Chromosome Split After All? What Does it Take?

Here is a chromosome. Parts of genome have been marked by lowercase letter a - m (counting j as i, after my Latin preference). Telomeres have been marked uppercase T, centromeres uppercase C.

T a b c d e f C g h i k l m T

Now, imagine it undergoes a single doubling event, we will italicise the area doubled, then show the result:

T a b c d e f C g h i k l m T

T a b c d e f C g h e f C g h i k l m T

If you have a split directly after this, as P. Z. Myers imagined, you would have it either between the two centromeres or within one of them.

Between two telomeres eliminates genome in between, since genome unprotected by a mere at each end will shrivel:

T a b c d e f C g h e f C g h i k l m T*

Result:

T a b c d e f C/T

T/C g h i k l m T

No parts of genome doubled, but two "incomplete", one armed chromosomes, like the Y-chromosome. It is called telocentric. Sometimes it is called "extreme acrocentric," but acrocentric is simply not having a longitudinal symmetry around centromere. Telocentric is having only two meres, like Y-chromosome, and one of them also functioning as centromere, i e connecting to centromere of other chromosome.

Let's imagine instead split goes through one of centromeres, both of the:

T a b c d e f C g h e f C/T

T/C g h i k l m T

One of the chromosomes is still telocentric. Part of genome is repeated between the telocentric and the normal one, namely g h, other part, namely e f, between the arms of the normal chromsosome.

This is the weakness I diagnosed in P. Z. Myers' imagination.* He had written it out like this:

a b c d e f C g h e f C g h i k l m

And pretended to split like this:

a b c d e f C g h

e f C g h i k l m

But the problem is, this would really be:

T a b c d e f C g h

e f C g h i k l m T

And this means:

T a b c d e f C g h

e f C g h i k l m T

And he might hope instead:

T a b c d e f C g h TEN

TEN e f C g h i k l m T

Where TEN stands for Telomere Ex Nihilo. While telomeres and centromeres are from telomerase and centromerase, which is a special sequencing of DNA, distinct from genes, or perhaps two different ones, it is still DNA and needs to be compied from an original. Splits between meres are known from cancer studies, from onkology.

Now, imagine instead there is a second doubling event.

T a b c d e f C g h e f C g h i k l m T

T a b c d e f C g h d e f C g h e f C g h i k l m T

T a b c d e f C g h d e f C/T

T/C g h e f C g h i k l m T

e f three times, of which d e f twice

g h three times

Or if the second doubling event comprises both centromere exemplars:

T a b c d e f C g h e f C g h i k l m T

T a b c d e f C g h e f C g h i d e f C g h e f C g h i k l m T

T a b c d e f C g h e f C/T

T/C g h e f C g h i k l m T

e f three times, g h three times.

I have ignored versions which would leave one armed chromosomes as results. Here I'll be complete, a doubling across both centrmeres could be followed by a split through one of the three:

T a b c d e f C g h e f C/T

T/C g h i d e f C g h e f (C) g h i k l m T

Where (pretty standard) (C) = disactivated centromere.

Repeated sequences:

d e f twice and apart from that e f twice, g h i twice and apart from that g h twice.

This is the kind of scrambling of genome you should look for, when claiming a chromosome number increase by chromosome split.

For any creature, trisomy enlarged to tetrasomy is unlikely, since the trisomies are disabling and since tetrasomies are even more so, and recombining the fetal development to use two of the chromosomes for one and two for another part is unlikely and therefore ruled out. Tetraploidy is highly unlikely for mammals. It exists in plants and in amphibians, as does octoploidy. For mammals, chromosome split is the way up, and it is if not ruled out, totally, at least very hard.

Hans Georg Lundahl
Bibl. Mouffetard
St Justin Martyr**
14.IV.2018

* His post is here:

Pharyngula : Basics: How can chromosome numbers change?
https://freethoughtblogs.com/pharyngula/2008/04/21/basics-how-can-chromosome-numb/

It is possible he has rewritten it, so as to answer some of my objections ... will take a look ...

** Sancti Justini, Philosophi et Martyris, cuius memoria pridie hujus diei recensetur. / oops, too late ...