Chromosome Fusion

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I need someone who has been keeping an eye on the biological literature.

It is by now an established fact that the human chromosome 2 originated as a fusion of two chimpanzee chromosomes (and I'm not about to dispute that, this post is in no way an advocacy of intelligent design :))

There are other species in nature who's number of chromosomes differ by two, suggesting that fusion (or splitting) of chromosomes is quite common event. However, it is a quite drastic change of the genome and it is well-known that species with different chromosome number have great difficulty in reproducing, and always produces infertile offspring. In light of this, the chromosome fusion is a bit bizare; It cannot happen gradually (two chromosomes are either fused or they are not) and unless the exact same fusion happens to occur in a whole number of individuals of the population at the same time, it could not spread because of the infertility of the offspring. At least in the laymans world.

Which brings me to my question: Has a mechanism for 1) how/why such chromosome fusions/splittings occur and 2) how such a fusion/splitting spreads in a population, been proposed?
 

Moonbear

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It is by now an established fact that the human chromosome 2 originated as a fusion of two chimpanzee chromosomes
Before getting to your question, can you provide a reference that supports this "established fact?" This is new to me.
 
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Before getting to your question, can you provide a reference that supports this "established fact?" This is new to me.
How about http://en.wikipedia.org/wiki/Chromosome_2_(human)" [Broken]'s references?
 
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Negative, that is incorrect.

Two chromosomes fused in the human genome during its genetic evolution, that is detectable by comparing a human chromosome map and a chimpanzee chromosome map, however none of the human genome originated from a chimpanzee, but rather from a common primate ancestor that did not have a fused chromosome.
 

Moonbear

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How about http://en.wikipedia.org/wiki/Chromosome_2_(human)" [Broken]'s references?
Did you follow their references? They just go to other websites, but not research articles. :rolleyes: But, at least one of those sites cites an article in PNAS, which is getting a bit closer to a credible source.

It seems Orion has an explanation that makes more sense.
 
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Did you follow their references? They just go to other websites, but not research articles. :rolleyes: But, at least one of those sites cites an article in PNAS, which is getting a bit closer to a credible source.

A quick search in PubMed gives a number of references to the fact of chromosome 2 fusion early in human evolution. It seems as if this has been fairly well known about since the early 1990's.


I take exception to the OP's statement that "it is well-known that species with different chromosome number have great difficulty in reproducing, and always produces infertile offspring.". "Always" is a poor choice of words here and would seem to be outright incorrect by the simple fact that our human ancestors did have such an event and were able to produce viable offspring....as well as many other species that have had similar events.

However, if he is trying to make the assertion that this somehow "disproves" evolution, then I will just have to sit back and chuckle. :smile:
 
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The reference you are looking for is: http://www.nature.com/nature/journal/v434/n7034/full/nature03466.html
Chromosome 2 is unique to the human lineage of evolution, having emerged as a result of head-to-head fusion of two acrocentric chromosomes that remained separate in other primates. The precise fusion site has been located in 2q13–2q14.1 (ref. 2; hg16:114455823–114455838), where our analysis confirmed the presence of multiple subtelomeric duplications to chromosomes 1, 5, 8, 9, 10, 12, 19, 21 and 22 (Fig. 3; Supplementary Fig. 3a, region A). During the formation of human chromosome 2, one of the two centromeres became inactivated (2q21, which corresponds to the centromere from chimp chromosome 13) and the centromeric structure quickly deterioriated.
Notice the precision of the identification of the fusion site (within 15 bases).

This fusion event plays a large roll in the defense of evolution by pro-science cell biologist Ken Miller, who has used it as evidence in his recent book "Only a Theory: Evolution and the Battle for America's Soul".

There is an excellent Youtube video of a lecture by Miller about this. Here is a small clip from it.

 
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I need someone who has been keeping an eye on the biological literature.

It is by now an established fact that the human chromosome 2 originated as a fusion of two chimpanzee chromosomes (and I'm not about to dispute that, this post is in no way an advocacy of intelligent design :))

There are other species in nature who's number of chromosomes differ by two, suggesting that fusion (or splitting) of chromosomes is quite common event. However, it is a quite drastic change of the genome and it is well-known that species with different chromosome number have great difficulty in reproducing, and always produces infertile offspring. In light of this, the chromosome fusion is a bit bizare; It cannot happen gradually (two chromosomes are either fused or they are not) and unless the exact same fusion happens to occur in a whole number of individuals of the population at the same time, it could not spread because of the infertility of the offspring. At least in the laymans world.

Which brings me to my question: Has a mechanism for 1) how/why such chromosome fusions/splittings occur and 2) how such a fusion/splitting spreads in a population, been proposed?

I have addressed this topic in a series of four essays on the blog "The Panda's Thumb". The last essay is here, and contains links to the previous three:

http://pandasthumb.org/archives/2009/09/the-rise-of-hum-3.html

Dave Wisker
 
I'd like to add to this topic.:biggrin:

Organism - Human (Homo sapiens)
Genome Size(Base Pairs) - 3.2 billion
Estimated Genes - 25,000
http://www.ornl.gov/sci/techresources/Human_Genome/publicat/primer2001/4.shtml
9/9/08
There is actually very good evidence that human chromosome 2 resulted from a fusion of two ape chromosomes, 2p and 2q. Each chromosome has distinct sequences at both ends called telomeres, as well as a unique central region called a centromere. If two ape chromosomes fused end to end (a phenomenon known as a Robertsonian translocation), we would expect to see two indicators of this translocation in the human chromosome. First, the fused chromosome should have two centromeres. Second, traces of telomeres should be evident in the middle of the chromosome where the fusion occurred. In fact, both of these predictions are supported. The normal centromere on human chromosome 2 corresponds with the centromere on chimpanzee chromosome 2p, while traces of a second centromere are observed in the region of the human chromosome that lines up with the centromere of chimp chromosome 2q. Also, the repetitive DNA sequences characteristic of telomeres are found precisely at the predicted fusion site, and the repeats reverse direction exactly as would be expected for two chromosomes joined end to end.

Despite having a different number of chromosomes, an individual carrying a Robertsonian translocation has not lost any genetic material and can therefore mate with a normal individual and produce viable, fertile offspring. A useful diagram (learn.genetics.utah.edu/units/disorders/karyotype/robertsonian.cfm) shows the possible chromosome combinations that can result from this pairing. One-fourth of the offspring would receive the standard combination of chromosomes and would of course be normal and would not be able to pass on the fused chromosome to future generations. Half the offspring would receive either only one copy or three copies of one chromosome; these individuals would be unlikely to survive or reproduce successfully. The remaining one-fourth of the offspring would inherit a balanced Robertsonian translocation. These individuals would also be viable and fertile and could found a lineage carrying only the fused chromosome if they mated amongst themselves.

A well-known example of this situation can be found in the Przewalski's wild horse, formerly extinct in the wild but recently reintroduced into its native habitat in Mongolia. This horse is closely related to the domesticated horse, but it has 33 pairs of chromosomes whereas the domesticated horse has only 32 due to a translocation. However, if the two are crossed, they can produce fertile offspring.
http://www.hhmi.org/askascientist/answers/if_humans_have_23_pairs_of_chromosomes_and_apes_have_24_pairs_of_chromosomes_then_there_must_have_b.html [Broken]
I'll have to continue later and add a touch more. (I have to go digging for my old notes.:tongue2:) Also, after reviewing #3 under "References" in Wikipedia, it appears to me to be correct. :smile:
 
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I take exception to the OP's statement that "it is well-known that species with different chromosome number have great difficulty in reproducing, and always produces infertile offspring.". "Always" is a poor choice of words here and would seem to be outright incorrect by the simple fact that our human ancestors did have such an event and were able to produce viable offspring....as well as many other species that have had similar events.

However, if he is trying to make the assertion that this somehow "disproves" evolution, then I will just have to sit back and chuckle. :smile:
Thank you *very* much for your reply - although you didn't quite grasp the question which I was asking or the premesies under which I asked it:

Troels said:
It is by now an established fact that the human chromosome 2 originated as a fusion of two chimpanzee chromosomes (and I'm not about to dispute that, this post is in no way an advocacy of intelligent design :))
I think an unconditional apology for hinting that I'm "trying to make the assertion that this somehow "disproves" evolution" is very much in order :) I was as a matter of fact merely trying to enlighten myself as this issue pussled me.

Now, I happened to discuss this matter with a biologist friend of mine some time after I made this post, and he pointed my errenous position out to me and explained that different cromosome numbers doesn't always mean that the offspring is infertile; such infertility more often originates from the subsequent diverging of the lineages.
 
Despite having a different number of chromosomes, an individual carrying a Robertsonian translocation has not lost any genetic material and can therefore mate with a normal individual and produce viable, fertile offspring. A useful diagram (learn.genetics.utah.edu/units/disorders/karyotype/robertsonian.cfm) shows the possible chromosome combinations that can result from this pairing. One-fourth of the offspring would receive the standard combination of chromosomes and would of course be normal and would not be able to pass on the fused chromosome to future generations. Half the offspring would receive either only one copy or three copies of one chromosome; these individuals would be unlikely to survive or reproduce successfully. The remaining one-fourth of the offspring would inherit a balanced Robertsonian translocation. These individuals would also be viable and fertile and could found a lineage carrying only the fused chromosome if they mated amongst themselves

This is only a general theoretical estimation. My essays describe the unique circumstances in mammals and the human lineage that show that the decrease in fertilty for the end-to-end fusion that resulted in human chromsome 2 was probably 10% or less.

Dave Wisker
 

Monique

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D. Wisker, instead of referring to your blog, could you please point to the scientific reference that backs up your statements?
 
D. Wisker, instead of referring to your blog, could you please point to the scientific reference that backs up your statements?
Monique, the blog entries themselves are fully referenced, so I'm not sure what your point is. Simply reading the blog (rather than having me reproduce the argument again here) makes more sense to me. But a list of the references used in the argument follows:


For a discussion of the dicentric issue:

Sullivan BA, DJ Wolff, and S Schwartz (1994). Analysis of centromeric activity in Robertsonian translocations: implications for a functional acrocentric hierarchy. Chromosoma 103(7):459-67

For the discussion of the fusion and its effect on fertility:

Bardhan A and T Sharma (2000). Meiosis and speciation: a study in a speciating Mus terricolor complex. J. Genet. 79: 105-111

Coyne, JA and HA Orr (1998).The evolutionary genetics of speciation. Phil. Trans. R. Soc. Lond. B 353: 287-305

da Mota LSLS and RAB da Silva (1998). Centric fusion in goats (Capra hircus): Identification of a 6/15 translocation by high resolution chromosome banding . Genet. Mol. Biol. 21(1): S1415-47571998000100012(online publication)

Nachman MW and P Myers (1989). Exceptional chromosomal mutations in a rodent population are not strongly underdominant. PNAS 86: 6666-6670

Spirito, F (1998). The role of chromosomal change in speciation. In Endless Forms: Species and Speciation, DJ Howard and SH Berlocher, eds. Oxford University Press.


For the probability of fixation within a deme (and evidence for meiotic drive in human fusion heterozygotes):

Bengtsson BO (1980). Rates of karyotypic evolution in placental mammals. Hereditas 92: 37-47.

Bengtsson BO and WF Bodmer (1976). The fitness of human translocation carriers. Ann. Hum. Genet London 40: 253-257.

Hedrick P (1981). The establishment of chromosomal variants. Evolution 35(2): 322-332.

Lande, R (1979). Effective deme sizes during long term evolution estimated from rates of chromosomal rearrangement. Evolution 33(1):234-251.

Pardo-Manuel de Villena F and C Sapienza (2001). Transmission ratio distortion in offspring of heterozygous female carriers of Robertsonian translocations. Hum. Genet. 108: 31-36.

Beyond the deme:

Fix AG (1978). The role of kin-structured migration in genetic microdifferentiation. Ann. Hum. Genet. Lond. 41: 329-339.

Hedrick PW & DA Levin (1984). Kin-founding and the fixation of chromosomal variants. Amer. Nat. 124(6): 789-787
 

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