"Stephen E. Jones" writes
in message <200004062207.GAA19256@popserver-02.iinet.net.au>:
<snip>
> Of course, since Maynard Smith is a very clever person and has all the vast
> resources of modern science at his disposal, he does manage to suggest
> some `just-so' stories which might explain why sex originated.
So he doesn't believe its an insourmountable problem for evolution,
either, eh? I guess he's using "problem" differently from the
way you're using it.
I think Stephen misrepresents the "problem" of sex and especially
current research on the topic. Here's an excellent summary
from http://coldfusion.discover.com/output.cfm?ID=67
| ...
| One of the arguments currently dominating the competition is championed
| by Michael Rose, a mathematical geneticist at the University of
| California at Irvine, and his colleague Donal Hickey of the University
| of Ottawa. Along with many others in their field, they believe
| that a bacterial phenomenon known as conjugation constitutes an
| ancient form of proto-sex. Conjugation is a property of some but
| not all bacteria in a given colony. It involves the extension of
| a projection called a pilus from one bacterium to another, and the
| journey along that bridge of a self- contained, parasitic loop of
| genetic material called a plasmid. ("There is a certain morphological
| similarity between conjugation and higher sex," notes Michod
| delicately.)
|
| The bacteria seem to gain nothing from this transaction. In fact,
| if this is proto-sex, it's proto-bad-sex, because neither bacterium
| can be described as consenting. The plasmid contains the quintessential
| selfish gene, a bit of DNA whose only mission is to reproduce
| itself, thus driving the plasmid to distribute as many copies of
| itself to as many hosts as possible. In the process, bits of the
| original bacterium's genome occasionally cling to the plasmid like
| foxtails on a dog's coat and find themselves in the new host.
| Eventually, explains Rose, some hosts begin to use and benefit from
| the inadvertent gift of another individual's DNA.
|
| Rose and Hickey have gone on to propose that selfish DNA could
| account for a primitive form of sex that's closer to sex as we now
| know it. In some early single-celled organisms, they theorize,
| selfish DNA didn't merely cause a bridge to form so that it could
| travel from one individual to another--it impelled the two organisms
| to actually fuse, in a primitive anticipation of what sperm and
| egg do during fertilization. This parasitic DNA could then spread
| contagiously until the whole population was committed to sex. How
| widely accepted is this scenario? "There are three stages in the
| life cycle of any scientific idea," says the 36-year-old Rose.
| "First, it's treated as a joke. Next, it's taken seriously but
| considered to be impossible. Finally, people admit that it's
| possible, but they insist that it's trivial." Rose says he came on
| the scene in 1983, during the second stage, performing the mathematics
| to demonstrate that selfish DNA is a powerful evolutionary force.
| "Now," he deadpans, "we're in the third."
|
| [Richard] Michod is thoughtful when asked to comment on the
| Hickey-Rose theory of gene transfer. "It is certainly a reasonable
| explanation for the origin of sex," he says. "In fact, I think it's
| the main competition to the DNA repair view. " Michod's idea that
| the reshuffling of genes from two organisms originated as a mechanism
| to mend damaged chromosomes is another of the theories in current
| contention. Influenced by Maynard Smith, Michod refused to buy the
| argument that genetic variation was enough justification for sex.
| "Look at us," he says, "adult organisms who have already passed
| muster, evolutionarily speaking. We've survived, so our genomes
| must be in reasonable shape." But what is the most striking effect
| of sexual reproduction? "It scrambles up that perfectly good genome.
| What are the odds that that will be an improvement? And even if it
| is, then what? You can produce a superkid, but she'll just reproduce
| and scramble the genome even more." Everything sex does, it partly
| undoes in the next generation.
|
| Michod reasoned that since DNA is a way of conveying information,
| perhaps sex was initially a way of getting the message straight:
| it might be about error correction, not variation. In 1988 he and
| his team demonstrated sex-for-DNA-repair in a bacterium called
| Bacillus subtilis. These microbes engage in an activity called
| transformation, which involves incorporating bits of DNA floating
| in their environment. (Not to be too lurid about it, but this DNA
| originates from the disintegrating corpses of neighboring B.
| subtilis.) Michod believes that they use this "spare" DNA to repair
| breaks in their own chromosomes caused by exposure to environmental
| insults, such as excessive oxygen or ultraviolet light. The evidence?
| Damaged bacteria use more DNA than undamaged bacteria, and repaired
| bacteria replicate more successfully than unrepaired bacteria.
| (Comments Rose, "Sex with dead bacteria is apparently better than
| no sex at all.")
|
| None of this means that either Rose or Michod underestimates the
| significance of variation. "Look," says Michod, "diversity is the
| fuel of evolution, and gene recombination produces diversity. We're
| just saying that recombination--proto-sex if you will--didn't come
| into existence to produce variation." Variation, in other words,
| is an effect of sex, one that's turned out to be extraordinarily
| useful, but it's not the original reason for sex. "There must be
| some short-term, individual benefit to recombination," says Michod,
| and in his view, it's DNA repair.
|
| Rosemary Redfield agrees about the short-term benefit, but she has
| her own ideas on what it might be. Redfield, 43, a biochemist at
| the University of British Columbia in Vancouver, dubs her take on
| bacterial proto-sex "Having your cake and eating it, too." ("I'm
| rather pleased with that description," she says.) Accepting her
| idea, which she plans to lay out for her colleagues next month at
| a conference on the evolution of sex, doesn't mean rejecting
| Michod's, she adds; the two can coexist quite nicely.
|
| Redfield agrees with Michod's observation that bacteria go to a
| great deal of trouble to incorporate external DNA, but she notes
| that the patching they achieve is hit-and-miss, as likely to be
| bad as good. What else, then, might motivate transformation, this
| DNA absorption that seems to be the harbinger of sex? In Redfield's
| opinion it's that other great physiological drive: hunger. The
| spine of the DNA molecule is made up of alternating sugars and
| phosphates, she explains, with a chemical base hanging off each
| sugar. "When DNA is broken down, it's really sugars and a base,"
| she says. "I think of it as molecular candy, rather like the candy
| on a string we used to eat as children." When a bacterium feels
| "hungry"--runs out of its usual sugar supplies--it becomes capable
| of taking up external DNA. Through a poorly understood mechanism--"Though
| it must be something like slurping spaghetti," Redfield says--it
| sucks a string of DNA though a pore in its wall and sets about
| digesting it.
|
| This explains only half of Redfield's catchy aphorism. The bacterium
| can eat its DNA confection; what about having it? DNA, recall,
| consists of a twist of two complementary strands. When a bacterium
| goes to work on a DNA fragment, it degrades one strand for the
| sugars, leaving the other one floating free. The second strand may
| subsequently be digested also. But if it matches a stretch of the
| bacterium's own DNA (especially a damaged bit), it knocks out that
| bit and replaces it. The discarded DNA can then be digested too.
| Redfield notes that while individual steps in this scenario have
| been observed, she has yet to prove the whole story. But she feels
| confident that the Haemophilus influenzae colonies in her petri
| dishes are going to confirm the benefits of proto-sex: a fill-up
| and, with luck, a tune-up.
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