Directional substitution and evolution of nucleotide content in the cytochrome oxidase II gene in earwigs (dermapteran insects)

The cytochrome oxidase subunit II (COII) gene was sequenced for six dermapt eran species. The nucleotide composition of this gene is biased in most ani mals. While the CG content of other insect orders is low (mean, 27.6%; rang e, 19.5%-33.1%), species from the Forficula genus showed unusually high val ues (mean, 42.4%; range, 37.3%-44.1%), mostly due to high CG frequencies at third codon positions: the mean CG content at these positions was around 4 5% (range, 43.9%-46.9%) for Forficula, compared with only 13.3% for other i nsects. This effect was so strong that in one species, Forficula lesnei, th ere was no significant difference between the frequencies of the four bases . During evolution, this loss of bias has involved a significant increase i n the synonymous substitution rate and an increase of transitions over tran sversions compared with other insects. A strong directionality of substitut ions has favored T-->C and A-->G changes. This phenomenon was also observed between two conspecific populations of Forficula auricularia. A species fr om a closely related genus, Anechura bipunctata, was intermediate between F orficula and other insects for these parameters, while two remotely related dermapteran species, Labidura riparia and Euborellia moesta, were similar to other insects. These results suggest that the evolution of Forficula DNA content has been both rapid and recent.