Geographical strains and selection for the diapause trait in Calliphora vicina

Abstract

The blow fly Calliphora vicina is widespread in the Palaearctic and Nearctic regions, where it overwinters in a photoperiodically induced diapause at the end of the third larval instar. Like many insects with a large geographical range, it presents latitudinal clines in diapause-related traits, such as critical day length (CDL). This chapter reviews a paper on geographical strains and selection for the diapause trait in Calliphora vicina. Five strains spanning 36° to 65°N were shown to display such clines in critical day length, diapause duration and, diapause related cold tolerance. Divergent selection for diapause incidence using a diapause-depleted stock restored both a high incidence of diapause and its complete elimination within less than five generations. Reciprocal crossings between northern (65°N) and southern (51°N) strains showed that males contributed nothing to the diapause status of their immediate offspring, and even in later generations and back crosses, the male contribution was less than that of the female. Diapause duration showed contributions from both sexes, suggesting that incidence and duration may be inherited in a different maimer. The chapter also reviews the central problem of photoperiodic time measurement, especially the evidence that night length measurement is a function of the insect's circadian system (Biirming's general hypothesis).