Evidence for a common ancestor sequence for the Balbiani ring 1 and Balbiani ring 2 genes in Chironomus tentans

The Balbiani ring (BR) 1 and BR 2 genes in Chironomus tentans are functionally related and are only expressed in the salivary gland cells. Here we reveal the principal structure of the BR 1 gene and analyze the structural and evolutionary relationship between the BR 1 and BR 2 genes. The properties of the BR 1 gene, 37 kilobases in size, are derived from the analysis of a cloned cDNA sequence, pCt 21. A considerable part of the BR 1 gene consists of one or a few blocks of a tandemly repeated 246-base-pair (bp) major repeat unit. About half of this major repeat unit is in turn built from four tandem repeats of a 33-bp sequence. This hierarchic arrangement of repetitive sequences within the BR 1 gene suggests that the gene has evolved through two major amplification steps, starting from a short primordial sequence. A similar evolutionary model has been put forward for the BR 2 gene [Sümegi, J., Wieslander, L. & Daneholt, B. (1982) Cell 30, 579-587]. The two putative primordial genes contain a similar, 102-bp-long sequence (86% nucleotide sequence homology), indicating that the BR 1 and BR 2 genes most likely arose from the same ancestor sequence. During the course of evolution the two genes diverged, mainly due to differences in the length and sequence of the gene segments involved in the two amplification steps. Moreover, at least one of the BR genes was translocated to another chromosomal locus.