Aphids (Hemiptera: Aphididae) are ancient pest crop insects (fossils go back to the Triassic, about 220-210 myr) that have conquered most of the biomes, including the arctic and subarctic regions and they infest a huge range of plants, usually monophagously, although some species are polyphagous.
Aphids reproduce primarily by apomictic parthenogenesis, a form of reproduction whereby adult females give birth to a female progeny without any male fertilization. It has been frequently suggested that in the parthenogenetic generations no genetic recombination occurs, so that it has been assumed that the offspring represents a genetically identical clone of aphids. In view of this genetic identity, Janzen (1977) refereed to aphids as a superorganism or as a single evolutionary individual able to exploit a much larger geographic region and its resources.
Actually, aphid lineages within a same species can differ for colour, size, intrinsic rate of increase, ovariole number, reproductive modes, ability to transfer pathogenic plant viruses and susceptibility/resistance to predators, parasites, pathogens and pesticides strongly suggesting that clonality in aphids have been overestimated and making interesting to evaluate the true nature and reality of the concept of clones in such intriguing insects.
In the last years a growing amount of molecular evidences suggested that aphid asexual lineages are not true clones, since they can rapidly mutate and this variation is selectable and may affect some phenotypic traits, such as the host choice, so that the real nature of clone in aphids is not simply semantics.
Molecular analyses, such as amplified fragment length polymorphism (AFLP), have provided unequivocal evidences of mutational changes within aphid lineages and most of them were due to somatic event. Similarly, microsatellite mutations within apparent clonal lineages have also been reported, as showed in the aphid Myzus persicae.
Some Authors have also revealed variations in the ribosomal intergenic spacer (IGS), as well as events of somatic recombination between the homologous nucleolar organizing regions (NORs) located on X chromosome supporting the presence of recombinational events in aphid clonal lineages.
The presence of genetic differences among clones could be very important for aphid evolution since several studies showed that cryptic sympatric speciation occurred in a wide range of aphid species, including evidences of rapid chromosomal changes affecting speciation events in the corn leaf aphid Rhopalosiphum maidis and in the peach potato aphid M. persicae.
In view of their impact in agriculture (in particular for virus transmission), aphids need to be controlled by pesticides and/or using biological control agents. However, in the absence of a thorough understating of the genetics of aphid populations/clones, it could be difficult to properly evaluate the presence of transmissible and adaptative variations that could make biological and chemical controls less effective.
The idea that aphid populations are genetically stable on time and space is therefore erroneous since aphid clones are not genetically homogeneous as previously expected and a single female lineage does not represent a single evolutionary individual, as stated by Janzen (1977). On the contrary aphid species seem to be the sum of populations that can have different karyotypes and could consequently give different responses to the selective environmental forces.
Reference
Janzen DH (1977) What are dandelions and aphids? Am. Nat. 111: 586-589.
