Biology Reference
In-Depth Information
Flies in which the
Shaker
gene is deleted still have K currents and
Butler et al.
(1989)
isolated three additional family members,
Shab
+
,
Shaw
+
, and
Shal
+
.
These four genes define four K-channel subfamilies in
Drosophila
and homolo-
gous genes isolated from vertebrates all appear to fall into one of these four
subclasses.
Other K-channel mutants, including
eag
(
Warmke et al. 1991
) and a cal-
cium-activated K-channel gene (
slo
) (
Atkinson et al. 1991
), have been isolated.
Another neurotransmitter,
γ
-aminobutyric acid (GABA), is a major inhibitory
agent in the insect nervous system. The synthesis of GABA is controlled by the
enzyme glutamic acid decarboxylase (GAD) (
Jackson et al. 1990
).
11.5.7 Divergent Functions of
Est-6
and
Est-5
in Two
Drosophila
Species: A
Cautionary Tale of Homologs
Evolutionary changes in gene regulation can be important in macroevolu-
tionary change and species divergence. One case study involves an analysis of
the
Esterase-6
gene in
Drosophila melanogaster
and its homolog (
Esterase-5
)
in
D. pseudoobscura
(
Brady and Richmond 1990
). Both influence behavior in
D. melanogaster
but have a very different function in the two species, indicating
that sequence homology may not be equivalent to behavioral homology.
The Esterase-6
(
Est-6
) gene influences male-mating speed and rate of remat-
ing by
D. melanogaster
females. Fast- and slow-variants of the Esterase-6 pro-
tein are produced in natural populations of
D. melanogaster
. More Esterase-6
protein is produced in adult males than in females. The enzyme is highly con-
centrated in the anterior ejaculatory duct of males and is transferred to females
during the first 2-3 minutes of the 20-minute copulation. Enzyme activity in
females can be detected up to 2 hours after mating and influences the timing
of remating by females. Males transfer a substance in the seminal fluid that is
converted in the females' reproductive tract by the Esterase-6 protein into a
pheromone that serves as an
antiaphrodiasiac
. The antiaphrodiasiac reduces the
sexual attractiveness and receptivity of females, reducing the likelihood she will
remate. Because the sperm from the most recent male takes precedence in fer-
tilizing a female's eggs, this behavior encourages monogamy in
D. melanogaster
females (
Richmond et al. 1986
).
The Esterase-6
gene also influences the rate of mating of males in
D. mela-
nogaster
. Males with the slow variant of the protein require 10.2 minutes to
achieve copulation with females, whereas males with the faster-moving protein
require only 5.7 minutes. Once the
Est-6
gene was cloned, it was used as a probe
to identify homologous genes in related species (
Brady and Richmond 1990
), and
