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Author Topic:   Chromosomal evolution
Codegate
Member (Idle past 60 days)
Posts: 84
From: The Great White North
Joined: 03-15-2006


Message 1 of 7 (357265)
10-18-2006 1:54 PM


I'm hoping for some expert opinions and help here.

Over the course of evolutionary history, life on this planet most likely had to move from a form that had only a single, unpaired chromosome to the vast variety we see today.

I'm curious as to the different causes and issues involved with this progression.

For asexual creatures, it's really not that big of an issue (or is it?) seeing as how they don't have to find mates. Sexual lifeforms however have a must more difficult time modifying their chromosome counts.

I found a great article discussing Robertsonian fusions which can reduce the number of chromosomes in a population. It was (according to the article) the method by which humans moved to 23 pairs from the 24 pair ape population.

What other methods are out there that change chromosome counts? How do the new creatures procreate? Is it through inbreeding with siblings with the same 'disorder'? If so, that is the ultimate population bottleneck encompassing a single family.

I look forward to learning something here.

Edited by Codegate, : No reason given.


Replies to this message:
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AdminJar
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Message 2 of 7 (357271)
10-18-2006 2:12 PM


Thread moved here from the Proposed New Topics forum.

  
kuresu
Member (Idle past 772 days)
Posts: 2544
From: boulder, colorado
Joined: 03-24-2006


Message 3 of 7 (357301)
10-18-2006 4:53 PM
Reply to: Message 1 by Codegate
10-18-2006 1:54 PM


don't knw the name of the process, but the effect is polyploidy. This happens most commonly in plants, and I know there are a couple of frogs that have this too.

Polyploidy is nothing more than having a duplicate of every chromosome pair. So you go from having ten chromosome pairs to 20 pairs, but the second ten are duplicates of the first ten.

as to those bacteria, they don't have chromosomes. A chromosome is a very long strand of DNA wrapped up around histone proteins, which is lacking in bacteria.


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This message is a reply to:
 Message 1 by Codegate, posted 10-18-2006 1:54 PM Codegate has responded

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Codegate
Member (Idle past 60 days)
Posts: 84
From: The Great White North
Joined: 03-15-2006


Message 4 of 7 (357461)
10-19-2006 1:07 PM
Reply to: Message 3 by kuresu
10-18-2006 4:53 PM


Polyploidy is nothing more than having a duplicate of every chromosome pair. So you go from having ten chromosome pairs to 20 pairs, but the second ten are duplicates of the first ten.

Can creatures that suffer from polyploidy still reproduce with other non-poly creatures?

If so, can the polyploidy be passed on to their children?


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NosyNed
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Posts: 8860
From: Canada
Joined: 04-04-2003
Member Rating: 4.8


Message 5 of 7 (357465)
10-19-2006 1:21 PM
Reply to: Message 4 by Codegate
10-19-2006 1:07 PM


Equine Chromosomes
I googled equine chromosome number and found this site:

http://www.madsci.org/posts/archives/may2001/989331026.Ev.r.html

One part says:

quote:
Now differences in chromosome number do not serve as reproductive barriers between all species. For example, lets look at some of the equine species ( horses and donkeys). Domesticated horses have 32 pairs of chromosomes and Donkeys have 31. Yet, they can produce offspring, mules, which have 31.5 pairs of chromosomes. One of the horse chromosomes goes unpaired. Wild mountain zebras have 16 pairs of chromosomes, while the last species of wild horse (Przewalski's Horse) has 33 pairs. However, all of these equine species can produce hybrid offspring. In all of these crosses but one, the offspring are sterile. It has long been argued that this sterility is due to the difference in chromosome number, but hybrids of the wild (33 pairs) and domesticated horse (32 pairs) are fertile, and have 32.5 pairs of chromosomes. So clearly, something more than just differences in chromosome number is contributing to the species interbreeding barrier.

This message is a reply to:
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RAZD
Member
Posts: 20119
From: the other end of the sidewalk
Joined: 03-14-2004
Member Rating: 3.8


Message 6 of 7 (357946)
10-21-2006 12:34 PM
Reply to: Message 5 by NosyNed
10-19-2006 1:21 PM


Re: Equine Chromosomes
Wild mountain zebras have 16 pairs of chromosomes, ...

That's quite a difference eh?

Compared to the other species it looks like polyploidy or something similar occurred at some point between WMZebras and other equines.

http://en.wikipedia.org/wiki/Zeedonk

quote:
A donkey has 62 chromosomes; the zebra has between 44 and 62 (depending on species). In spite of this difference, viable hybrids are possible provided the gene combination in the hybrid allows for embryonic development to birth. A hybrid has a number of chromosomes somewhere in between. The chromosome difference makes female hybrids poorly fertile and male hybrids sterile due to a phenomenon called Haldane's Rule. The difference in chromosome number is most likely due to horses having 2 longer chromosomes that contain similar gene content that contain the same genes as 4 zebra chromosomes.

So are there hybrids between the different zebra species? Are they fertile?

Seems to me that when there are fused chromosomes in some species closely related to others without the fusion, that they could - occasionally - have both of the non-fused versions match up to the one fused one in hybrids.


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RAZD
Member
Posts: 20119
From: the other end of the sidewalk
Joined: 03-14-2004
Member Rating: 3.8


Message 7 of 7 (357948)
10-21-2006 12:44 PM
Reply to: Message 3 by kuresu
10-18-2006 4:53 PM


polyploidy in a number of species
and in rodents

http://www.intl-pag.org/pag/8/abstracts/pag8374.html

and to answer codegate

quote:
Since fertility barriers likely isolated this tetraploid lineage from its ancestors, instantaneous speciation, although rare is possible in mammals, and a role for doubling series variation in genome size to trigger evolutionary novelties is suggested by this unique tetraploid rodent. Funded by Fondecyt 1970710 and the Fulbright Commission.

It certainly poses a problem for reproduction to have some paired chromosomes and some unpaired ones, as demonstrated by the numerous cases of infertile hybrids between species with only 1 or 2 differences in numbers.

Would it be possible for a tetraploid egg to be fertilized by two diploid sperm? A diploid egg to be fertilized by one tetraploid sperm and one diploid sperm? Both could generate matched pairs of chromosomes in the zygote that would now be also tetraploid without necessitating sibling mating.

Just my rambling thoughts.


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