The photoperiodic counter, quantitative day-length effects and scotophase timing in the vetch aphid Megoura viciae

Abstract

The development of parthenogenetic or sexual female forms (virginoparae and oviparae, respectively) in the aphid Megoura viciae is determined by the photoperiodic conditions perceived by the mother at any stage in her development from late embryo to young adult. A sensitive period for this response was selected using standard long (16 h light-8 h dark) and short days (12 h light–12 h dark) such that the form of the first ten progeny born was affected. At 12°C this sensitive period covered the first 11 days of maternal postnatal development, at 15°C it was 10 days while at higher temperatures (17 and 20°C) the short-day response (ovipara production) was attenuated. The required day number for a 50% response at 15°C was 3 days for long days but 7–8 days for short days indicating that long days are photoperiodically more potent. Experiments at 12 and 15°C showed that the accumulation of long-day information was more temperature sensitive than the accumulation of short-day information. The photoperiodic response of M. viciae depends primarily on the length of the dark period (scotophase). Novel experimental protocols and non-24 h light-dark cycles revealed that the photoperiodic counter accumulated long nights (short days) irrespective of photophase length. Short nights (long days), on the other hand, were not summated; moreover their effect depended on the length of the photophase. It appears that, although short nights are photoperiodically stronger, they serve only to modify the counter's accumulation of long nights. The quantitative effects (i.e. photoperiodic strength) of different 24-h light-dark cycles was investigated over experimental periods close to the required day number. As expected, photoperiodic strength decreased when scotophase lengths were close to critical (9.5 h). Long nights of 12–20 h were equally effective but short nights were maximally effective at 8 h. The four stages of dark-period measurement in the hour-glass clock were also reflected in these experiments.