The role of photoperiod and temperature in the determination of parthenogenetic and sexual forms in the aphid Megoura viciae Buckton—I : The influence of these factors on apterous virginoparae and their progeny

Abstract

Apterous virginoparae from long-established parthenogenetic clones of Megoura are capable of producing three types of daughters (winged and wingless virginoparae and sexual oviparae) in addition to males. This polymorphism is regulated by the environment which, by acting through a series of maternal “switch” mechanisms, directs the growing oöcytes and embryos into different developmental pathways. Although there is little reason to doubt that sex is determined genetically (presumably at the oöcyte stage), the sex ratio of the progeny is also under physiological control. In certain environments, notably at high or low temperatures (although not in long photoperiods), male production is inhibited. But even under the most favourable conditions males are relatively scarce and are born predominantly in the middle of the reproductive life of the parent. The later differentiation of part-grown female embryos is also governed environmentally. Long photoperiods and high temperatures induce them to become virginoparae, the reverse conditions, oviparae. The action of photoperiods of intermediate or “critical” length is particularly instructive as providing evidence of the maternal, and perhaps humoral, control of these forms. Their differentiation begins when the parent is still in the early larval instars and is completed in the adult stage. The progression of determination can be explained in terms of the competence of embryos in different stages of growth to respond to alternative maternal stimuli.