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Author | Topic: The origin of new alleles | |||||||||||||||||||||||
Fosdick  Suspended Member (Idle past 5530 days) Posts: 1793 From: Upper Slobovia Joined: |
RAZD wrote:
One thing important to this discussion is the role of meiosis and how that may play into the survival of new alleles that are passed along to the progeny by way of sex. There is a mechanical obstacle for new alleles in the first prophase of meiosis in the form of "crossing over," in which alleles may "jump" from one chromosome to the other. This may act as a homologous filter against new alleles and prevent them from eventually entering the gametes. The reasoning behind this is based on a well-reasoned theory that sex evolved to enable genes to escape their parasites (i.e., mutant codes). In this way at least some of new alleles could be treated as hostile to the genome. Recombination IS a mutation. Calling it something else does not stop it from happening. ”Hoot Mon
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Fosdick  Suspended Member (Idle past 5530 days) Posts: 1793 From: Upper Slobovia Joined: |
Please correct me if I'm wrong, but I was looking a crossing-over as a way to force processes such as nucleotide replacement and enzyme search-and-correction. These filter-like operations would effectively screen out mutated alleles. That way the genes avoid their parasites.
”Hoot
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Fosdick  Suspended Member (Idle past 5530 days) Posts: 1793 From: Upper Slobovia Joined: |
WK wrote:
I'm not a geneticist, only a reader in that field. My readings of sereval good thinkers (e.g., Hamilton, Dawkins, Margulis, Hartl) have tweeked my interest in the biological implications of allelic origination, sex, death, and "genetic parasitism" (i.e., transposons or plasmids as digital "parasites"). I have taken sex to imply that advantages accrue for the genes, but not necessarily for their organisms, if there are other means of reproduction besides simple cellular fission. Sex might possibly be that advantage, and meiosis might be where genetic escape strageties take place; fertilization, too. Crossing-over seemed to me like a good bet. But I could just as easily be tripping through the tulips here, because most of this area of genetics is highly theoretical. AS far as I understand it you are wrong, but you aren't really saying anything specific enough to tell you how wrong.Perhaps you are getting crossing-over confused with mismatch repair. Some of the same enzymes or related ones are involved in both processes but the two processes remain distinct. As far as an allele being parasitic goes, that would only apply to deleterious mutations since if the mutation was not deleterious then the relationship would be mutualistic or possibly commensal rather than parasitic. Nevertheless, I am persuaded by Hamilton's arguments that genes are strategic in their efforts to avoid hostile configurations and survive. If deleterious plasmids, for example, enter the genome, the genes may have better options to avoid them if they can segregate themeselves through conjugation other more-complex measures of sex. That's the direction I was limping toward in with my previous post. ”Hoot Mon
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Fosdick  Suspended Member (Idle past 5530 days) Posts: 1793 From: Upper Slobovia Joined: |
WK wrote:
I certainly could be. I was looking for some specific explanation that crossing over occurs as a means to enhance genetic survival. The mechanics of crossing over seem like a lot trouble just to scramble the genes around; something else might be involved. So from there I speculated, foolishly perhaps.
That's all moderatley tenable speculation but you did make one very specific claim that crossing over in some way acted as a filter for alleles during meiosis? Do you think you were mistaken about this? As I pointed out previously crossing over and gene conversion can lead to recombinations of genes which will cause any offspring to die early in development, but this is not the same as preventing them from becoming gametes in the first place.
Do you have a model or a theory to explain the evolutionary value of crossing over? ”Hoot
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Fosdick  Suspended Member (Idle past 5530 days) Posts: 1793 From: Upper Slobovia Joined: |
crashfrog wrote:
Frog, are you familiar with transposons? They are sometimes referred to as "mariner genes," but they are actually plasmids. The tsetse-fly genes we carry around are good examples. I'm not sure which organisms you're talking about (or which planet, for that matter) where plasmids enter the genomes of sexually-reproducing eukaryotes. ”Hoot
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Fosdick  Suspended Member (Idle past 5530 days) Posts: 1793 From: Upper Slobovia Joined: |
crashfrog wrote:
If you care to look it up, frog, you can find many references to mariner genes, transposons, plasmids, and neutral endosymbionts jumping around all over the place. You could start by considering this one: In an article in New Scientist (6/24/200) titled “Look Before It Leaps, Rob Edwards wrote:
...(I've never heard of them being called "mariner genes." "Jumping genes" is the common term.) They're genetic sequences that move from chromosome to chromosome within a cell. A lot of them are the result of viruses (retrotransposons), and about 45% of our genome is comprised of them and their remnants.Are you maybe just making things up? Why would we have tsetse fly genes? Insects aren't viruses; they don't inject their DNA into hosts. (The worst thing they do is inject the parasitic Trypanosoma protozoan into the host's bloodstream, causing the much-feared "sleeping sickness.") An ancient mariner might help genes jump species
You might be interested in what kinds of plasmids and transposons are jumping inside Barbara's lab these days. Check it out. A "JUMPING GENE" being used to genetically engineer organisms has crossed the species barrier at least seven times in evolutionary history, in one instance between flies and humans, according to a study commissioned by the British government. If organisms modified using this mobile element are released, there will be a risk of genes spreading to other species, the report says... ”Hoot
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Fosdick  Suspended Member (Idle past 5530 days) Posts: 1793 From: Upper Slobovia Joined: |
Technically, you are right. But you're splitting hairs here: "mariner elements" = "jumping genes". I should not have called them "mariner genes." My mistake, goodness sake! But what's the big deal, anyway? You've missed the entire point about endosymbionic genetic elements. Your pedantry is now evident on this tread, as well as on others.
”Hoot Mon
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Fosdick  Suspended Member (Idle past 5530 days) Posts: 1793 From: Upper Slobovia Joined: |
crashfrog wrote:
I missed it by a single letter”an "n" for a "t"”"endosymbiotic." So, so, very sorry. "Endosymbionic" isn't even a word. This is what I mean about your pedantry. ”Hoot
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Fosdick  Suspended Member (Idle past 5530 days) Posts: 1793 From: Upper Slobovia Joined: |
Hawks wrote: It actually appears as if T. cruzi DNA can insert in the host DNA. But Hoot Mon's claim that WE carry around tsetse-fly genes seems off the mark. I didn’t make this up. An article in New Scientist titled Look before it leaps is the one mentioning tsetse fly genes in humans:
quote: I can't seem to locate the referenced British study, but here's another report that connects to the New Scientist article: Safe Online Gambling & Casino Guide | Reliable Casino Comparison
quote: ”Hoot Mon
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Fosdick  Suspended Member (Idle past 5530 days) Posts: 1793 From: Upper Slobovia Joined: |
...but neither one of your two articles proposes a mechanism of horizontal gene transfer between insects and humans, and several entomologists that I spoke to thought this was a tenuous possibility at best.
Perhaps you're right about this, frog. Maybe there is nothing at all to "horizontal gene transfer." But I have carefully read Frederic Bushman's Lateral DNA Transfer/Mechanisms and Consequences (2002), and he makes an awfully good case for the mechanisms you are questioning. You should check it out of the library. Pay particular attention to Chapter 10 "Lateral Transfer in Eukaryotes: Fluidity in the Human Blueprint." Lateral genetic transfer is fascinating, and many of the mechanisms are well understood. Upthread I posted a muse of mine that crossing over in the first prophase of meiosis is also evidence of an escape mecahism used by genes to avoid their parasites. Hamilton, Williams, Dawkins et al. are proponents of genetic "strategies" that give meaning to the need for sex. So I became interested in lateral DNA transfer from that POV. Please tell me what you think is invalid about gene flow as a means to accomplish microevolution. ”Hoot
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Fosdick  Suspended Member (Idle past 5530 days) Posts: 1793 From: Upper Slobovia Joined: |
So, Hoot Mon's link and claim that DNA has been transferred between humans and tse-tse's doesn't seem all too farfetched.
I think horizontal genetic mobility connects to this topic relevantly, because lateral DNA transfer may be associated with the origin of new alleles. It may also associate with their durability and transposability once they become new alleles. Now I just have to try to figure out why this is actually important for this thread. ”Hoot
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Fosdick  Suspended Member (Idle past 5530 days) Posts: 1793 From: Upper Slobovia Joined: |
I don't see how it works in sexual metazoans.
These are good questions. The answers are not so easiliy summarized, but I'm serious about Frederic Bushman's book, referenced upthread. He brings forward a great deal research about such topics (chapter titles) as: "A transposon progenator of the vertebrate immune system," "Lateral DNA transfer and the AIDS epidemic," "Genes floating in a sea of retrotransposons," "Controlling mobile element activity," and "Lateral DNA transfer: Themes and evolutionarey implications." I think you would find answers to your questions in this book. (I found it so important I purchased a copy for myself.) For instance, a biting fly pierces my skin and begins to feed off my blood. How does any of it's genes get from the cells of its digestive tract (the only cells currently in contact with my body) all the way down to the protected gametes in my genitals? And I'm male. How would it work in my wife, who at birth had already generated all the gametes she'll ever have? Meiosis is over by the time she could possibly be exposed to these parasites. ”Hoot
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Fosdick  Suspended Member (Idle past 5530 days) Posts: 1793 From: Upper Slobovia Joined: |
Hawks wrote: But there are no absolute barriers against it either. crashfrog replies: I disagree. The barriers that seperate spermatozoa from the rest of the body are so tight they can screen out antibodies, which are much smaller than bacteria. How is your nomad bacterium supposed to get through that?I suppose that lack of direct observation might be called hand-waving but there are mechanisms that can do it. I'm still waiting to hear what those are. ”Hoot Mon
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Fosdick  Suspended Member (Idle past 5530 days) Posts: 1793 From: Upper Slobovia Joined: |
But what possible circumstances could result in a DNA leap between the mouth of an insect and a sperm cell that, for all intents and purposes, might as well be on the other side of the planet? This is the part that horizontal gene transfer advocates gloss over. "Eh, it just happened." It's just highly unlikely to me, is all.
You're knee-jerking again, frog. Your question is at least partially answered in my last post (#52):
quote:The bloostream is a river of opportunity for any blood-borne agent. The same bloodstream that circulates through your skin also circulates through your testicles (if you got 'em). I don't see why this is so difficult to grasp. Even if it is unlikely that a gene in the saliva of an insect can travel through the bloodstream all the way into a meiotic event, it is not impossible. repeating your question:
What? "...other side of the planet?" Come on, frog, by your ridiclous analogy your skin covers a parking lot in Lincoln, NE, while your nuts are somewhere in the Australian outback. Not quite so. If you got a penicillin shot in your arm for a testicular infection I think the drug could get down there without having to travel around the world to do it. But what possible circumstances could result in a DNA leap between the mouth of an insect and a sperm cell that, for all intents and purposes, might as well be on the other side of the planet? ”Hoot
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Fosdick  Suspended Member (Idle past 5530 days) Posts: 1793 From: Upper Slobovia Joined: |
crashfrog, your barrier hypothesis is good. But what if the barrier fails for some reason? Things do jump barriers, you know. When any fly injects its saliva into the bloodstream of its host there could be a considerable amount of fly DNA that comes along with it each time. Do you deny this? Maybe your barrier is good most of the time, but maybe, just maybe, once in a while things don't work right and fly DNA jumps over the barrier and into a spermartozoan. What's impossible about that?
”Hoot
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