Biology Reference
In-Depth Information
molecular biology and evolution of TEs have been obtained from the study of
TEs from a wide assortment of nonhuman organisms. We attempt to draw upon
these diverse systems throughout our review to illustrate important concepts in
the host-element relationship.
Advances in the field of TE research over the past two decades have
effectively put to rest lingering questions about their overall relevance within
biology. Although once relegated as mere evolutionary curiosities, clear and
abundant evidence of their far-reaching impact now extends across diverse
disciplines, ranging from evolutionary biology to medical genetics. This new
perspective on mobile elements is perhaps best summed up by Lynch (2007),
“
they are so ubiquitous, so diverse, and have such a profound effect on
eukaryotic chromosomal architecture that one can reasonably argue that an
overview of genomic evolution ought to start with them, before moving on to
the host genes themselves” (Lynch, 2007). Among other things, the activity of
TEs has been an expansionary force in the genome that provided abundant
fodder with which natural selection could experiment. TEs have been exapted
to perform functional biological roles numerous times during the history of life
(reviewed in Bowen and Jordan, 2007; Dooner and Weil, 2007). The fact that
such an important role was played by TEs is no longer matter of debate. That this
role was cultivated and maintained by natural selection to promote “evolvabil-
ity” is a stronger claim, and one for which both theoretical and empirical support
remains lacking. This last claim is closely related to what has been an enduring
question in TE research. Namely, how do we best describe the unusual relation-
ship between host genomes and their TE residents? Is it mutualistic, symbiotic,
parasitic, or something that defies our typical classification schemes? Views range
from, on the one extreme, TEs as vital components of organismal biology to, at
the other extreme, completely parasitic selfish genes undermining organismal
fitness. The true nature of the host-TE dynamic no doubt rests somewhere
between these polar extremes. A more nuanced view is offered by Kidwell and
Lisch (1997), when they write, “the idea that TEs are primarily parasitic is not at
all inconsistent with a role for these elements in the evolution of their hosts.”
Throughout this review, we indicate empirical evidence supporting what is
ultimately an ambivalent role for TEs in organismal biology.
...
II. THE HUMAN ELEMENTS
The TE families remaining active in modern humans include the LINE-1 (L1)
element, Alu, SVA, and human endogenous retroviruses (HERVs). All of these
lineages belong to the Type 1, retrotransposon class (Fig. 6.1). Their basic
structure is outlined below; a more detailed description of their molecular biology
is provided in Belancio
et al.
(2010a).
