Deadlines and delays as factors in aphid sex allocation
In this paper we use the concept of the evolutionary individual as the basis for an attempt to characterise sex-allocation patterns in aphids. We then examine the various selection pressures involved in the evolution of aphids' sex ratios, and propose a novel explanation for biased sex allocation in host-alternating aphidines, in which inbreeding is impossible. Their production of gynoparae (females that migrate from secondary to primary hosts to produce the sexual females) before males is the clonal equivalent of sex reversal - sequential hermaphroditism. Selection on the timing of the reversal, and thus the overall sex-allocation ratio, should depend largely on the rate of decline of the populations on the secondary hosts in autumn and on how long it takes oviparae (sexual females) to reach maturity. The longer the nymphal period of the oviparae, the sooner investment in gynoparae becomes futile, since both gynoparae and oviparae must mature and oviparae must oviposit before leaf-fall. It is the combination of a deadline - leaf-fall - and a delay - the two generations that must be completed between allocation to gynoparae and oviposition - that determines the selection on the allocation ratios of host-alternating aphidines. Data on Rhopalosiphum padi in Scotland and Sweden show a strong female bias, as the model predicts.