The Jonah genes: a new multigene family in Drosophila melanogaster.

The Jonah gene family consists of approximately 20 genes, distributed in small clusters at eight or more widely dispersed chromosomal sites. Gene clusters differ in the number of genes per cluster, ranging up to four, and in the arrangement of genes within clusters, which include direct repeats, inverted repeats, and combinations of direct and inverted repeats. In the third-instar larva the Jonah genes are abundantly expressed as transcripts of a single size class, located exclusively in the midgut. The Jonah genes were initially defined by homology to a reference cDNA clone. However, the structural heterogeneity within this gene family is so large that Jonah genes that fail to hybridize to the reference cDNA, yet hybridize to other members of the family that do hybridize to that cDNA have been subsequently identified. These findings raise questions regarding the sharpness of gene family boundaries.