THE MOLECULAR-BASIS OF INSTABILITY OF THE SINGED(VERY WEAK) MUTATION IN DROSOPHILA-MELANOGASTER

The singed(very weak) mutation was created by the sequential addition of two P transposable elements to the singed gene. The mutation can be somatically unstable through the action of a dominant maternal effect mutation on the second chromosome. It is also unstable in the germ li ne in these conditions. Sequencing of the region of the P insertions i n the mutation reveals that the two inserted elements have single inte rnal deletions, and the larger of the two is a copy of the KP element. The mutation will generate, at high frequencies, strongly singed and pseudo-wild type products by reversions occurred in the germline. Thes e are the result of the precise excision of the smaller and the larger elements respectively. By PCR amplification of dissected thoraces we show that the somatic instability of the mutation, from a weak to a st rong singed phenotype, is also caused by the excision of the smaller o f the two elements.