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have eventually allowed the diminution of the haploid generation
because a gene silencing or control mechanism would have allowed
the somatic developmental program to produce both heterochronic or
progressive differentiation and genome quality checking. The RNA-
mediated gene silencing pathways are considered to play an important
role in just such metastable gene suppression in ontogeny (Cox et al.
1998; Allis et al. 2007; Havecker et al. 2012). Although haploid and
diploid genomes of extant species can have distinct transcriptomes
(Dilkes and Comai 2004; Autran et al. 2011), also indicating that a major
role still exists for gene silencing in alternation of generations, there
remain many unknown details of how this is achieved (Feng et al. 2013).
It is interesting to note that the original discovery of the mechanism of
microRNA-mediated gene silencing occurred by the identi
cation of a
heterochronic locus (Lee et al. 1993).
Although there are several interpretations regarding the function and
evolutionary signi
cance of invasion of viral DNA into host genomes
(Bock 2010), an unusual bold perspective holds that host function of TEs
(such as in gene silencing) may have evolved before interorganism
invasiveness. Thus, TEs, may have resulted in the evolution of a
mechanism to transfer DNA from one individual host to another.
Although the possible origins of TE function are a bit like the chicken
and the egg, the evolution
first of a host function for interorganism, the
DNA transfer also could have produced a natural, selectable system to
move DNA horizontally and coordinate and spread information between
cells of different individual hosts.
A focus on the mobility of TEs outside in the physical environment (if
for only brief periods) may have strongly fostered the prevalent idea that,
in that context, they (invasive DNA viruses) evolved
first as self-entities
that sought reentry into hosts solely for multiplication. This view is in
stark contrast to the other possibility that since they had to originate in
host genomes, they also use reentry mechanisms to accomplish primar-
ily a function in the host genome, for example, speci
c activation or
repression of host genes (Kathiria et al. 2010). When a DNA transfer
system such as this becomes too active (invasive and random), it could
also lead subsequently to a sex-mediated recombination process of TE
DNA elimination, or silencing to control invasive movements as now
more commonly accepted. However, from either perspective, of which
came
city of movement could have
allowed TEs to establish a mechanism of horizontal transfer of non-
random genetic changes that, importantly, affect phenotype (Kathiria
et al. 2010). Three signi
first, the gaining of site speci
cant characteristics of such a process could
emerge and be extremely signi
cant
to adaptive evolution. First,
