Biology Reference
In-Depth Information
B. A positive note
The previous section highlighted several avenues by which TE activity can
potentially compromise genomic integrity and, as a consequence, human health.
At the same time, it is also safe to say that evolution of humans would have taken
a dramatically different course—if it could even have been possible at all—were
it not for the periodic “intervention” of TE activity. There are now numerous
established examples of TEs being coopted for functional roles across many
different taxa. Perhaps most famously, the vertebrate adaptive immune system
was made possible, in part, by the invasion of immunoglobulin-coding genes by a
RAG transposon (reviewed in Flajnik and Kasahara, 2010). The recombination
signal sequences (RSS), introduced into these genes by the transposon conferred
the ability for directed somatic recombination, increasing the ability to rapidly
generate antibody diversity. A second example is that of the syncytin gene,
which is derived from the envelop gene of the human endogenous retrovirus
(HERV-W; Mi
, 2000). Yet another important example of the contribution
of TEs to human evolution was the discovery that the SETMAR gene was
derived from the fusion of the SET histone methyltransferase gene with a
mariner-like Hsmar1 transposon (Cordaux
et al.
, 2006). SETMAR was observed
to have inherited several biochemical properties from its transposon source,
although its molecular function remains poorly understood (Liu
et al.
, 2007).
In addition to the creation of novel genes by the direct contribution of genetic
sequence, TEs activity can result in the rearrangement, duplication, and/or
reshuffling of existing exons or entire genes (Goodier
et al.
et al.
, 2000; Moran
et al.
,
1999; Pickeral
et al.
, 2000). A demonstration of this process in humans was
provided by Xing
(2006), where the authors observed that three copies of
the AMAC gene in primates had been generated by L1 transduction (Xing
et al.
,
2006). Several lines of evidence, including intact ORFs and active expression,
suggest these newly emerged copies play a functional role in the genome. These
are but a few examples from an increasing number of examples of TE domestica-
tion events. Across the entire genome, it has been reported that TE sequences
are incorporated in as much as 4% of human protein-coding genes (Kim
et al.
,
2010a,b). As our knowledge of the genome increases, demonstrations of TE
contributions to organismal biology will continue to mount. There nevertheless
remains a large theoretical and empirical leap to be made between the observa-
tion of extensive evidence of TE domestication, to the conclusion that TE
lineage activity is maintained by natural selection fulfill such an evolutionary
role. Once again, it is important to be reminded of Kidwell and Lisch's observa-
tion that the role of TEs in the evolution of its host is not at odds with their
having an essentially parasitic relationship to the genome. As discussed below,
how precisely to characterize that relationship depends, in part, on empirical
data that remains to be collected.
et al.
