II. WHY HAVE
SO MUCH REPETITIVE DNA?
Charlesworth, B., Sniegowski, P. and Stephan, W.
(1994) The evolutionary dynamics of repetitive DNA in eukaryotes.
1. Mid-rep. DNA
makes it easier to generate genetic diversity.
Genetic diversity can be
generated by unequal crossing over. If there are two copies of a
gene, some crossover events can lead to duplication of the gene on
one chromosome, and deletion on the other. Successive duplication
or deletion of copies can lead to increases or decreases in the
size of multigene families.
If a gene duplication does occur, one copy of the gene is free
to mutate, while the other can retain its original function.
This allows evolution to experiment with variants of a protein
until it stumbles upon a beneficial variant.
2. Perhaps most
important reason: Decreases the potential risks of
Unequal crossing over
could have deleterious effects if it occurs within a coding
region. If all recombination occurred in coding regions,
then you would have a high frequency of inactivation of genes.
This may be tolerated in unicellular organisms (bacteria and
fungi) that have very little repetitive DNA. In this case their
high reproductive rates my be such that, a) even if a fraction of
individuals gets deleterious mutations due to recombination, the
rapid growth rate is sufficient to prevent a population
bottleneck: b) Furthermore, there is probably a selective
advantage for having a small genome if you have a rapid
3. Mid rep. DNA
could play a role in chromatin packaging.
By definition, things
like matrix attachment sites, and perhaps sites (if they exist)
that govern higher levels of chromatin packageing, must be present
in hundreds or thousands of copies. Therefore, such sites fall
into the middle repetitive fraction of the genome.
4. Mid rep. could simply be selfish DNA.
Some sequences may just
be more efficiently duplicated by the cellular machinery that
duplicates sequences. Those sequences that lend themselves to
being duplicated by the DNA replication machinery will tend to
propogate throughout the genome. Darwinian selection operates even
at the molecular level.Transposons are a good example of selfish
5. Mid. rep DNA might play no significant role
Not everything has to
have a selective advantage to get fixed in the population.
Point about equilibrium: Even if a particular trait or structure
in the genome is selectively disadvantageous, it may take time to
lose it after speciation occurs (speciation usually implies a
population bottleneck). Caveat: Most of the domesticated species of plants and
animals are not at equilibrium. In many cases, their
genetic diversity is greatly limited by artificial selection. We
have to be careful about inferring much about evolution from