MITOCHONDRIAL-DNA DIVERSITY IN THE PEA APHID ACYRTHOSIPHON-PISUM

The pea aphid Acyrthosiphon pisum has been established in North Americ a for at least a century and occurs on a broad range of host plants. D espite its importance as a crop pest, there is little understanding of the genetic structure of its populations or the extent of genetic div ergence associated with different host plant utilization. This study e xamined the extent of mitochondrial DNA (mtDNA) diversity among 35 clo nes of pea aphids from alfalfa. Polymorphisms were detected at only 2 of 126 restriction sites, suggesting the same impoverished level of ge netic diversity found in prior allozyme studies. However, length varia tion was common and apparently arose as a result of tandem repeats in two regions of the mtDNA molecule. Region 1 occurred in proximity to t he control region of the molecule, while region 2 was close to a clust er of tRNA genes flanking the NAD-dehydrogenase subunit 3 gene on the opposite side of the mtDNA molecule. Each of the aphid clones was homo plasmic for a single length variant in region 1, which varied with res pect to the number of copies of a 120-bp repeat. By contrast, one-thir d of the clones were heteroplasmic at region 2, where they possessed a variable number of copies of a 210-bp repeat. Reanalysis of clones af ter 30 generations of parthenogenetic reproduction established the sta bility of length variants over this interval, suggesting their value i n studies of the genetic structure of aphid populations.