Re: [Killietalk] N. kafuensis Kayuni, etc.

[Brian Watters <bwatters at shaw_ca>]

Bruce, thank you for that "unemotional" and reasoned account. As you say, no
one is calling anyone a liar and, if that is the impression created by my
original post, then I humbly apologize. I am aware of much of what Bruce has
written but made my comments and expressed an opinion based on my personal
experience with numerous populations of N. kafuensis.

Fred, if you read my post again you will see that I used the word
"inadvertent" when referring to the possibility of a cross. The reason I put
a question mark after it was not to imply that Karl would have deliberately
crossed populations, but to leave in the possibility that the fish may have
been crossed prior to Karl acquiring the strain. In retrospect, I probably
should have been more explicit. Oh, and by the way, I might be a scientist
but I am also an "aquarist" and have been since long before I ever became a
scientist.
___________________________
Brian Watters
6141 Parkwood Drive
Nanaimo, BC, V9T 6A2
Canada
Ph: (250) 760-0564
E-mail: bwatters at shaw_ca

> -----Original Message-----
> From: killietalk-bounces+bwatters=shaw_ca at aka.org
> [mailto:killietalk-bounces+bwatters=shaw_ca at aka.org]On Behalf Of Bruce
> J. Turner, Dept. Biol. Sci., VPISU, Blacksburg, VA 240
> Sent: December 9, 2007 5:46 AM
> To: killifish discussion list
> Subject: [Killietalk] N. kafuensis Kayuni, etc.
>
>
> I don't think there is any need for emotional arguments in
> situations like this.
> So far as I can see, no one is calling anybody else a "liar."
> The situation is
> something like this:  An experienced and knowledgeable
> specialist has raised some individuals, or a strain, of a Notho.
> population that
> have a (partially) "red" or orange appearance.  People familiar
> with the natural
> population that was likely the original source of the stock have
> observed that
> there
> were no orange males in those populations.  Since other
> populations of the same
> species are known to have orange males, it is natural to raise
> the possibility
> that there was some inadvertent crossing among stocks from
> different geographic
> origins.  As a general rule, that explanation is not unreasonable, for
> experience has taught us that it is sometimes difficult to
> maintain genetic
> "purity" of our various lines without serious diligence, and even
> with that
> diligence, accidents do happen.
>
> However, there is another explanation which may be appropriate here:
>
> 1.  Simply because orange males weren't observed in the natural population
> doesn't mean that the genes for the orange color weren't there,
> merely that they
> were present at some low frequency.  This is especially so if the trait is
> encoded by recessive alleles.  They can persist in a population
> indefinitely in
> heterozygous form and but the phenotype is not expressed because
> the frequency
> is so low that very few homozygotes are ever produced.  In addition, a
> conspicuous phenotype like an orange body color might be selected
> against in
> certain populations, though this is speculative.
>
> 2.  It is illusory to think that our aquarium bred lineages are
> unbiased samples
> of the variation present in natural populations.  Unless the
> natural population
> is unusually low in variation, and/or we obtain a large sample
> initially and
> keep the stock at high effective population sizes, our aquarium
> lineages are
> biased samples (relative to the natural populations) from the get
> go, and that
> bias increases the longer we maintain the stock, even without deliberate
> inbreeding or line breeding.  The larger the natural population
> that we sampled,
> the more genetic variation it likely contains, and the greater
> the likelihood
> that our initial (usually small) sample was biased.  To my
> knowledge, no one has
> ever measured this for Notho populations, but in many cases their
> populations
> are large, and
> I suspect they have much variation, some of it doubtless not
> visible simply by
> observing the animals.   When we take a sample of a natural
> population with a
> large effective size, the kind of bias we introduce is what evolutionary
> biologists call the "founder effect."  If we then maintain the
> stock at low
> effective sizes, we compound the original bias in the sample with
> continual
> sampling error, the effect known as "genetic drift."
>
> 3.  Aquarists are familiar with genetic drift from a conservation genetics
> perspective.  A certain proportion of the average overall variation
> (heterozygosity) is lost every generation; the loss is inversely
> proportional
> to the effective population size at which the stock is
> maintained. But that is
> a description of the AVERAGE effect, and the average
> tends to make us forget that the frequencies of individual
> alleles (alternative
> forms of genes) are fluctuating sharply from generation to
> generation.  The
> smaller the effective size at which we maintain the stock, the more
> fluctuation.  It is in the nature of sampling ANYTHING that this kind of
> fluctuation sometimes means that genes that were initially in low
> frequency can
> reach very high frequencies quite by accident.  It doesn't happen very
> frequently, but it does happen.  The world of population genetics
> is rich in
> examples of individual, small, often isolated populations having
> otherwise rare
> alleles at high frequency.  If you want a striking example, do a
> Google search
> on the word "guevedoces."
>
> This is what could have happened with the case that we are dealing with.
> Through founder effect and/or genetic drift, a rare gene reached high
> frequencies and its expression became visible in some one's
> stock. Unless we
> know categorically that this rare gene did not exist in the
> natural progenitor
> population (and this would put us in the position of  trying to prove a
> negative, not a good place to be) this explanation cannot be
> ruled out.  On
> average, it is probably less likely an explanation than inadvertent stock
> hybridization. but it will happen from time to time.
>
> As long as we understand that the orange phenotypes were not
> obvious in the
> presumptive progenitor natural population, let us simply enjoy
> these gorgeous
> animals.  And maybe we shouldn't get so exercised about issues like "stock
> purity" until we are able to measure genetic variation within and
> among the
> natural
> populations that we sample.  From a statistical viewpoint, biased sampling
> begins as soon as one puts a net in the water.
>
> Bruce J. Turner
> Dept. Biol. Sci.
> VIRGINIA TECH
> Blacksburg, VA 24061
> (540)-231-7444 (V)
>                       "...We are Hokies.  We will prevail..."
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