Supplementary Material for: Phylogenetic Distribution and Evolutionary Dynamics of the Sex Determination Genes <b><i>doublesex</i></b> and <b><i>transformer</i></b> in Insects

Sex determination in insects is characterized by a gene cascade that is conserved at the bottom but contains diverse primary signals at the top. The bottom master switch gene <i>doublesex</i> is found in all insects. Its upstream regulator <i>transformer</i> is present in the orders Hymenoptera, Coleoptera and Diptera, but has thus far not been found in Lepidoptera and in the basal lineages of Diptera. <i>transformer</i> is presumed to be ancestral to the holometabolous insects based on its shared domains and conserved features of autoregulation and sex-specific splicing. We interpret that its absence in basal lineages of Diptera and its order-specific conserved domains indicate multiple independent losses or recruitments into the sex determination cascade. Duplications of <i>transformer</i> are found in derived families within the Hymenoptera, characterized by their complementary sex determination mechanism. As duplications are not found in any other insect order, they appear linked to the haplodiploid reproduction of the Hymenoptera. Further phylogenetic analyses combined with functional studies are needed to understand the evolutionary history of the <i>transformer</i> gene among insects.