Epigenetic transmission of phase in the desert locust, Schistocerca gregaria: determining the stage sensitive to crowding for the maternal determination of progeny characteristics.

Desert locust female adults respond to crowded conditions by changing progeny characteristics such as egg size, clutch size (no. of eggs per pod), hatchling body size and coloration. This study was conducted to determine the stage sensitive to crowding in this locust. Reproductively active females reared in isolation increased egg size and decreased clutch size and the proportion of green hatchlings after exposure to crowded conditions (in which each female was kept with four male adults). These changes depended not only on the timing of exposure to crowded conditions during the reproductive cycle but also on the length of the exposure. By varying the time and length of the exposure, it was found that crowding had no influence on progeny characteristics during the last two days of egg development at 31 °C and that there was a four-day sensitive stage before this period. The sensitive stage coincided with the time when the affected oocytes were 1.5-4mm long, while the sensitivity to crowding appeared to be constant over the sensitive stage. The larger the magnitude of the increase in egg size after exposure to crowding, the smaller the proportion of green hatchlings (and the larger the proportion of gregarized dark hatchlings); there was a sigmoidal relationship between the two variables. Based on these results, we propose a model for determining the stage sensitive to crowding in both the female parent and the oocytes.