Coordinately expressed members of two chorion multi-gene families are clustered, alternating and divergently orientated

Abstract

The chorion (eggshell) of silkmoths consists of more than 100 distinct proteins. Individual components are synthesised in characteristic and overlapping sequence, at early, middle, late or very late developmental periods of choriogenesis (which lasts 2 days in total)^1,2. Protein sequencing^3,4 indicates that components which belong to two size classes, A and B, are encoded by two respective families of evolutionarily homologous genes (multi-gene families). Genetic analysis in Bombyx mori^5,6 has shown that chorion genes are tightly linked in three clusters on a single chromosome (n = 28). Total chorion mRNA of the wild silkworm, Antheraea polyphemus, has been copied into double-stranded cDNA and cloned by recombinant DNA procedures⁷. Many distinct cDNA clones have been selected and identified as members of the A and B multi-gene families by detailed cross-hybridisation⁷ and sequencing⁸ analysis. Stringent conditions which minimise cross-hybridisation permit use of these clones as specific probes. In many cases, we know the protein encoded and the developmental period when the clone sequence can be found in cytoplasmic RNA^7,8. Thus, we may now use these cDNA clones as probes to determine how genes of known developmental properties and evolutionary history are arranged within the chorion chromosomal region. We report here that two cloned, 14-kilobase chromosomal DNA segments contain multiple chorion genes. In each segment, the genes represent both A and B families, but only one developmental class: one segment contains only middle-period A and B genes and the other only late-period A and B genes. In both segments, members of the two families are arranged in alternating sequence and divergent orientation.