Figure 3

Processes affecting population structure in Myzus persicae. The figure illustrates processes that influence the genetic structure of the aphid population at local, regional and global scale, with emphasis on selection due to host-plant and insecticide pressure. A. Represents the basic population of M. persicae living in and around peach trees and orchards. The emerging spring population is diverse and this diversity is maintained because no lineage dominates during the summer months and gametes have equal opportunities for mating at the end of the return migratory phase. These populations can produce asexual clones from time to time. B. Represents the situation where tobacco cultivation is close to peach trees. Tobacco selects for particular gene combinations and this in turn carries alleles associated with these genes. Over time, tobacco selection pressure has generated breeding barriers and a distinct aphid form. The tobacco form shares many characteristics of the main group being capable of growing on a range of host plants and globalisation. C. Represents the process that occurs when an insecticide resistance allele occurs in a population for the first time. There will be an immediate advantage for individuals in a clone carrying this resistance allele and within one season their numbers would increase rapidly. At the end of that season a sexual lineage will return to peaches in vast numbers, where it will mainly inbreed. Over time two events are likely, that the resistance allele will spread into more genotypes and that some of these genotypes will become asexual clones and then superclones capable of globalisation.