<i>Drosophila miranda</i> is a model species to study sex chromosome evolution.

<p><b>A.</b> Male (left) and female (right) <i>D</i>. <i>miranda</i>. <b>B.</b> Mitotic chromosome squashes of male <i>D</i>. <i>miranda</i>. Both the ancestral X (XL/XR) and the Y chromosome (YD/neo-Y) show large blocks of dark staining (Giemsa), indicative of heterochromatin. The acrocentric rods are the neo-X, and chromosomes 2 and 4. <b>C.</b> Polytene chromosomes of a female <i>D</i>. <i>miranda</i> stained for <i>HP1</i> (heterochromatin protein 1). Note the large blocks of heterochromatin (arrows) on chromosomes 2 and 4. <b>D.</b> Karyotype evolution in <i>D</i>. <i>miranda</i>. Chromosomal fusions between the sex chromosomes and autosomes have resulted in both the reversal of Y_<i>anc</i> to an autosome as well as the independent de novo formation of new sex chromosomes from autosomes at two distinct evolutionary time points (XR and YD were formed about 15 MY ago, and the neo-X and neo-Y originated about 1.5 MY ago). Genome analysis allows us to reconstruct the temporal dynamics and molecular processes involved in sex chromosome evolution in this species. chr, chromosome; dot, dot chromosome; XL and XR, left and right arm of the X chromosome; YD, Y chromosome resulting from the unfused D element; <i>HP1</i>, heterochromatin protein 1; MY, million years; MYA, million years ago; Y_<i>anc</i>, ancestral Y chromosome.