Biology Reference
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surface for pupation. The second gene,
pre-intermolt
+
, also is expressed in larval
salivary glands. Genes homologous to
dunce
+
have been identified in mice, rats,
and humans, and the mammalian counterpart of
dunce
+
functions in regulating
mood (
Tully 1991a
).
dunce
+
is expressed in the mushroom bodies in the brain of
D. melanogaster
(
Figure 11.1
). This was discovered because the mushroom bodies can be stained
with an antibody to the
dunce
+
-encoded protein (
Figure 11.1
)
.
The activity of the
dunce
+
gene was identified by the
enhancer trap
method (
O'Kane and Gehring
1987
), a technique that involves placing a
reporter gene
(such as
β
-
galactosidase
which turns the fly's brain tissues blue when the substrate is added) into the
P
element under the control of a weak constitutive promoter. When this
P
ele-
ment is brought in proximity to a tissue-specific enhancer after the
P
inserts into
a chromosome,
β
-
galactosidase
expression will be regulated by the “native”
enhancer in a tissue- and stage-specific pattern. Ideally,
β
-
galactosidase
will be
expressed in a manner similar to the native gene. To determine which genes are
expressed in the mushroom bodies, fly brains were screened and some 50 learn-
ing mutants were identified, including several alleles of
rutabaga
+
. Mutations
of
rutabaga
+
cause decreased expression of cAMP and the rutabaga protein was
identified as an adenylate cyclase (
Han et al. 1992
,
Table 11.3
).
Mushroom bodies are important for olfactory learning and memory. In
D. melanogaster
these structures are paired and consist of
≈
2500 neurons (
Davis
1993, Heisenberg 1998
,
Figure 11.1
). Mushroom bodies receive olfactory infor-
mation from the antennal lobes. Mushroom bodies house part of the short-term
memory for odors, are required for courtship conditioning memory, and are
necessary for context generalization in visual learning, as well as regulating the
transition from walking to rest (
Zars 2000
).
Learning requires other brain centers, including the antennal lobes, the cen-
tral complex, and the lateral protocerebrum in insects (
Davis 1993, Hansson
and Anton 2000
). During metamorphosis, the nervous system of holometabo-
lous insects such as
Drosophila
changes significantly. A controversy has existed
as to whether flies retain learned behavior after metamorphosis from larvae to
adults. There is no evidence that larval conditioning induces a change in adult
olfactory responses (
Barron and Corbet 1999
). This is not surprising, because
larval sense organs undergo histolysis during the pupal stage and adult sense
organs are formed
de novo
from imaginal discs. The mushroom bodies of the fly
brain are extensively rewired during metamorphosis.
Drosophila
carrying a mutant version of the
turnip
+
gene have difficulty in
olfactory discrimination, conditioning of leg position, larval, visual and reward
learning (
Table 11.3
,
Choi et al. 1991
). Additional mutated genes, including
