Comparative genomics analysis of metallothioneins in twelve Drosophila species

Metallothioneins (MTs) are a superfamily of Cys-rich polypeptides that bind heavy metal ions, both for physiological and detoxification purposes. They are present in all organisms, but their origin is probably polyphyletic, so that MT evolutionary studies are rather scarce. We present a thorough search and analysis of the MT coding sequences in the 12 Drosophila genomes completely sequenced, taking as reference the features reported for D. melanogaster, where four isogenes (MtnA to MtnD) are known and deeply characterized. We include a fifth isoform in this study, named MtnE, and recently annotated. The MTs polymorphism pattern is essentially the same for the 12 Drosophila species. Invariably, a MtnA form and an MtnB-cluster, comprising the MtnB-to-MtnE forms in tandem array, are observed. The whole set of genes are kept in the same synteny element (Muller E), but implicated in rearrangement events (mainly inversions), encompassing all or some of the isogenes. Gene exon/intron architecture, and cDNA and protein sequences appear highly conserved through Drosophila speciation, concordantly with an essential function for MT isoforms in flies, even for those previously considered as minor products. Data presented here will be comprehensively analyzed to provide a valuable guide for future MT evolutionary, structure and function studies.