Biology Reference
In-Depth Information
VI. CONCLUSIONS
A. The human model
The human organism, in many respects, serves as an exemplary system within
which to consider the interplay between host genomes and their TE inhabitants.
As detailed above, we find in our own species clear examples of both the positive
and negative attributes of TE activity. We have discussed at length the extensive
systems the host organism has evolved to curtail TE activity. This serves a strong
indicator that TE activity, left unchecked, presents a significant danger to the
host organism. We must, however, consider that the unchecked activity of many
“functional” biological processes would also be detrimental to the organism.
When such processes are themselves curtailed, we are apt to say they are “kept
in balance” or “regulated,” as opposed to “defended against.” The presence of
host repression mechanisms, in and of themselves, is not sufficient to exclude a
possible functional role for persistant TE activity, at least among some taxa.
Perhaps what distinguishes TEs from these more essential systems is the
ambiguity concerning the extent to which they are components of the “organis-
mal machine,” as opposed to separate entities pursuing their own agenda. As
Dawkins illustrated in his
(1976), any gene can be conceived of
as pursuing its own agenda of propagation within subsequent generation, poten-
tially at the expense of what might appear to be the wellbeing of its host
organism. When the gene happens to be fundamental to a species' survival,
however, we tend to have a more tolerant view towards this sort of scheming.
The ability of some TEs to be horizontally transferred across species boundaries
strongly suggests that their presence can be nonessential for organismal function.
Presumably the host species receiving the horizontally transferred TE was getting
along well enough before the TE arrived on the scene. At the same time, we
perceive TEs to be diligently pursuing their own agenda of proliferation, occa-
sionally generating negative consequences for the organism. From this perspec-
tive, their behavior is more like a parasite, one that just so happens to reside in
our own germline. But, as discussed above, TEs have occasionally made positive
functional contributions to organismal biology during evolution. The overall
frequency of such contributions, however, remains to be determined. Is it
sufficiently high enough to pay the rent, so to speak. For further insight, we
can look to the study of the human microbiome. The increasing realization that
mammals and other multicellular eukaryotes function as super-organisms, com-
prised of a number of interacting entities exhibiting various degrees of interde-
pendence. In any such system, the line between self and nonself is not so easily
delineated. In the case of the human organism, a spectrum emerges wherein, on
the one extreme, there is the mitochondria organelle, an entity fully integrated
and essential to organismal survival. On the other side are members of the
The Selfish Gene
