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a range of heterochronic phenotypes, including the precocious presence
compound eyes and a pupal-like cuticle. The
Tribolium
Met protein binds
JHwith high affinity, and forms heterodimers in a ligand-dependent manner
with another bHLH-PAS protein, Taiman, which is orthologous to the
above-mentioned SRC (aka FISC) (
Charles et al., 2011; Li, Mead, &
Zhu, 2011
).
Taiman is not the only known partner for Met. The Raikhel laboratory
recently identified the circadian clock protein CYCLE as a binding partner
for Met in
Aedes aegyti
, using yeast-two-hybrid screening (
Shin et al., 2012
).
Cycle encodes, like Met, a member of the bHLH-PAS protein family, rais-
ing the interesting possibility that circadian circuits intersect with JH
response pathways by direct protein-protein interactions. The interaction
of these two proteins required the presence of JH, similar to what has been
observed for Taiman. Interestingly, Taiman was also shown to be important
for the JH-dependent action of the Cycle-Met heterodimer. In particular,
the expression of two JH-responsive genes,
Kr-h1
and
Hairy
, was impaired
when RNAi against Met, Cycle, or Taiman was used. Interestingly, the
transcripts of both
kr-h1
and
hairy
showed clear periodicity when animals
were kept on a LD cycle, but this oscillatory behavior was lost in DD con-
ditions, indicating that
kr-h1
and
hairy
are regulated by both light and JH.
Consistent with this, the expression levels of both transcripts were highly
elevated, but still rhythmic when JH was applied topical (
Fig. 1.5
). This
observation raises the interesting question whether JH levels change rhyth-
mically during a particular developmental window, and whether JH could
act as an internal zeitgeber for peripheral clocks.
The authors then tested whether CLOCK (
Yu & Hardin, 2006
), the
heterodimeric partner of Cycle in the canonical circadian pathway, modu-
lates JH induction of
Kr-h1
and
hairy
. RNAi-mediated knockdown of
clock
did not affect the rhythmic expression of
kr-h1
and
hairy
, suggesting that the
canonical function of CYC/CLK might not be directly involved in the
JH-dependent regulation of
kr-h1
and
hairy
. Deciphering the precise
crosstalk between JH-mediated regulation and the circadian clock represents
an exiting goal for future experiments, which could pave the way for a better
understanding how the endocrine system interacts with the circadian clock.
11. OUTLOOK
Since the first evidence of circadian gating of developmental transi-
tions in
Manduca
, significant progress has been made that allow for a better
