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can hook up a gene of interest to the UAS promoter,
then the gene of interest will be expressed wherever
GAL4 is. This enhancer trap system was used to
express the transformer gene, a feminizing signal, in
olfactory neurons that might be processing the
pheromonal signal (Ferveur et al., 1995).
Transformed males were presented with flies of
either sex to see whether they would selectively court
the female. Surprisingly, some strains of flies courted
males with as much vigor as they did females (Figure
10.19C). The behavior of transformed animals may
be due to their failure in discriminating the female
pheromone. In fact, when the enhancer trap technique
is used to make male flies that secrete only female
pheromones, these flies are courted as if they are
females (Ferveur et al., 1997). Thus, in flies, specific
brain regions must have a gender if animals are to
accurately interpret sensory information and produce
sexually appropriate motor responses.
A separate tack has been used to explore what kinds
of genes must be expressed in male or female nerve
cells in order to produce correct sexual behaviors (Hall,
1994). For example, a gene product called fruitless is
expressed in about 500 neurons of male flies only, and
mutations of this gene also cause males to court one
another. A mutation of the dissatisfaction gene leads
virgin females to resist males during courtship, and
they fail to lay mature eggs (Finley et al, 1997). Most
mutations that affect sexual activity in flies are also
found to affect other behaviors. Mutations of the period
gene affects circadian rhythms, but they also change
the temporal properties of the courtship song.
Depending on the precise mutation, the interval
between wing-beats can be shorter or longer than
normal. That is, the song will have a lower or higher
frequency, respectively.
The P[GAL4] enhancer trap system
Enhancer-trap element
b -galactosidase
wild type
strain 2
expresses transformer
in the same location as
b -galactosidase
Tested with:
decapitated male
decapitated male
Strain 2
( tra )
Courtship (%)
FIGURE 10.19 Enhancer traps and the expression of the trans-
former gene. A. An enhancer trap element inserts into the fly
genome between an enhancer region and the gene that it normally
controls. Whenever the enhancer is activated by a transcription
factor, a reporter gene within the enhancer trap is expressed. In this
example, a yeast transcription factor called GAL-4 gene is expressed.
To visualize the anatomical location of GAL-4 expression, the
enhancer trap flies are crossed to flies that have a UAS-lacZ gene.
Since GAL-4 is a transcription factor that activates UAS (blue), the
lacZ gene (red) is expressed, and it encodes a protein (b-galactosi-
dase) that can be stained for (red). Thus, labeled cells are known to
have GAL-4 expression. B. The enhancer trap line can also be used
to drive the expression of native genes, such as transformer (green).
Expression occurs only in cells with an activated enhancer. C. When
an enhancer trap line was used to express transformer in olfactory
neurons, the male flies courted males and females equally. Normal
males only court females. (Adapted from Ferveur et al., 1995)
One of the most striking correlations between
sexual behavior and brain anatomy is found among
several species of songbirds. Male birds attract a mate
of the same species with vocalizations, or songs, that
are commonly learned during juvenile development
(see below). Zebra finches learn one song during the
first 80 days after hatching, while canaries add new
phrases to their song each breeding season. When sci-
entists first looked at the brains of these animals, they
were startled to find brain regions of remarkably dif-
ferent size in each sex (Nottebohm and Arnold, 1976).
The sexual dimorphism occurs in brain nuclei that are
known to participate in song production (RA, HVc),
the brain is female (Figure 10.19). A piece of DNA,
called PGAL4, is randomly inserted in the genome of
many flies. By chance, it will occasionally insert next
to an enhancer, and this enhancer will then activate the
GAL4 gene in the enhancer trap element. If the
enhancer is only active in one part of the body, then
GAL4 will be expressed in that same part of the body.
How does this help feminize the brain? It turns out
that GAL4 can activate another promotor, called
upstream activating sequence (UAS). If an experimenter
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