Geographical variation in diapause development in eggs of Sympetrum frequens (Odonata: Libellulidae)

Abstract

Geographical differences causing variations in the egg period and the effects of environmental factors on diapause development were examined in Sympetum frequens (Selys), a univoltine species with an obligatory egg diapause for overwintering. Eggs were obtained from females collected from 11 localities in Japan and incubated under six different combinations of photoperiod and temperature. No clear geographical trends were found in the average egg period under any experimental treatment. Average hatch period (i.e., period from the date when 10 % of the eggs were hatched to the date when 90 % of the eggs were hatched) did not display any geographical trend at 25 and 20 °C. However, at 15 °C, a significant negative correlation was observed between the hatch period and the latitude of the collection site. Similarly, a significant correlation was also detected between the coefficient of variation in the egg period and the average annual temperature near the collection site, but only at 15 °C. Because each egg batch was divided into six groups which were then incubated under different experimental conditions, it was possible to discern that the rate of diapause development at 15 °C varies among eggs from southern populations. The large variations in the egg period in the southern populations at 15 °C were considered to be a risk-spreading strategy: a certain proportion of the eggs were able to maintain diapause until winter, even if the adults laid the eggs early in the season. These differences in the rate of diapause development within a population may be an adaptation to the unpredictable length of the summer–autumn period.