Neuroanatomical Approaches to the Study of Insect Photoperiodism†

Abstract

The anatomical locations of three components of insect photoperiodism--the photoperiodic photoreceptor, photoperiodic clock and hormonal effector--are summarized and compared between species. Among photoperiodic photoreceptors, either the retinal or extraretinal types or both are operative, and there is no general relationship between phylogeny and photoreceptor type. The photoperiodic clock comprises time measurement and counter systems. Currently, it is generally accepted that circadian oscillators are involved in the photoperiodic clock. Several recent studies have raised the possibility that timeless, a circadian clock gene, plays a role in the photoperiodic clock in flies. The dorsal protocerebrum has been identified as an important region regulating the endocrine system for adult, pupal and embryonic diapause controlled by photoperiod. In the blow fly Protophormia terraenovae, neural connections between circadian clock neurons and indispensable neurons in the pars lateralis for diapause induction in the dorsal protocerebrum have been demonstrated. This neural network may provide the access needed to investigate the neural components of the photoperiodic clock.

Roles of PER immunoreactive neurons in circadian rhythms and photoperiodism in the blow fly, Protophormia terraenovae

“…Actually, removal of pars lateralis neurons causes non-diapause phenotypes even under diapauseinducing conditions in some insects, including P. terraenovae (e.g. Shiga and Numata, 2000;Shimokawa et al, 2008) (for a review, see Shiga and Numata, 2007). The pars lateralis neurons innervating the corpus cardiacum or corpus allatum have been considered to inhibit hormonal events necessary for reproduction or development under diapause-inducing conditions.…”

Neural control of gene recruitment in metazoans

“…Induction of production of diapausing eggs in the maternal organism starts with reception of the external stimuli by exteroceptors, their conversion into trains of electrical signals and their processing in neural circuits in the brain. In the silkworm, Bombyx mori, the processing inactivates a GABA-ergic inhibitory mechanism, thus allowing secretion of a neuropeptide, the diapause hormone (DH), the product of the DH-PBAN (DH-pheromone biosynthesis activating neuropeptide) gene, by a pair of labial secretory neurons in the midline of the subesophageal ganglion (Sato et al, 1994;Ichikawa, 2003;Shiga and Numata, 2007). Axons of these neurons reach corpora cardiaca, from where they release DH in the hemolymph (Ichikawa et al, 1995).…”

What Makes a Fly Enter Diapause?

“…[20][21][22] Along with this, much of the neuroanatomy of clock gene expressing cells has been elucidated. 10,[23][24][25][26] Some of the circadian clock neurons can be divided on a functional basis into dark-dependent morning cells (M-cells) and light dependent evening cells (E-cells). 27,28 These two sets act as oscillators with the E-cells controling the rhythm in light (daytime) and the M-cells dominating in the dark (night time).…”

“…54 and 55) the same neurons that are implicated in diapause in other insects and are probably innervated by "clock" neurons. 10,24 Ablation of these DILP-expressing PI neurons results in adults showing similar effects on sugar levels and lifespan along with increased resistance to stress and starvation. 56 It seems that reducing insulin-like signaling affects many of the processes that would need to be adjusted by a fly entering diapause and that this is mediated through the same cells in the brain.…”

“…10 These signal, either directly or humorally, to the corpus allatum (CA) and ovaries. In Drosophila this abrogates Juvenile hormone (JH) synthesis in the CA and ecdysteriods in the ovaries 11,12 resulting in a decrease of yolk protein synthesis in the fat body and uptake by the ovaries leading to pre-vitellogenic immature oocytes.…”