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Circadian clocks regulate many aspects of behavior, including
sleep-wake cycles, sensory function, and reproduction. Genes that code for
circadian clock components have been discovered by screening a variety of
model organisms for mutations that alter the period or amplitude of
circadian rhythms. We have initiated a behavioral screen for clock
mutants in zebrafish in an effort to identify new vertebrate circadian
clock genes and to gain insight into the functions of known clock genes.
Our primary screening assay is the circadian rhythm in spontaneous swimming
activity of 9-18 day old larvae, measured by an automated infrared video
image analysis system. We are screening for dominant mutations that alter
the timing of behavioral rhythms in the progeny of ENU treated males
crossed with wild-type females. We recovered two confirmed clock mutants
from the first 1275 animals tested. Both of these mutant alleles are
incompletely dominant, shortening the freerunning period of behavioral
rhythms by 0.5-0.8 h in heterozygotes and 1-1.5 h in homozygotes. One of
the mutations maps to Linkage Group 7 and the other maps to LG 20. We have
so far confirmed that one of these mutations also shortens the period of
the melatonin release rhythms measured from cultured pineal glands,
indicating that the mutant gene product affects tissue-level rhythmicity as
well as behavior. We are continuing to screen for new mutants in order to
identify additional alleles of these genes, as well as mutations in other
vertebrate clock genes. Our experience so far indicates that the zebrafish
will be a productive model system for genetic analysis of the biological
clocks that regulate vertebrate behavior.
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