THE CYTOCHROME-B REGION IN THE MITOCHONDRIAL-DNA OF THE ANT TETRAPONERA-RUFONIGER - SEQUENCE DIVERGENCE IN HYMENOPTERA MAY BE ASSOCIATED WITH NUCLEOTIDE CONTENT

Polymerase chain reaction (PCR) followed by sequencing of single-stran ded DNA yielded sequence information from the cytochrome b (cyt b) reg ion in mitochondrial DNA from the ant Tetraponera rufoniger. Compared with the cyt b genes from Apis mellifera, Drosophila melanogaster, and D. yakuba, the overall A + T content (A + T%) of that of T. rufoniger is lower (69.9% vs 80.7%, 74.2%, and 73.9%, respectively) than those of the other three. The codon usage in the cyt b gene of T. rufoniger is biased although not as much as in A. mellifera, D. melanogaster, an d D. yakuba; T. rufoniger has eight unused codons whereas D. melanogas ter, D. yakuba, and A. mellifera have 21, 20, and 23, respectively. Th e inferred cyt b polypeptide chain (PPC) of T. rufoniger has diverged at least as much from a common ancestor with D. yakuba as has that of A. mellifera (similar to 3.5 vs similar to 2.9). Despite the lower A T%, the relative frequencies of amino acids in the cyt b PPC of T. ru foniger are significantly (P < 0.05) associated with the content of ad enine and thymine (A + T%) and size of codon families. The mitochondri ally located cytochrome oxidase subunit II genes (CO-II) of endopteryg ote insects have significantly higher average A + T% (similar to 75%) than those of exopterygous (similar to 69%) and paleopterous (similar to 69%) insects. The increase in A + T% of endopterygote insects occur red in Upper Carboniferous and coincided with a significant accelerati on of PPC divergence. However, acceleration of PPC divergence is not s ignificantly correlated with the increase of the A + T% (P > 0.1). The high A + T%, the biased codon usage, and the increased PPC divergence of Hymenoptera can in that respect most easily be explained by direct ional mutation pressure which began in the Upper Carboniferous and sti ll occurs in most members of the order. Given the roughly identical A + T% of the cyt b and CC-II genes from the other insects whose DNA seq uences are known (A. mellifera, D. melanogaster, and D. yakuba), it se ems most likely that the A + T% of T. rufoniger declined secondarily w ithin the last 100 Myr as a result of a reduced directional mutation p ressure.