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related to retroviruses that have long-terminal repeats (LTRs). They also include
elements that lack long-terminal repeats (non-LTR retrotransposons).
Class II elements transpose directly from DNA to DNA. They include elements
with short-inverted terminal repeats and have a coding region for a transposase.
They also include elements with long-inverted repeats. Many TEs have been dis-
covered in D. melanogaster ( Bowen and McDonald 2001 ). A diversity of TEs is
known from other insects as well.
Another group of transposable elements, called Helitrons , have been found
( Kapitonov and Jurka 2001, 2007, 2008 ). Helitrons , a new class of TEs, are found
in eukaryotes ( Kapitanov and Jurka 2001 ). These unusual TEs, first found in
plants and the nematode Caenorhabditis elegans but now known to occur in
many organisms, including insects, seem to replicate by a rolling-circle method.
Transposition is thought to occur by movement of one strand from one genomic
site to another, where it is the template for DNA repair, resulting in a double-
stranded DNA insert ( Kapitonov and Jurka 2007 ).
At least half of all spontaneous mutations in D. melanogaster are due to inser-
tions of TEs. For example, P elements in D. melanogaster cause excisions, chromo-
some rearrangements, and insertions. The foldback ( FB ) transposon (the names of
most TEs are italicized) is associated with deletions, inversions, reciprocal translo-
cations, and insertional translocations in which normally unique Drosophila DNA
is flanked by two FB elements. All well-characterized, highly unstable genes in
D. melanogaster contain either the P element or FB elements ( Berg and Howe
1989 ). Members of the HeT-A and TART families of TEs are found at telomeres
and in centromeric heterochromatin and never in the euchromatin regions of
chromosomes in D. melanogaster ( Mason et al. 2000 ). Kapitonov and Jurka (2003)
analyzed the newly sequenced genome of D. melanogaster and discovered addi-
tional, previously unknown TEs called Transib . They also found ancient, 5-million-
year old ancestors of the P element, as well as Helitrons and they estimated that
TEs are “three times more abundant than reported previously, making up to
~22% of the whole genome.” Biemont and Vieira (2006) reported “ 50-80% of
mutations are due to such insertions” and that TEs are “ more powerful produc-
ers of the raw material of evolution than is the classical nucleotide-base substitu-
tion rate, which is around 10 8 to 10 9 per nucleotide per generation.”
TEs may carry genetic information, regulate genes, or initiate genetic changes
( Britten 1997, Miller et al. 1997, Shapiro 1999, Biemont and Vieira 2006 ). Wilson
(1993) suggested that TEs could lead to resistance to pesticides, although he
did not provide any direct evidence for this conclusion. Agarwal et  al. (1993)
found a TE named Juan associated with amplification of an esterase gene in
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