Photoreception in decapitated larvae of silkworm Bombyx mori

Abstract

To investigate the photoreception that controls daily oscillations at the periphery in insects, we decapitated larvae of the silkworm Bombyx mori (Lepidoptera: Bombycidae) by ligature, and observed rhythms in their peripheral tissues under several light conditions. We measured the mRNA expression of period (per) and timeless (tim), which are homologues of Drosophila clock genes that function in the core oscillator of the circadian clock system. The expression of both per and tim significantly changed in the midgut, Malpighian tubules and silk glands of decapitated larvae exposed to photophase and scotophase that were reversed from the original daily light–dark cycle under which the larvae were housed. Under constant darkness, the daily expression of tim mRNA persisted for at least one cycle in the midgut and silk gland. In addition, an appropriate light stimulus under constant darkness induced a significant phase shift in the endogenous timing system (probably a circadian clock) that determined peak levels of tim mRNA expression in the midgut and silk glands of decapitated larvae. Since light regulated the gene expression rhythm in peripheral tissues of decapitated silkworm larvae, neither the brain nor eyes were essential for photoreception to control daily oscillations in these tissues. Thus, peripheral tissues in insects might directly use light even at the larval stage.

Involvement of the Clock Gene period in the Circadian Rhythm of the Silkmoth Bombyx mori

“…Expression peaks occurred in the early scotophase, and expression levels were low in the photophase. Similar expression trends were reported in the adult brain and larval midgut of B. mori (Iwai et al, 2006; Nobata et al, 2012). Tao et al (2017) showed in the embryo of this species that the expression peak of per also resides in the earlier scotophase but that of tim occurred in the later scotophase.…”

The Circadian Clock in Lepidoptera

“…Although extensive and standardized studies have not always been performed, several tissues have been shown to express multiple clock factors during larval development in species such as B. mori , S. litura , and S. littoralis . Specifically, last (5th) instar larvae of B. mori actively transcribe the clock genes Bm tim , Bm per , Bm Clk , and Bm Bmal1 in the head, midgut, fat body, and silk glands ( Markova et al, 2003 ; Iwai et al, 2006 ; Xu et al, 2011 ; Nobata et al, 2012 ). In the midgut, a significant cycling expression of Bm tim was demonstrated in both LD and DD, while Bm per mRNA levels tended to be higher during the night compared to the day ( Nobata et al, 2012 ).…”

“…Specifically, last (5th) instar larvae of B. mori actively transcribe the clock genes Bm tim , Bm per , Bm Clk , and Bm Bmal1 in the head, midgut, fat body, and silk glands ( Markova et al, 2003 ; Iwai et al, 2006 ; Xu et al, 2011 ; Nobata et al, 2012 ). In the midgut, a significant cycling expression of Bm tim was demonstrated in both LD and DD, while Bm per mRNA levels tended to be higher during the night compared to the day ( Nobata et al, 2012 ). Among the multiple cell types composing the silkworm midgut, BmPER staining was limited to goblet cells, known for their role in the active transport of potassium from hemolymph into the gut lumen and in the release of digestive enzymes ( Daimon et al, 2008 ; Park et al, 2014 ).…”