KnightWhoSaysNi

Most of us know about human chromosome 2. We find an extra centromere and telomeres within the chromosome indicating that at some point in our ancestry, a fusion had occurred. This explains why we have 46 chromosomes as opposed to our closest relative, the chimpanzees, that have 48.

Since we know that a fusion happened, it's reasonable to assume that this wasn't the only chromosome fusion event in our ancestry if we go back even further. Do we in fact find some ancient remnants of other artifact centromeres and telomeres in our DNA? And better yet, can these artifact centromeres and telomeres be matched with other organisms like fish and reptiles when comparing chromosomes?

Jason

Emma Peel

I haven't heard about other chromosome fusions, but I found 2 good articles on duplications & rearrangements in our past over at the Cr/Evo:TED archive:

• Genome Evolution | First, a Bang Then, a Shuffle
  For three decades, based largely on extrapolations from known gene families in humans, researchers have hypothesized two complete genome doublings--technically, polyploidization--modified by gene loss, chromosome rearrangements, and additional limited duplications. But that view is changing as more complete evidence from genomics reveals a larger role for recent small-scale changes, superimposed on a probable earlier single doubling. Ken Wolfe, a professor of genetics at the University of Dublin, calls the new view of human genome evolution "the big bang" followed by "the slow shuffle."
  
• Humans made by genetic accident, claim researchers
  [The researchers] found evidence that the chromosomes of our common ancestors had become accidentally rearranged during reproduction.
  
  They could still mate and swop genes but the rearrangements prevented characteristics on some parts of the chromosomes being exchanged.
  
  Advantageous mutations in these areas could not be swopped between the two emerging species. The result, according to the new account, was the creation of two distinct gene pools.
  
  Professor Barton said: "It appears that different parts of the species population diverged at different rates, with some in each group continuing to mate with each other until they gradually separated forming their own species."

judge

By what means do we know that a fusion happened?

How can we be sure it was not always in the form it is in now?

RufusAtticus

This for instance:



The fusion point in chromosome 2 has been sequence and the researchers found telomeres there. Teleomeres are distinctive nucleotide sequences that are only found on the ends of chromomes where they have a functional purpose. Now if our chromosome 2 has always been like it is, why does it have a teleomer in the middle of it at precisely the fusion point suggested by comparative karyotyping.

Oolon Colluphid

Or to put it simply: The chimpanzee chromosomes have remarkably similar sequences to the single human chromosome 2.

Hypothesis: the chromosomes fused in our lineage.

Test: look for evidence of the fusion. Suitable evidence would be endy-bits -- telomere sequences -- in the middle of the chromsome, where they have no business being.

Result: telomere sequences found in the middle.

Conclusion: the chromosomes did indeed fuse.

How else to explain the presence and location of these sequences? Was god just messing with our heads?

Note, also, that the chromosome 2 also apparently (apparently, cos it’s the telomeres that get talked about most, so I don’t recall it being discussed before) contains a couple of extra centromere sequences -- that’s the bits in the middle, used during mitosis -- just where the ‘fusion hypothesis’ predicts, too. A chromosome should have one centromere; this one in effect has three.

Therefore, we know that the chromosome 2 wasn’t always like that. It used to be two pieces. Just like it still is, in the creatures which for a myriad other, separate reasons are considered our close relatives.

Cheers, Oolon

God Fearing Atheist

This page discusses the evidence in a bit more detail.

-GFA