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15105-15110. 
MOLECULAR GENETIC STUDIES OF BEHAVIOR AND DEVELOPMENT IN DROSOPHILA 
Michael W. Young, Ph.D., Investigator 
Studies of Biological Rhythms 
Certain behaviors in Drosophila melanogaster 
have a circadian rhythm. Locomotor activity and 
eclosion both occur rhythmically with 24-hour pe- 
riodicities. The rhythms must reflect action of an 
endogenous clock, as persistent environmental cy- 
cling is not required to maintain the rhythms. The 
laboratory has recently completed a study of the on- 
togeny of Drosophila's clock(s) controlling circa- 
dian behaviors. 
Drosophila born and reared in constant darkness 
and at constant temperature exhibit circadian loco- 
motor activity rhythms as adults. Thus exposure to 
environmental cycling does not appear to be needed 
to establish biological rhythmicity. However, the 
circadian rhythms of the individual flies composing 
these dark-reared populations have many different 
phases. The absence of synchrony among flies ex- 
tends to those commencing development at the 
same time. The latter results show that a biological 
clock(s) controlling these rhythms in Drosophila 
begins to function without a developmentally im- 
parted phase. 
The phases of rhythms produced by dark-reared 
flies can be synchronized by supplying light treat- 
ments during development. Experiments of this sort 
have been used to map the earliest detectable func- 
tion of a Drosophila clock(s). Light treatments 
initiated and ending as early as the developmen- 
tal transition from embryo to first-instar larva can 
synchronize adult locomotor activity rhythms. Be- 
cause the behavior of adult flies is seen to be syn- 
chronized by treatments ending in early develop- 
ment, a clock controlling circadian rhythms should 
function continuously from the time of larval hatch- 
ing to adulthood. 
The flies' circadian rhythms are influenced by 
mutations of the period (per) gene. Three mutations 
were characterized in earlier studies. Two of these, 
per^ and per\ are associated with single-amino acid 
substitutions. The first, per^, shortens the period to 
19 hours, while per^ lengthens it to 28 hours. The 
third mutation, per", results in arrhythmic flies and 
a truncated, presumably functionless protein. 
In dosage studies using transgenic Drosophila 
that express different levels of per RNA, the labora- 
tory has shown that period length is correlated with 
abundance of the per product. Those flies produc- 
ing lower levels of per RNA have longer-period cir- 
cadian rhythms. Because per^ and per'' mutants pro- 
duce wild-type levels of per RNA, it was thought that 
the mutants might produce hypoactive and hyperac- 
tive per proteins, respectively. Dr. Young and his 
colleagues have been interested in determining if 
and how a change in per protein structure might 
increase the protein's activity. 
Genes carrying new per mutations have been pro- 
duced by in vitro mutagenesis and returned to Dro- 
sophila by P-element-mediated transformation. A 
wide variety of missense mutations, each altering 
the same region of ~20 amino acids around the site 
of the original per^ mutation, have been character- 
ized. The mutations predominantly generate short- 
period phenotypes. In many cases double- and tri- 
ple-amino acid substitutions were generated. In 
fact, the line with the shortest period rhythm, ~ 1 .5 
hours shorter than the original per^ mutant, was as- 
sociated with a triple-amino acid substitution. 
These results indicate that short-period pheno- 
types result from loss rather than gain of function in 
a limited domain of the per protein. Possibly this 
region of the protein suppresses activity of the per 
protein, and therefore plays a regulatory role, in 
wild-type flies. 
The laboratory has been characterizing a new 
chromosome-linked clock mutation. The timeless 
(tim) mutation produces arrhythmia for both eclo- 
sion and locomotor activity. In addition to its effects 
on behavioral rhythms, the tim mutation affects ex- 
pression of per. Although the per locus is found on 
the X chromosome, tim alters circadian timing of 
/>er transcription. Since the arrhythmic phenotypes 
GENETICS 291 
