Text 1206, 94 rader
Skriven 2004-12-29 06:21:00 av Wirt Atmar (1:278/230)
Ärende: Re: The "fuel" of evoluti
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Bill writes:

>Thanks for your input, Wirt, but you may have misunderstood my argument. 
>The original poster (and I have now forgotten who that was) noted the often 
>quoted 2:1 advantage of parthenogenesis over sexual reproduction. This has 
>been brought up frequently as a conundrum, since most of us know that in 
>fact strict parthenogenesis is extraordinarily rare in nature. The point of 
>my "handwaving" argument was that in organisms with large numbers of 
>offspring, the supposed twofold advantage of parthenogenesis is 
>nonexistent, and that in fact only a 1% advantage of sexual reproduction is 
>sufficient to overcome the additional offspring of parthenogens.
>
>Even if my argument is correct, that still leaves the question of why 
>parthenogenesis does not arise more frequently in organisms with few 
>offspring. Your "big three" are fine, but they all require some time for 
>their effects, so I might expect that any one point in time we would see a 
>significant number of parthenogenetic species, even though we would expect 
>them all to be short-lived. Regarding this your last point regarding 
>complex populations may be the most germane. The organisms that practice 
>extensive parental care of a few offspring are mostly quite complex, so 
>even though parthenogenesis has more of a potential benefit in these 
>organisms, the rapid interference of germline decay with an extended 
>developmental sequence makes parthenogenesis a very short lived phenomenon.

Let me argue with your presumptions, William. Clonal, apomictic parthenogenesis
(which are three words meaning much the same thing) is not rare. Indeed, it is
the most common of all reproductive modes. For all but a very tiny minority of
cells in your body, it is the basic mode of cellular reproduction that you
employ. It is also the reason that you die.

While the benefits of sexual reproduction may take a little while to become
apparent, the downside to apomixis becomes readily obvious in just a few
generations. In the 1960's, Leonard Hayflick demonstrated that human cells can
replicate for only about 60 generations, regardless of their chronological age.
Replication count determined a lineage's viability, not age.

While we now know that a part of this senescence is preprogrammed by clipping
teleomeres off the ends of the chromosomes with each replication, that's almost
certainly an evolved response to insure to the cell the integrity of the
information residing on the chromosome.

I mentioned earlier regarding the experiment that every young biologist should
perform with a xerox machine. If you do it, you'll quickly find that 60
replications is a little more than astounding. Using the best xerox machine
that you can find, you will almost certainly not be able to read your page by
the 60th replication.

The primary reason why parthenogenetic populations are not common has nothing
much to do with their demographics, but it does have everything to do with
their capacity to accurately transmit their genomes in indefinite replication.


>But it does appear that facultative parthenogenesis is not that rare (based 
>on a quick search on Google Scholar), so it might be interesting to pursue 
>the conditions for facultative parthenogens which favor asexual vs. sexual 
>reproduction. 

Facultative parthenogenesis is primarily an ecological response, most useful to
herbivores, and reaches its pinnacle in the aphids. 

The vast majority of aphids are holocyclic (parthenogenetic reproduction in the
spring, but where the last generation of the season is fully sexual). 

In this system, single mother aphid can found an entire population very
rapidly, long before the inevitable populations of predators and parasites
arise. However, aphids have exaggerated even this ecological response, evolving
a process called "paedogenesis," where one adult mother has her daughter
developing inside her, who is herself pregnant with her daughter. In the
absence of the requirements of sex, you can "telescope" generations and build
species with explosive reproductive capabilities.

The reproductive advantage for these facultative parthenogenetic aphids is
substantially greater than 2:1. But the problem is that they can't do it for
very long. Somewhere, at sometime, they have to clean out the accumulated
replicative defects, and aphids perform this task at the end of the season by
producing a generation of males.

This one generation of reproductives in aphids seems sufficient to keep the
germline alive forever, but the various species of aphids are probably pushing
the "design limits" of an implementable parthenogenetic strategy. You, on the
other hand, have never devised a similar method to expose and expurgate defects
from your somatic tissue, thus I am saddened to relay news of your impending
doom.

Wirt Atmar
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