Biology Reference
In-Depth Information
genes — genes in the same protein complex or metabolic pathway —
exhibited common patterns of loss, and genes with high expression lev-
els were less likely to be lost, implying that the retention of duplicated
genes is influenced far more by important gene dosage effects than by the
processes of subfunctionalization or neofunctionalization.
In addition to the dynamics of gene repertoires and their arrange-
ments, there is a continual gain and loss of genomic DNA that appears
to be mostly responsible for the extensive variation in genome sizes of
even relatively closely related species. The gain of DNA is chiefly attributed
to the amount of repetitive DNA arising from the activity of “selfish” or
“parasitic” transposable elements (TEs), which are counterbalanced by
the efficacy of the host's defense and clearance systems including the
inhibition of transposition by RNAi
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and cytosine methylation.
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The
outcome of the battle between the host genome and the mobile genetic
elements is variable and depends on the history of the species population
dynamics.
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For example, recognizable TE sequences make up almost
half of the genome of the dengue/yellow fever mosquito,
Aedes aegypti
,
and are thought to have made a major contribution to the approximately
fivefold larger genome size in comparison to the malaria mosquito,
Anopheles gambiae
. Much of the remaining species-specific junk DNA
may in fact consist of ancient disabled TE copies that have diverged
beyond recognition. In fact, there are usually only a handful of active TE
elements and the rest are the neutrally decaying disabled copies, which
are generally assumed to be unconstrained by selection, therefore pro-
viding an opportunity to calibrate an intragenomic molecular clock for
comparative studies. The random insertion of proliferating TEs and their
subsequent random pseudogenization lead to the effect of active ele-
ments jumping around the genome from generation to generation. This
can be exploited to recognize genes originating from TEs whose func-
tion might have been recruited by the host genome, where the purifying
selection of their function has kept these genes intact and immobile
(thereby preserving synteny) over a long evolutionary timescale.
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TE
activity is mostly deleterious, but it provides a rich source of sequences
for evolutionary innovations and may sometimes duplicate whole genes,
create chimeric genes through exon shuffling, or influence gene expres-
sion by altering regulatory regions.
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It also leads to increased frequency
