think there's a "gotcha" here, I suggest they think (and I mean "think") twice.
The apparent species skipping is just not so.
Your (BMCDA) post is quite good.
js1138 (?) also showed that the problem could well be associated with incorrect taxonomy, something that is always being updated and corrected. Science is not static.
The other possibility is that the gene is really not "missing", just cryptic or recessive or modifed (could even be post translation modification, or mutated simply so that it can revert to wildtype at a later stage.
All of these are much more probable and fit Occam's Razor much better that ID or some other mystical mush.
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