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Summary
Polyphenism, which allows one given genotype to produce several discrete phenotypes, is an extreme case of phenotypic plasticity and is mainly found in arthropods. Social insects are the canonical example of polyphenism with the development of castes in the colonies. However, aphids display one of the largest range of polyphenisms, notably by producing winged or wingless, as well as asexual or sexual forms, depending on environmental conditions. During spring and summer, aphids reproduce by viviparous parthenogenesis, whereas in autumn they enter sexual reproduction. This switch in reproductive mode is triggered by changes in photoperiod and temperature. Here, the data accumulated since the 1960s on the identification of photoperiodic clocks, counter and putative neural photoreceptors that participate in this reproductive shift are reviewed. After perception, the photoperiodic signal is transduced through the secretion of hormones (juvenile hormones may well be involved) which, in turn, may act on the target cells, namely the oocytes. In short-day conditions, oocytes enter meiosis and produce haploid eggs which develop a 2n embryo after fertilisation. By contrast, in long-day conditions, a single maturation division produces 2n oocytes which immediately enter parthenogenetic embryogenesis. A physiological model of the determination of sexual vs. asexual reproduction in aphids is proposed and viewed from the perspective of newly initiated molecular studies.
