Why are there so few aphid clones?

In Europe, aphids contribute significantly to the so-called 'aerial plankton' during the spring to autumn months (growing season), although individual flight behaviour has been found, especially from molecular ecological studies, to be species-specific in terms of migratory range (ambit). Many of these species individuals may be assumed to be clonal in origin, that is, derived from a single asexual foundress. We are presently studying two specialist aphid species on Tansy, Tanacetum vulgare L. from samples collected in Jena, Germany - Macrosiphoniella tanacetaria (Kaltenbach) and Metopeurum fuscoviride Stroyan, using microsatellite markers. On plotting the number of sets of different multilocus genotypes or MLGs (i.e. multiple clonal repeats: 1, 2, 3 copies, etc.), against the frequency of their occurrence, a negative exponential relationship was found, with populations of both species consisting mostly of single (i.e. unique) or low number repeats rather than larger multiple copy (clonal) MLG repeats. To test this further, microsatellite data collected from a previous study on M. tanacetaria in Jena in the year 2000 and on samples of the Grain aphid, Sitobion avenae (F.), collected in the UK in 1997/8, the latter both in the field and from 12.2 m high suction traps, were examined in the same way. Again, similar relations were found, with most MLGs occurring as unique or low copy number repeats. The data are briefly discussed in the light of our evidence, as well as that of other similar studies on other aphid species, relating aphid molecular genetic data to aphid life cycle, behaviour and ecology.