Evolutionary links between circadian clocks and photoperiodic diapause in insects

Abstract

In this article, we explore links between circadian clocks and the clock involved in photoperiodic regulation of diapause in insects. Classical resonance (Nanda-Hamner) and night interruption (Bünsow) experiments suggest a circadian basis for the diapause response in nearly all insects that have been studied. Neuroanatomical studies reveal physical connections between circadian clock cells and centers controlling the photoperiodic diapause response, and both mutations and knockdown of clock genes with RNA interference (RNAi) point to a connection between the clock genes and photoperiodic induction of diapause. We discuss the challenges of determining whether the clock, as a functioning module, or individual clock genes acting pleiotropically are responsible for the photoperiodic regulation of diapause, and how a stable, central circadian clock could be linked to plastic photoperiodic responses without compromising the clock's essential functions. Although we still lack an understanding of the exact mechanisms whereby insects measure day/night length, continued classical and neuroanatomical approaches, as well as forward and reverse genetic experiments, are highly complementary and should enable us to decipher the diverse ways in which circadian clocks have been involved in the evolution of photoperiodic induction of diapause in insects. The components of circadian clocks vary among insect species, and diapause appears to have evolved independently numerous times, thus, we anticipate that not all photoperiodic clocks of insects will interact with circadian clocks in the same fashion.

Fig. 1

The circadian clock model in Drosophila and other insects. (A) In the nucleus of D. melanogaster, circadian cells CLOCK (CLK) and CYCLE (CYC) proteins form a heterodimer that acts as a transcriptional activator by binding to the E-box promoter region of the period (per) and timeless (tim) genes. per and tim mRNA are translated in the cytoplasm of the cell. PER and TIM proteins then form a heterodimer and translocate back into the nucleus where PER inhibits the action of CLK:CYC, thereby suppressing the transcription of per and tim. The CRYPTOCHROME1 (CRY1) protein degrades TIM and itself in the presence of light. This results in increasing levels of per and tim mRNA throughout the day when CLK:CYC activity is uninhibited, and decreasing levels of per and tim mRNA during the night. (B) The circadian clock in most other insects, such as monarch butterflies, and mosquitoes, differ from the Drosophila clock in that they possess a light-insensitive CRY2 protein that acts as the major negative transcriptional regulator of the core circadian clock. In this case, PER appears to assist CRY2 in nuclear translocation, whereas TIM helps to stabilize PER and CRY2.

Fig. 2

Photoperiodic response curve of diapausing pupae of the flesh fly, Sarcophga bullata, from populations in Illinois and Missouri. Each point represents the mean (±SE) incidence of diapause in progeny of 12–14 females (from Denlinger 1972).

Fig. 3

(A) Schematic representation of the brain of the blow fly, Protophormia terraenovae (adapted from Shiga and Numata 2001, 2009), showing connections between PDF-positive, small lateral ventral neurons (sLN_v) that innervate neurosecretory cells in the pars lateralis (PL). These neurosecretory cells innervate the corpora cardiacum (CC) and corpora allatum (CA) that produce and/or release key hormones involved in development and diapause. (B) An hypothetical model showing the potential role of PDF in initiating diapause. Under short days, high amounts of PDF are transmitted to neurosecretory cells in the PL, promoting the production of allatostatin, which prevents the CA from producing juvenile hormone (JH), thereby resulting in adult reproductive diapause. Under long-day conditions, PDF levels are low and the neurosecretory cells in the PI produce allatotropin, which stimulates the CA to produce JH, leading to reproductive maturation.