A transposon as a cytogenetic marker in Drosophila melanogaster

We have analyzed the behavior of a transposing element (TE) in Drosophila melanogaster. The TE carries the structural genes white (wa or waR = white apricot reversed) and roughest (rst+), which corresponds to the bands 3C2-6 and a genetic distance of approximately 0.7 map units. Due to the large size, TE can often be visualized in the polytene chromosomes as extra bands at the site of the transposon. We have identified over 100 different transpositions, most of which are situated in the large autosomes; genetic and cytological information is presented for 41 of these positions. Excision of TE may occur once in 1,000 chromosomes, while insertion in a new position is more rare, about once in 10,000 animals or less. The structure of TE itself is variable: regions within it may be lost, genes located adjacent to the site of insertion may transpose with the TE ("hitch-hiking" genes) or the TE may be duplicated. Possible mechanisms for transposition of the TE and its relation to "dispersed gene families" are discussed. Paro et al. (1983) have studied the end segments of the TE and isolated so-called FB elements (FB-NOF), which are responsible for its ability to transpose. A careful analysis of the many insertion points for TE will result in a more accurate correlation between the genetical and cytological maps for the two large autosomes of Drosophila melanogaster.