LIFE-CYCLE OF THE CEREAL APHID SITOBION-AVENAE F - POLYMORPHISM AND COMPARISON OF LIFE-HISTORY TRAITS ASSOCIATED WITH SEXUALITY

1. Clones of Sitobion avenae from the west of France exhibited four ty pes of responses in growth chambers to short days and low temperatures : in addition to permanently parthenogenetic clones (anholocyclic) and clones producing both males and parthenogenetic females (androcyclic) , lineages producing both mating females and males were categorized in to holocyclic and intermediate types, the latter investing into both s exual and parthenogenetic reproduction. 2. Holocyclic, intermediate an d androcyclic clones were tested outdoors for sexual production in aut umn and results were consistent with those obtained in growth chambers . Eggs from holocyclic and intermediate clone selfings were laid and h atched for most crosses. 3. Reciprocal crosses within and between life cycle variants showed no effect of the type of cross on the number of eggs laid per mating female, their hatching success, or the survival and fecundity of the parthenogenetic females born from eggs (fundatric es). 4. Eggs from intermediate x intermediate crosses hatched 20 days earlier than those involving at least one holocyclic parent. It is sug gested that this short diapausing time gives a selective advantage to intermediate clones in oceanic climates, which could compensate for th eir lower sexual production in autumn. 5. Selfed offspring showed inbr eeding depression, which seems to be common in non host-alternating ap hids. 6. Mating with androcyclic males affects percentage (lower) and time (longer) of egg hatching, but not the other investigated traits. 7. The significance of intragenotypic strategy mixing in androcyclic a nd intermediate clones is discussed as an overall adaptation to the un certainty of winter climate in oceanic zones.