Fig. 5. Suggested origins of the three monoamine transporter types in the Bilateria. Homologues of the serotonin transporter gene, SERT, are expressed in all phyla examined to date. Although presumed to be ancestral, the invertebrate DAT gene (iDAT) may have been lost in some invertebrate phyla (Annelida, possibly Mollusca) and in the Chordata. The ancestral catecholamine/phenolamine transporter (MAT) persists today as invertebrate OAT and the chordate paralogues NET and cDAT. The chordate NET and cDAT genes appear to have arisen through duplication of a MAT gene following the loss of an invertebrate-type DAT gene in a basal protochordate. The loss, or absence of expression, of some monoamine transporter genes in different phyla (iDAT in Annelida, iDAT and MAT in Mollusca) or in sub-phyletic taxa (DAT in Urochordata, OAT in some Insecta, SERT in Hymenoptera) appears to be a commonly repeated theme in the Metazoa.