Foldback (sound engineering)

Abstract: Chromosomal inversions are the most common type of genome rearrangement in the genus Drosophila. Although the potential of transposable elements (TEs) for generating inversions has been repeatedly demonstrated in the laboratory, little is known on their role in the generation of natural inversions, which are those effectively contributing to the adaptation and/or evolution of species. We have cloned and sequenced the two breakpoints of the polymorphic inversion 2q7 of D. buzzatii. The sequence analysis of the breakpoint regions revealed the presence in the inverted chromosomes of large insertions, formed by complex assemblies of transposons, that are absent from the chromosomes without the inversion. Among the transposons inserted, the Foldback-like element Galileo, that was previously found responsible of the generation of the widespread inversion 2j of D. buzzatii, is present at both 2q7 breakpoints and is the most likely inducer of the inversion. A detailed study of the nucleotide and structural variation in the breakpoint regions of six chromosomal lines with the 2q7 inversion detected no nucleotide differences between them, which suggests a monophyletic and recent origin. In contrast, a remarkable degree of structural variation was observed in the same six chromosomal lines. It thus appears that the two breakpoints of the inverted chromosomes have become genetically unstable hotspots, as was previously found for the 2j inversion breakpoints. The possibility that this instability is caused by structural properties of Foldback elements is discussed.

Abstract: A few foldback (FB) transposable elements have, between their long terminal inverted repeats, central loop sequences which have been shown to be different from FB inverted repeat sequences. We have investigated loop sequences from two such FB elements by analyzing their genomic distribution and sequence conservation and, in particular, by determining if they are normally associated with FB elements. One of these FB loop sequences seems to be present in a few conserved copies found adjacent to FB inverted repeat sequences, suggesting that it represents an integral component of some FB elements. The other loop sequence is less well-conserved and not usually associated with FB inverted repeats. This sequence is a member of another family of transposable elements, the HB family, and was found inserted in an FB element only by chance. We compare the complete DNA sequences of two HB elements and examine the ends of four HB elements.