Investment in Gynoparae and males by Myzus-persicae (Sulzer)
Recent studies of sex allocation in aphids have shown, in one case, a constant investment in males, and in another, an equal investment in males and females. We show here that in the host-alternating aphidine, Myzus persicae (Sulzer), investment fits neither of these patterns. In a series of laboratory experiments, we investigated the effect of manipulating an environmental cue (day length) on the timing of sexual offspring production and the relative investment in males and gynoparae. The latter are winged parthenogenetic females capable of giving birth to sexual female offspring (oviparae). Exposure of wingless parthenogenetic females to long nights brings about endocrine changes resulting in the production of gynoparae and males on the summer (secondary) host plant. Gynoparae and males then fly to the winter (primary) host, where the oviparae are born, mating occurs and overwintering eggs are laid. Our experiments confirm that gynoparae precede males among the progeny of an individual, and show that the sex ratio on the secondary host is male biased. Because each gynopara contains four to five oviparae, the ultimate sex ratio on the primary host is potentially slightly female biased. The field sex ratio, however, will depend on survival time of the mothers of the sexual forms, those that survive longer producing relatively more males. The patterns of variation in sex ratio are discussed in relation to current theory and other recent studies on aphid sex allocation.