Cloning and characterization of the highly polymorphic Ser2 gene of Bombyx mori

Three alleles of the sericin (Ser) 2-encoding gene (Ser2), called L, C and mC, were isolated from a Bombyx mori genomic library, and two related ones, called mCL and Cv, were also characterized in B. mori European strains. The Ser2 gene gives rise to two middle silk gland mRNAs by differential splicing. The size of a short mRNA (3.1 kb) is constant, but the length of a longer one ranges from 5 to 6.4 kb depending on the Ser2 allele. These length variations probably result from unequal recombinations in a region which contains about 30 well conserved 45-bp repeats coding for a Ser-like peptide. Furthermore, the L allele (and probably the mCL one) contains a 4.4-kb retrotransposon, resembling the copia-like ones of Drosophila.