Internal structure of the silk fibroin gene of Bombyx mori. II. Remarkable polymorphism of the organization of crystalline and amorphous coding sequences.

Alleles of the silk fibroin locus from 22 inbred stocks of Bombyx mori were compared. Nineteen alleles differing from one another in length and internal sequence organization were distinguished. Individuals from a single stock generaly are homozygous for a particular allele, as judged by their gene restriction pattern and the length of the fibroin protein produced. Restriction with endonucleases having four base recognition sequences revealed no variation with respect to these particular coding sequences among the alleles tested. Furthermore, digestion with endonucleases specific for amorphous coding sequences indicated that all the alleles tested had amorphous coding sequence domains alternating regularly with crystalline domains just as was found for the L allele. The stocks differed considerably in their fibroin length, and in the total length of the fibroin coding regions of their genes. These differences were accounted for by variation in the lengths of crystalline coding domains when compared to the ends of the genes. Several characteristics of the alleles indicates that this variation results from recombination between the highly repetitive coding sequences of misaligned genes (homologous unequal crossing-over). Polymorphism of the fibroin gene in B, mori appears to be greater than for any other gene for which data are available.