Evidence for clonal selection and the need for the maintenance of sexual recombination: A reappraisal and overview

Aphids are renowned for their performance as global pests of a wide variety of plants of agricultural, horticultural and forestry importance, causing damage physically and in many plant species, by transmission of pathogenic plant viruses. Their role is exacerbated by their prodigious reproductive abilities, involving asexual propagation, with, in many species, a single sexual phase in the autumn (cyclic parthenogenesis). Sexual reproduction has many advantages: recombination of new favourable alleles, crossing over and the elimination of deleterious alleles. However there are disadvantages too, mainly the breaking up of existing favourable allele associations, which may be locally adapted, and difficulties of finding a mate. Asexual reproduction, the main reproductive mode for most of the growing season, allows quick exploitation of ephemeral food resources, but it is assumed aphids are not so adaptive during this reproductive mode due to the nature of clonality, i.e., that offspring are largely genetically identical with their stem mother and clone mates (although individuals do undergo mutational changes of one kind and another), and have an essentially fixed (non-recombinant) genome. In this paper, we suggest that there are indeed costs to being clonal. We present data showing that in semi-natural habitats at least, particular aphid clonal lineages are not that abundant, new genotypes being continually created by sexual recombination (annual) and mutation (on going) and are then mainly eliminated by natural selection/drift and competition for limited resources. Hence the retention of sex, even rare sex, is important in generating variation, more than is apparently possible by mutation alone within asexual lineages. Such variation is in turn important in an ever changing world, more especially to keep pace with evolving natural enemies, locked with their aphid hosts in an ancient life and death struggle, a so-called 'arms race'. Lastly, we suggest that this may have implications for aphid control in integrated pest management (IPM) scenarios.