Biology Reference
In-Depth Information
pesticide-resistant
Culex
mosquitoes, but a direct involvement in inducing
gene amplification was not demonstrated.
Waters et al. (1992)
suggested the
TE called
17.6
is involved in susceptibility to pesticides in
Drosophila
associated
with a P450 gene. However,
Delpuech et al. (1993)
screened colonies of
D. mela-
nogaster
and
Drosophila simulans
from around the world, and they found no
relationship between the presence or absence of
17.6
and resistance.
Daborn
et al. (2002)
, however, found a P450 allele associated with resistance to DDT due
to overtranscription of the gene and associated with the insertion of an
Acord
TE into the 5
′
end of the gene.
Aminetzach et al. (2005)
found that insertion
of long interspersed elements (LINEs) is associated with “increased resistance to
organophosphates,” by the generation of a new protein.
One example of TEs containing genetic information may be found in
Drosophila hydei
males. TEs and repetitive DNA sequences comprise the majority
of the Y chromosome of
D. hydei
. Apparently, the lampbrush loop-forming fer-
tility genes on the Y chromosome consist, at least in part, of retrotransposons of
the
micropia
family (
Huijser et al. 1988
).
R1 and R2 are class I TEs originally found in some of the 28S rRNA genes of
the silk moth
Bombyx mori
, and several dipterans. A survey suggested that R1 and
R2 elements occur within the rRNA genes of many insects (
Jakubczak et al. 1991
).
Forty-three of 47 species surveyed, including species in Odonata, Orthoptera,
Dermaptera, Hemiptera, Coleoptera, Hymenoptera, Lepidoptera and Diptera,
contained the insertions in 5-50% of their 28S genes (
Burke et al. 1993
). The
broad distribution of these elements raises the question of whether they could
have been present in insects before their radiation. The R2 elements block the
production of functional 28S rRNA by the inserted gene, but this is not usually
deleterious to the organism because insects contain many more rRNA genes than
are needed (
Eickbush and Eickbush 2011
).
Ye et al. (2005)
found that R1 and R2
non-LTR retrotransposons compete; R1 elements insert into a site that is 74bp
downstream of the R2 insertion site, so the presence of an R2 element may inhibit
the expression of R1. As a result, the R1 element of
D. melanogaster
rarely inserts
into 28S genes already containing an R2 element.
Very little is known about the origin and evolutionary history of TEs. A
TE family might originate in a species, or TEs might be acquired by
horizon-
tal transmission
from another species. Normally, DNA or RNA sequences are
transmitted
vertically
from parent to progeny, but in horizontal transfer, DNA
sequences are transferred laterally across species, taxonomic borders that were
once thought to be inviolable (
Daniels et al. 1990, Kidwell 1992, Plasterk et al.
1999
). For example, the
hobo
transposable element of
D. melanogaster
has a
