Biology Reference
In-Depth Information
are nearly overwhelming, and
Hall (1998b)
questioned how it is possible “to
review an over-reviewed subject--one whose reviews have even been reviewed.”
11.5.1.1 The
period
+
Locus of
Drosophila
The
Drosophila period
+
locus (
per
+
) is on the X chromosome and mutations of it
influence eclosion, locomotor activity, and the length of the interpulse interval of
the courtship song (
Table 11.2
). Eclosion of wild-type flies (emergence of adults
from the pupal case) typically occurs around dawn, when the presence of dew
and high relative humidity increases their survival rate (
Figure 11.2A
). Locomotor
activity then decreases during midday and is followed by increased activity again
in the evening. Three classes of mutant alleles exist; they shorten (
per
S
mutants have
19-hour eclosion rhythms), lengthen (
per
L
mutants have 29-hour eclosion rhythms),
or abolish circadian eclosion and locomotor activity rhythms (
per
0
mutants). Flies
with the
per
0
mutation eclose arrhythmically, but periodicity in eclosion can be
restored by
P
element-mediated transformation of arrhythmic flies using the
wild-type
per
+
allele (
Bargiello et al. 1984
,
Figures 11.2B, 11.2C
).
The
per
+
gene is
≈
7 kb long, encodes a 4.5-kb transcript with eight exons, the
first of which is noncoding (
Figure 11.3
). One of the most striking features of
the protein is a series of threonine-glycine (Thr-Gly) repeats in the middle of the
gene (
Costa et al. 1992, Guantieri et al. 1999
). The region encoding the Thr-Gly
repeat is polymorphic in length within and between
Drosophila
species and
plays a role in the thermal stability of the circadian phenotype. For example, 17,
20, or 23 repeats are found in
D. melanogaster
populations, and a clinal pattern
occurs along a north-south axis in Europe and North Africa, with the shorter
sequences in southern Europe (
Costa et al. 1992
).
Costa et al. (1992)
suggested
that the length polymorphism cline is maintained by natural selection under dif-
ferent temperature conditions.
A large number of tissues express the
per
+
product, including embryonic,
pupal, and adult nervous systems, as well as the esophagus, gut, and ovaries.
Liu et al. (1992)
, demonstrated that the
per
+
gene product (PER protein) is pre-
dominantly found in cell nuclei in adult
Drosophila
, and
Hardin et al. (1992)
showed that
per
+
mRNA levels undergo daily fluctuations, which constitutes
a
feedback loop
in which the PER protein affects the oscillations of its own
mRNA. The fluctuation in
per
+
mRNA is due to fluctuations in gene transcription
because the
per
+
mRNA has a relatively short half-life (
Zerr et al. 1990
), which is
consistent with the hypothesis that PER acts as a transcription factor (
Table 11.2
).
The
per
+
genes from
D. simulans
,
D. virilis
,
D. pseudoobscura
, and
D. yakuba
have been cloned and sequenced. Parts of the gene are conserved and parts
are highly diverged, which suggests that conserved regions may encode basic
