ADAPTIVE SIGNIFICANCE OF CHANGES IN MORPH PRODUCTION DURING THE TRANSITION FROM PARTHENOGENETIC TO SEXUAL REPRODUCTION IN THE APHID RHOPALOSIPHUM-PADI (HOMOPTERA, APHIDIDAE)

Many aphid species have life cycles which alternate between asexual re production during spring and summer and sexual reproduction in autumn. This paper describes the sequences of morphs produced by clones of Rh opalosiphum padi (Linnaeus) in response to a range of photoperiods and temperatures in the laboratory, and under semi-natural conditions at the time of year when the transition from parthenogenetic to sexual re production occurs. By using similarities in the order in which the var ious morphs were born, clones of R. padi were found to produce eight p rogeny sequence types. A clone of X. padi will produce these sequence types sequentially as it changes from parthenogenetic to fully sexual reproduction. The progeny sequence produced by clones under conditions corresponding to those of spring and summer consist of parthenogeneti c females only. During late summer, production of parthenogenetic fema les is followed by males. Later in the year, a clone produces progeny sequences consisting of parthenogenetic females, gynoparae and males. The asexual phase of reproduction is terminated in late autumn when on ly gynoparae and males are born. A minority of clones terminate parthe nogenetic reproduction by producing only gynoparae. The function of th ese changes in progeny sequence in maximizing the reproductive success of a clone of X. padi is discussed.