The chorion genes of the medfly. II. DNA sequence evolution of the autosomal chorion genes s18, s15, s19 and s16 in Diptera.

We present a total of approximately 15 kb of DNA sequences, encompassing four chorion genes Ccs18, Ccs15, Ccs19, Cc16 and their flanking DNA in the medfly C. capitata. Comparison of coding regions, introns and intergenic sequences in five Dipteran species, D. melanogaster, D. subobscura, D. virilis, D. grimshawi and C. capitata documented an extensive divergence in introns and coding regions, but few well conserved elements in the proximal 5' flanking regions in all species. These elements are related to conserved regulatory features of three of the genes, including tissue- and temporal regulation. In the fourth, gene s15, significant alterations in the 5' flanking region may be responsible for its changed temporal regulation in C. capitata. One long intergenic sequence, located in the distal 5' flanking region of gene s18, is homologous to ACE3, a major amplification control element and contains an 80-bp A/T-rich sequence, known to stimulate strong binding of the origin recognition complex (ORC) in D. melanogaster. Analysis of the nucleotide composition of all chorion genes in C. capitata and D. melanogaster showed that C. capitata exhibit less biased representation of synonymous codons than does D. melanogaster.