How do we explain Gene Families?
popa0206 at po-box.mcgill.ca
Sat Apr 15 14:11:00 EST 1995
It is a prevalent theory, and one that comparative mapping is based
on, that in the past during the evolution of vertebrates a series of whole
genome duplications had occured. The best guess would be 2-3 times, for example
explaining the presence of four HOX gene clusters... i.e. one precursor
gene that was duplicated in one tetraploidization and then two upon the
second tetraploidization to produce 4 genes.
Now it is very attractive to say that this is a valid model as it explains
very easily the many gene families found on different chromosomes. In plants
there would be no need to question the mechanism as plants (such as wheat) can
survive great changes in there genome without leading to death of the individual
plant. Unfortunately animals are not so hardy when it comes to polyploidy and
wide spread mutation of the genome.
Remember, we are dealing with a process that would have to be resolved in one
generation. Tetraploidy occurs... now you have the problem of two identical
sets of chromosomes, to ensure proper disjunction during mitosis and meiosis
diploidization (or differentiation) of the two sets of homologs must occur.
In addition the tetraploidization must occur in the zygote at an early stage
as fertilization requires two compatible gametes, and it is unlikely that two separate
duplications could occur in both gametes (and identically resolved) prior to
fertilization. Therefore, how could the individual zygote survive such a
process of chromosome differentiation (which would include inversions,
deletions and translocations) without suffering so many lethal mutations
that it would kill the organism.
Although single events of duplication by gene conversion, primed by DSB's
near repeated elements (alus and line-1 sequences for example) could account
for a relatively "safe" mode of gene duplication, the shear number of duplicated
gene families and the fact that they are usually in 2's and 4's makes the whole
genome duplication theory more attractive, even though improbable.
Perhaps there could be some compromise...
any thoughts and comments are welcome
Exp. Med. Mcgill
Red Cross, Montreal
e-mail popa0206 at po-box.mcgill.ca
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