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Abstract

Circadian behavior is widely observed in insects; however, the mechanisms that drive its evolution remain a black box. While circadian activity
rhythms are well characterized in adults within the order Lepidoptera (i.e.,
most butterfly species are day active, while most moths are night active), much
less is known about daily activity and clock gene expression in the larval stage.
Additionally, direct comparison of clock gene expression between day-active
and night-active species reared together has not been quantified. Our study
characterized the daily rhythms of caterpillar feeding and activity in addition
to the gene expression of 2 central circadian clock genes, period (per) and timeless
(tim), in larvae and adults of the day-active butterfly Danaus plexippus and the
night-active moth Heliothis virescens. We found that neither Danaus nor Heliothis
caterpillars are strictly diurnal or nocturnal like their adult counterparts; however,
we found that slight rhythms in feeding and activity can arise in response
to external forces, such as temperature and host plant. Expression levels differed
between genes, between butterfly larvae and adults, and between butterfly
and moth species, even though expression levels of both per and tim
oscillated with a similar phase over 24 hours across all treatments. Our study,
the first of its kind to investigate circadian timekeeper gene expression in 2 life
stages and 2 species, highlights interesting differences in core clock gene
expression patterns that could have potential downstream effects on circadian
rhythms.