Structure, organization and evolution of developmentally regulated chorion genes in a silkmoth

We describe in detail two cloned chromosomal DNA segments from Antheraea polyphemus, each bearing multiple chorion genes. Two types of genes are found per segment, each in two or three copies: they belong to both major chorion multigene families (A and B), but are expressed during the same developmental period (middle choriongenesis in one segment, late in the other). A and B genes alternate and are paired, lie in divergent transcriptional orientation and are in very close proximity to each other (mRNA cap sites are separated by a 5' flanking sequence 264 or 325 bp long). Each AB pair is embedded within a large, tandemly repeating unit. Within that unit, the homologous 3' flanking sequences that separate gene pairs evolve rapidly and are frequently interrupted by long segments representing inserts (or deletions). These segments would decrease unequal crossing-over, facilitating rapid evolution of the chorion multigene families. A total of 10.4 kb of DNA has been sequenced, permitting detailed comparisons of genes, their introns, 5' flanking and immediately 3' flanking regions. Genes range over two orders of sequence similarity, from highly releated to very disparate (gene copies; members of the same multigene family; members of different families). Among their universal features are a common cap-site sequence, a single intron invariably located at the same position within the signal peptide-encoding region, and a Hogness box 21 to 23 nucleotides upstream from the cap site. Features of possible paired promoters occupying the short 5' flanking region are discussed. Genes evolve both by base substitutions and by segmental mutations, which are almost invariably deletions/insertions related to small, direct repeats.