Submitted to: Biological Journal of the Linnean Society, London
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 9/15/2003
Publication Date: 12/1/2003
Citation: Shufran, K.A., Mayo, Z.B., Crease, T.J. 2003. Genetic changes within an aphid clone: homogenization of rDNA intergenic spacers after insecticide selection. Biological Journal of the Linnean Society. 79:101-105.
Interpretive Summary: Aphids reproduce by a unique method called parthenogenesis, whereby adult females produce all female young from unfertilized eggs. There is presumably no recombination between chromosomes, the mechanism which produces genetic variability. Because of this, all descendants of a single female aphid are theoretically identical, i.e. clones. Recently, some studies have shown that mutations can and do occur within the DNA sequence of aphid clones. Generally these are minor and involve only one or a few changes in the genetic code of the aphid. However, we detected a large scale change within an aphid (the greenbug, Schizaphis graminum) clone that had undergone over 200 generations of asexual reproduction (4 years). We found loss of specific segments of DNA from within a gene coding family, accompanied by an increase in frequency of other segments. These changes could only be explained by recombination events that had occurred during parthenogenesis in the aphid. Previously, it was believed that the generation of genetic variation in the greenbug was mostly due to recombination during the sexual reproductive phase. This study concludes that genetic variation can be created and maintained in the absence of recombination, and allows for another mechanism by which genetic variation in greenbug populations can be generated and maintained.
Technical Abstract: A single asexual maternal lineage (i.e., clone) of the greenbug aphid, Schizaphis graminum (Rondani), was repeatedly selected with the insecticide disulfoton. A parallel colony of the non-selected clone was also maintained. After approximately 200 generations (4 years) of continuous selection, both the selected and non-selected clones were assayed for changes in intergenic spacer (IGS) length variants of the rRNA cistron. No changes in sets of IGS variants were detected in the non-selected clone. However, the selected clone was found to have lost three variants present in the non-selected clone. This probably occurred by unequal cross-over between sister chromatids, whereby the cistron was becoming homogenized by an increase of frequency of two smaller variants. This documents a large scale genetic change occurring within the rRNA cistron in a parthenogenetically reproducing aphid.