The silkmoth late chorion locus. II. Gradients of gene conversion in two paired multigene families.

The late chorion locus of Bombyx mori is composed of two multigene families arranged as divergently oriented pairs irregularly spaced over 140 X 10(3) bases of DNA. The average sequence homology is 91% for the 15 members of each family. DNA sequence analysis reveals: that most of the sequence variants found in individual members of a family are shared by two or more members of that family; that family members sharing a particular variant are scattered throughout the locus; and adjacent variants in individual genes are often shared by different subsets of family members. The large number of shared variants scored and their distribution is most simply explained by numerous sequence transfers similar to gene conversions. These sequence transfers are not uniformly distributed along each gene pair. Sequence transfer is highest near the 3' end of each gene and lowest in the common 5' region between the divergent genes. A model is presented that explains these gradients by assuming that the events leading to gene conversion preferentially initiate in a simple-sequence DNA repeat. The resulting heteroduplexes extend to distances influenced by features of the sequence affecting their stability. The simple-sequence DNA is part of the major exon in both gene families and encodes tandem copies of the sequence Cys-Gly-Gly and Cys-Gly. Striking identity in the precise pattern of codon biases of these repeated sequences in the two families suggests that repeated sequence transfers occur also between families in this simple-sequence region.