How does Head To Head Telomere Fusion express itself?

[GaryGaulin]

I am working on human chromosomal speciation but cannot confirm how a "head to head telomeric fusion" (not a Robertsonian translocation) such as human chromosome 2 would express itself in the first generations. About 2/3 down there is a paper with a Punnett square of the event in the first generation:

http://www.c-s-p.org/Flyers/9781847182647-sample.pdf

Another paper (closed access):

http://www.springerlink.com/content/a1551p74t67641l2/

It would seem that a fused chromosome that still has two functional centromeres would align then divide properly with the unfused pair, but that's just my educated guess. I'm hoping there is something I missed that would make it possible to know for sure what the Punnett squares should look like.

[kolean]

As in the Punnett Square that is the first paper, it is the word balanced that you should take into account for meiosis. This reminds me of the X chromosome, in which to be balanced in the female, one chromosome needs to be inactivated (into a Barr body), and thus already balanced in the male which has only one.

A fused chromosomal gamete is equivalent to the two normal chromosomal gamete - BALANCED. You can have viable offspring as long as you have the BALANCED count of 4 chromosomal gametes. If you have the normal 2 chromosomal gametes and the 1 fused chromosomal gamete, then you get viable with a count of 4.

If you have the normal chromosomal gametes (2) and a unbalanced gamete combination of one fused (count of 2) and 1/2 the normal 2 count chromosomal gamete (count of 1), then you have a count of 5, which is not viable.

Hope this helps.

[GaryGaulin]

Thanks Kolean, the first paragraph about inactivation of the X chromosome is interesting. I didn't know that before.

You have the right idea with balanced and unbalanced. What I am most wondering is whether during meiosis a fully balanced fusion that only involves the telomeres could even result in unbalanced germ cells. It would seem that the chromosomes would still properly align along the mid-line with the fused pair always going one way and the unfused always going the other way. Where the spindles on opposite poles of the fused chromosomes are each pulling on one of the two still viable centromeres there would be a tug-of-war type situation causing an extreme pulling force related imbalance, and where there is enough motor protein force could pull the poles to the center line which would result in unsuccessful meiosis possibly cell death. Where the spindles are bringing one unfused chromosome to one side and the other fused and unfused copy to the other there is another large force imbalance in the system. It seems like the meiosis mechanisms would be able to detect this much pulling force imbalance in the system then adjust spindles accordingly.

There does not seem to be any research where a head to head telomere fusion was induced then how it divided down in meiosis documented. But I could be wrong and was simply unable to find the paper that fully explains the result of this kind of fusion.

[kolean]

Maybe looking at the cell cycle during mitosis and seeing all the checkpoints might help you out. It could possibly end in cell death if the cell couldn't proceed thru its cell cycle.
It is microtubules that connects the centromeres to the poles, and they begin to pull by way of letting tubulin go from the pole end: thus pulling on the centromeres as they shorten. Are they strong enough to pull a fused chromosome apart? Depends on how condensed the chromosome at the fused part in order for it to either break off and freely go with the rest of the chromosome, or stay together and stall the cell cycle.
Now in Meiosis, there are chiasmata/crossingover going on, but that is just when the chromosome is decondensed and supporting proteins help in the process. After the process is done, the proteins are released and the cross over DNA is let loose to be a single strand, and then the condensing of the DNA into chromosomes begins. I don't know what proteins are found to help support a fusing of chromosomes, and then to hold them together during metaphase. It would have to be an intricate orchestrated process that would unfold into a viable offspring.
(Though telomeres do have unique proteins that do like to bind together, especially Sir2, in yeast models so far.)