The possible evolutionary significance of repeat elements near and within an early chorion gene in the late chorion locus of Bombyx mori.

Three beta-type early chorion gene copies (6F6.1, 6F6.2, and 6F6.3) are dispersed in the late region of chorion locus Ch1-2. Detailed analysis of the 5'-flanking region and the intron of 6F6.1 shows that they contain sequences that are homologous to Bombyx mori Bm1 repeat elements. Interestingly, the Bm1-type segment of the intron is interrupted by the insertion of a sequence that shows significant similarities with part of an intron of B. mori and Bombyx mandarina fibroin genes, and with part of the 3'-flanking region of B. mori prothoracicotropic hormone and tRNA-Glu genes; this sequence may represent a new repetitive, possibly transposable, element of B. mori. Following the Bm1-homologous sequence of the 6F6.1 5'-flanking region and preceding the gene promoter region, a short DNA segment shows sequence motifs that are also present in the ErA.1 promoter region. The occurrence of these sequences near one end or within the Bm1 repeat element is suggestive of complex sequence transfer events. Comparative analysis of known B. mori chorion alpha-gene promoters and of Bm1 repeat elements suggests, with marginal statistical significance, that these two sets of sequences contain common elements.