SYNONYMOUS SUBSTITUTIONS IN THE XDH GENE OF DROSOPHILA - HETEROGENEOUS DISTRIBUTION ALONG THE CODING REGION

The Xdh (rosy) region of Drosophila subobscura has been sequenced and compared to the homologous region of D. pseudoobscura and D, melanogas ter. Estimates of the numbers of synonymous substitutions per site (Ks ) confirm that Xdh has a high synonymous substitution rate. The distri butions of both nonsynonymous and synonymous substitutions along the c oding region were found to be heterogeneous. Also, no relationship has been detected between Its estimates and codon usage bias along the ge ne, in contrast with the generally observed relationship among genes. This heterogeneous distribution of synonymous substitutions along the Xdh gene, Which is expression-level independent, could be explained by a differential selection pressure on synonymous sites along the codin g region acting on mRNA secondary structure. The synonymous rate in th e Xdh coding region is lower in the D, subobscura than in the D. pseud oobscura lineage, whereas the reverse is true for tile Adh gene.