Clonal variability in sequences of morph production during the transition from parthenogenetic to sexual reproduction in the aphid Rhopalosiphum padi(Sternorrhyncha : Aphididae)

Winter climate determines the success of the two main reproductive strategi es employed by aphids. Permanent parthenogens survive as parthenogenetic fe males in mild winters, but are regularly eliminated by low temperatures; wh ile cyclical parthenogens, which switch to sexual reproduction by the end o f summer, produce every year fertilised diapausing eggs resistant to frost. We have studied the variation in sexual morph production of several clones of the cereal aphid Rhopalosiphum padi (L.) showing both strategies. Twenty clones of this species differing by their geographic origin and their mode of reproduction were placed in two laboratory environments mimicking the c hanges of photoperiod and thermoperiod occurring naturally from the end of summer and during the autumn in oceanic and continental conditions. The ana lysis of clonal responses in both climatic conditions showed (i) a wide var iation in investment of clones in sexual reproduction with, in particular, evidence for a mixed strategy employed by clones producing both sexuals wit hout ceasing parthenogenetic reproduction, (ii) no geographic adaptation am ong clones belonging to cyclical parthenogenetic populations, (iii) an earl ier production of sexuals in continental conditions and a higher production of males in oceanic conditions. Furthermore, we have compared the dates of first appearance of sexuals in o ur experiments with those occurring in the field based on a suction trap da tabase and found that sexuals were caught in nature at least four weeks ear lier than in the lab. These results underline the need for a better underst anding of the influence of the whole array of environmental factors inducin g the transition from parthenogenetic to sexual reproduction in aphids.