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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.
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