Parsimonious reconstruction of the evolution of the centromeric region of the Y chromosome of D. melanogaster. (a) Schematic representation of the sequence of the centromeric region h18 of the Y chromosome, not to scale. The sequence of BACR26J21 was analyzed by BLAST through FlyBase and by dot plot analyses. Slim orange arrows represent HeT-As decayed by deletions and/or few small inversions. A yellow arrow (or fragment of it) represents 3′ UTR TART sequences. Pentagons (rightward open arrowheads) including HeT-A and TART sequences represent units of the 18HT satellite (c). Each colored triangle represents a different transposon, with its orientation indicated by a thin black arrow, and its name displayed on top. All transposable elements are degenerate in comparison to the canonical element (http://flybase.org/), except 1731, which has been recently inserted. Green squares represent a small segmental duplication from the euchromatic region 42A. (e) to (a) is the order of the inferred evolutionary steps. Amplifications and deletions of various regions are indicated by two different types of discontinuous lines. For example, the region including sequences of four telomeric elements went through amplification, giving rise to the 18HT satellite, (e) to (d). Insertions of transposable elements are shown with triangles and the insertion of the sequence from 42A is indicated by a blue dotted line, (d) to (c). The outcome of three deletion events within the units of the 18HT satellite is indicated by contraction of the pentagon, (c) to (a). Violet parentheses have been used to reduce the diagram of the 18HT satellite region. (e) Schematic representation of the ancestral telomeric region. The telomeric array was composed of nine different telomeric elements, numbered from 1 to 9: four truncated HeT-As (1, 2, 3 and 5), one truncated TART () and four full-length HeT-As (6, 7, 8 and 9). Numbers have been preserved all along the scheme in order to help visualize the amplified regions.