🚀 go-pugleaf

(Ted H. Emigh) writes:
>In article <2052@arthur.cs.purdue.edu> (Colin Jenkins) writes:
>>> I imagine speciation in the following way (here I am way out of my line):
>>> Some event occurs (eg. a chromosome re-arrangement) such that an offspring can
>>> not mate productively, or, at least not have fertile offspring, with any
>>> members of the group except its parent or perhaps its siblings.
>>> Offsping of these matings are also constrained in there mating.  

First of all, you incorrectly attribute the above to colin.  I was the one who
wrote the above.  I don't mind the incorrect attribution, but colin might.
>>
>
Ted:
>I have before me a diagram (SCIENCE 1982, 215:1525-1530) which shows gives a
(thanks for references.)
>

[discription of banding patterns of chromosomes supporting the notion that
major re-arrangements were involved in speciation.]
The model is not dead yet!

>However, for all but the smallest chromosomes (19-Y) there are obvious
>inversions.  Now recalling your elementary genetics class, the important
>features to remember about inversions are:  Inversions have no detectable
>effect on the individual with an inversion (with the possible exception of
>regulatory groups, etc); Individuals who are heterozygous for the inversion
>(that their two chromosomes have one "normal" and one inverted) have a much
>lower reproductive fitness (crossing over within an inversion lead to large
>chunks of chromosomes either deleted or duplicated).
>
>-- 
>Ted H. Emigh, Dept. Genetics and Statistics, NCSU, Raleigh, NC

I should hardly be the one to argue against my own suggestion, but, what you
say about inversions is not true for all organisms. 
Recalling the upper division molecular
genetics course in which I taught this portion:  What you say about inversions
is correct for yeast and Chlamydomonas and other organisms that go through
a transient diploid phase then sporulate to produce tetrads.  50% of spores
produced in tetrads that had a cross over are inviable.  People who looked
for of offspring in flies that had Paracentric inversions found no evidence of
it (you can tell inversions in flies from the polytene chromosomes).
They incorrectly concluded that there was no crossing over in these flies.
Sturtevant and Beadle (Genetics '36 or '37 i believe) came up with proof that
there was crossing over in inversions inthese flies.
The explanation for the lack of inviable ova came from cytological observation.
In females, only one of the 4 nucleii of each meiosis becomes an ovum.
The choice is made by physical location--the one at the "bottom" of the
folicle is the ovum.
Crossovers produced chromosome "bridges" between the poles, which physically
prevented the crossover products from getting into the position where the
ovum was made.  There was therefore no reduction in fertility.

Now in humans, to my knowledge, there is no selection that would prevent the
damaged DNA from getting into the ovum.

I checked with our local expert on mammalian fertilization and early
development, Jonathan Von Blerkom.  It seems that comparison of egg nucleii
in humans shows alot of variation in banding patterns.  Many diploid egg
nucleii (recall meiosis II occurs after fertilization in mammals) show
translocations, inversions and other problems.  Most of these eggs are never
fertilized and many that are turn out to have cell-lethal re-arrangements.
Many are viable (this goes counter to what I was taught years ago, but
He swears it's true--even showed me some pictures).
Fertility in humans is pretty flippin' lousy to start with.  This may account
for alot of that.  That inversions and translocations are common could
be used to argue both sides of the current dispute.

I certainly have learned alot about different models to account for speciation.
My initial, naive, view was exactly that.  It seems now like most of the
models currently put forth as well as others not yet developed may be
correct for a subset of events.  "Heterochronic" mutations affecting the
timing of expression of batteries of genes in development are all the rage
in the molecular-development field.  These are reletivley minor DNA changes
that cause gross changes in the organism.  A new(ish) wrinkle in evolution.

Thanks for all the comments

-tony