P. Sunnucks et Df. Hales, NUMEROUS TRANSPOSED SEQUENCES OF MITOCHONDRIAL CYTOCHROME-OXIDASE I-II IN APHIDS OF THE GENUS SITOBION (HEMIPTERA, APHIDIDAE), Molecular biology and evolution, 13(3), 1996, pp. 510-524
Polymerase chain reaction (PCR) products corresponding to 803 bp of th
e cytochrome oxidase subunits I and II region of mitochondrial DNA (mt
DNA COI-II) were deduced to consist of multiple haplotypes in three Si
tobion species. We investigated the molecular basis of these observati
ons. PCR products were cloned, and six clones from one individual per
species were sequenced. In each individual, one sequence was found com
monly, but also two or three divergent sequences were seen. The diverg
ent sequences were shown to be nonmitochondrial by sequencing from pur
ified mtDNA and Southern blotting experiments. All seven nonmitochondr
ial clones sequenced to completion were unique. Nonmitochondrial seque
nces have a high proportion of unique sites, and very few characters a
re shared between nonmitochondrial clones to the exclusion of mtDNA. F
rom these data, we infer that fragments of mtDNA have been transposed
separately (probably into aphid chromosomes), at a frequency only know
n to be equalled in humans. The transposition phenomenon appears to oc
cur infrequently or not at all in closely related genera and other aph
ids investigated. Patterns of nucleotide substitution in mtDNA inferre
d over a parsimony tree are very different from those in transposed se
quences. Compared with mtDNA, nonmitochondrial sequences have less cod
on position bias, more even exchanges between A, G, C and T, and a hig
her proportion of nonsynonymous replacements. Although these data are
consistent with the transposed sequences being under less constraint t
han mtDNA, changes in the nonmitochondrial sequences are not random: t
here remains significant position bias, and probable excesses of synon
ymous replacements and of conservative inferred amino acid replacement
s. We conclude that a proportion of the inferred change in the nonmito
chondrial sequences occurred before transposition. We believe that Sit
obion aphids (and other species exhibiting mtDNA transposition) may be
important for studying the molecular evolution of mtDNA and pseudogen
es. However, our data highlight the need to establish the true evoluti
onary relationships between sequences in comparative investigations.