Silkmoth chorion gene families contain patchwork patterns of sequence homology

The late chorion locus of Bombyx mori, containing paired members from two multigene families, has been analyzed in detail. The 15 gene pairs, irregularly spaced over 140 kilobases, exhibit an identical structure and a high overall sequence homology, while the flanking DNA (intergene regions) varies considerably. Segments of DNA of 150-300 base pairs from a reference gene pair were used as probes in a series of DNA hybridization experiments. It was found that the sequence variants within the locus are in a "patchwork" arrangement. Each gene pair contains a unique pattern of regions or "patches" that are highly homologous to the reference gene pair, interspersed with regions exhibiting less homology. We suggest that phenotypic selection is unable to account for the observed patchwork patterns; rather, sequence exchange between genes must be postulated. The nature of the DNA flanking the gene pairs would suggest that most of this sequence exchange is by means of gene conversion rather than unequal crossover events.