Agriculture Reference
In-Depth Information
spreading a genetic change laterally within a population could occur
without the need to reproduce new generations. Second, such rapid
genetic changes could occur within even very small species populations
compared with Hardy
Weinberg (Stern 1943)-type slow population
shifts. Finally, the rapid time frame for the population to acquire the
genetic change would dramatically increase the adaptation rate of
organisms with long reproductive cycles, especially in very challenging
environments.
The most striking evidence that the TE silencing system actually did
evolve to participate in the control of somatic ontogeny may be in the
1 billion year old ciliate protozoans. Considerable work with ciliates has
revealed that prior to the postmeiotic somatic ontogenic gene expression
control programs, evolution apparently
-
first produced a remarkably
related molecular process to preferentially remove repetitive and poten-
tially mobile sequences. Members of the ciliates, such as Tetrahymena,
have an intriguing system of double, micro- and macronuclei (Meyer
and Chalker 2007). The micronuclei are diploid and are maintained as
the germ line genome (the protected totipotent program) that undergoes
meiosis to form haploid germ cells or gametes. After fusion of meiotic
products some diploid micronuclei differentiate into highly rearranged
polyploid macronuclei
that guide somatic development
through a
process of gene-speci
c expression achieved by DNA elimination, while
micronuclei remain intact and silent. In Tetrahymena, DNA rearrange-
ments or
have, by several approaches, been linked
to the prevention of the potentially highly deleterious class I-type
TE sequences from becoming active during somatic cell ontogeny
(Mochizuki and Gorovsky 2004; Yao and Chao 2005). Tetrahymena
does this by using a genome surveillance mechanism that compares
the highly conserved germ line genome to the soma development
mediating genome using nucleotide pairing mediated by small RNAs
(sRNAs) or scan RNAs (scnRNAs) (Schoeberl et al. 2012). The connec-
tion of this DNA sequence elimination process in ciliates to chromatin
marking at the very ancient and highly conserved histone target H3K9
(Liu et al. 2004) also strongly implies that multicellular organisms have
advanced this gene expression control from DNA removal to DNA
silencing by using in part, altered chromatin structure that has evolved
from the same conserved marks directed by nucleotide pairing. The
“
recombinations
”
findings that double-stranded RNA(dsRNA) gene silencing components,
such as Argonaute and Dicer, are required for soma genome loci targeting
for chromatin modi
cations, with marks directed by DNA pairing and
genome rearrangements (Mochizuki et al. 2002; Malone et al. 2005;
Mochizuki and Gorovsky 2005), have provided an even more
