Three copies of the early gene 6F6 are interspersed in and around the late chorion gene cluster of Bombyx mori

Abstract

The developmentally regulated chorion genes of the silkmoth, Bombyx mori, are clustered in two genetic loci (Ch1-2 and Ch3) of chromosome 2, which are separated by approximately 4 centiMorgans. Early genes are clustered in chorion locus Ch3, whereas Ch1-2 contains all of the late genes in a continuous region of 140 kb, which is sandwiched between two regions containing most of the middle chorion genes. The late-gene area has been extensively studied and was considered to contain only late genes coding for chorion proteins of exceptionally high cysteine content organized in tightly clustered alpha/beta gene pairs. In the present paper we report the unexpected presence of three dispersed copies (6F6.1, 6F6.2, 6F6.3) of an early beta-gene, which disrupts the continuity of the late locus. Hybridization data indicate that 6F6.2 corresponds to the previously characterized m6F6 cDNA clone and that 6F6.1 and 6F6.3, but not 6F6.2, are adjacent to alpha-type genes. Determination of the complete sequence of 6F6.1 and of the major exon of the A-gene near it shows that these two genes have a convergent rather than a divergent direction of transcription and thus do not constitute a typical gene pair. The sequence data further suggest that 6F6.1 is transcriptionally active. The three dispersed 6F6 gene copies are localized at points of inversion of the polarity of neighboring gene pairs and their locations in this locus indicate complex gene rearrangement events.