Solitary and Gregarious Locusts Differ in Circadian Rhythmicity of a Visual Output Neuron

Abstract

Locusts demonstrate remarkable phenotypic plasticity driven by changes in population density. This density dependent phase polyphenism is associated with many physiological, behavioural and morphological changes, including observations that cryptic solitarious (solitary-reared) individuals start to fly at dusk, whereas gregarious (crowd-reared) individuals are day-active. We have recorded for 24-36h from an identified visual output neuron, the descending contralateral movement detector (DCMD) of Schistocerca gregaria, in solitarious and gregarious animals. DCMD signals impending collision and participates in flight avoidance manoeuvres. The strength of DCMD's response to looming stimuli, characterised by the number of evoked spikes and peak firing rate, varies approximately sinusoidally with a period close to 24h under constant light in solitarious locusts. In gregarious individuals the 24h pattern is more complex, being modified by secondary ultradian rhythms. DCMD's strongest responses occur around expected dusk in solitarious locusts, but up to 6h earlier in gregarious locusts, matching the times of day at which locusts of each type are most active. We thus demonstrate a neuronal correlate of a temporal shift in behaviour that is observed in gregarious locusts. Our ability to alter the nature of a circadian rhythm by manipulating the rearing density of locusts under identical light-dark cycles may provide important tools to investigate further the mechanisms underlying diurnal rhythmicity.

Opsin expression, physiological characterization and identification of photoreceptor cells in the dorsal rim area and main retina of the desert locust,Schistocerca gregaria

“…Various differences in the visual system of solitarious and gregarious locusts have been reported, including differences in eye size, number of ommatidia and sensitivity to motion stimuli (Matheson et al, 2004;Ott and Rogers, 2010;Rogers et al, 2010;Gaten et al, 2012), while polarization-sensitive interneurons were not noticeably affected by locust phase (el Jundi and Homberg, 2012). Here we show differences in photoreceptor spectral sensitivities, particularly in the ventral eye as revealed by ERG recordings and in peak wavelength of blue peaking receptors found in intracellular recordings.…”

Background complexity affects response of a looming-sensitive neuron to object motion

“…All animals, at least 3 wk past the imaginal molt, were obtained from a crowded colony maintained in the Department of Biology at the University of Saskatchewan (25-28°C, 12:12-h light-dark cycle). Experiments were carried out at ϳ25°C during similar times of the animals' light cycle to avoid potential variations in responsiveness when locusts fly at night (Gaten et al 2012).…”

Bilateral flight muscle activity predicts wing kinematics and 3-dimensional body orientation of locusts responding to looming objects

“…Experiments were carried out at room temperature (~25°C) during similar times of the animals' light cycle to eliminate potential variation in responsiveness known to occur when locusts fly at night (Gaten et al, 2012).…”