Action spectra for the photoperiodic control of polymorphism in the aphid Megoura viciae

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Abstract

Energy compensated action spectra are given for the photoperiodic control of polymorphism in Megoura. The production of ‘long day’ parthenogenetic virginoparae and ‘short day’ oviparae mainly depend on the night length. Light has three different effects. ‘Early’ interruptions of the dark phase in a long night cycle reverse the time-measuring dark response. The action spectrum for a 1 hr interruption placed 1.5 hr after the onset of darkness (during dark stage 1) shows a relatively narrow band of activity, mainly in the blue (450–470 nm). The threshold is ca. 0.25 μW cm−2. ‘Late’ interruptions placed 7.5 or 8 hr after dark hour 0 (during dark stage 3) strongly promote the production of virginoparae without causing a reversal of the response. The action spectrum has the same blue maximum but sensitivity extends into the yellow and red spectral regions. The third photosensitive component, the main photoperiod itself, is required for initiating the dark timing response and has an intermediate action spectrum. Time/intensity curves for a single wavelength (471 nm) show that the responses during stages 1 and 3 depart markedly from reciprocity. Short durations cannot be compensated by high intensities. The shape of the reciprocity curve for an ‘early’ interruption suggests that the stage 1 response is complete after ca. 1.25 hr.

The action spectra are believed to be compatible with the view that the photoreceptor is a caroteno-protein. It is suggested that all three pigment forms are related and that time measurement is largely a function of spontaneous ‘dark reaction’ changes in the pigment system. Stage 1 may represent the reversible conjugation phase of the protein/chromophore moieties. In Stage 2 the pigment is presumably photorefractory and is transitional to the highly sensitive broad spectrum form of stage 3.

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