Biology Reference
In-Depth Information
genome. As a result, it is desirable to examine the genome of the population
undergoing transformation to be sure that endogenous TE elements related to
the vector are lacking before conducting transformation experiments with a TE
vector. For example, tephritid flies transformed with
hobo
vectors were unstable
and excision was stimulated by heat shocks that presumably elicited the produc-
tion of an endogenous
hobo
-like transposase (
Atkinson et al. 1993
). The excision
rate was 8- to 10-fold higher than that seen for hosts lacking endogenous trans-
posase. It appears that it may be insufficient to examine the target insect for
endogenous versions of
hobo
, however. Laboratory assays indicated that
hobo
transposase functioned with both
hobo
and
Hermes
substrates (
Sundararajan
et al. 1999
). The TE family that contains
hobo
includes elements from plants,
fungi, fish, insects, and humans. Thus, it appears to move horizontally relatively
easily and
hobo
appears to have invaded
D. melanogaster
populations some-
time after 1960 (
Bonnivard et al. 2000
). The broad host range makes
hobo
desir-
able as a vector, but this is considered a negative attribute from the point of
view of risk assessment.
The
mariner
element initially was isolated from
Drosophila mauritiana
, but
is extremely widespread among arthropods (
Robertson 1993, 1995, Robertson
and Lampe 1995
) and is present in nematodes (
Grenier et al. 1999, Leroy et al.
2000
), flatworms (
Garcia-Fernandez et al. 1995
), and hydras (
Robertson 1997
).
It is found in mammals, including humans (
Auge-Gouillou et al. 1995, Oosumi
et al. 1995, Robertson and Martos 1997
). A
mariner
vector was used to trans-
form the chicken (
Sherman et al. 1998
), the zebrafish (
Fadool et al. 1998
), and
the protozoan
Leishmania major
, which indicates that
mariner
has a general
ability to “parasitize the eukaryotic genome” (
Gueiros-Filho and Beverley 1997
).
The broad host range of
mariner
raises the question as to whether there is a
risk that active
mariner
elements (or other TEs) purposefully released in insect-
control programs as “drivers” could move horizontally, even to humans? At
least two different subfamilies of
mariner
were found in the human genome,
which suggests our genome was invaded more than once. In fact,
Plasterk et al.
(1999)
engineered a
mariner
element to make it more active in human cells. It
(
Sleeping Beauty
) has 25-fold higher levels of activity in human cells than the
standard
mariner
. However, the presence of ancient and degenerated
mariner
elements in the human genome indicates that humans developed resistance to
mariner
, suggesting the risk could be low. Rates of transformation of arthropods
with
mariner
have been low (
Coates et al. 1995
,
Lampe et al. 2000, Wang et al.
2000
), perhaps because many insect species have, over evolutionary time, devel-
oped the ability to suppress its damaging effects. Genetic modification of the
mariner
element could improve it as an arthropod vector (
Lampe et al. 1999
).
