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Background  Aromatic L-amino acid decarboxylase (AADC) enzymes catalyze the synthesis of biogenic amines, including the neurotransmitters serotonin and dopamine, throughout the animal kingdom. These neurotransmitters typically perform important functions in both the nervous system and other tissues, as illustrated by the debilitating conditions that arise from their deficiency. Studying the regulation and evolution of AADC genes is therefore desirable to further our understanding of how nervous systems function and evolve.    Results  In the nematode  C. elegans , the  bas-1  gene is required for both serotonin and dopamine synthesis, and maps genetically near two AADC-homologous sequences. We show by transformation rescue and sequencing of mutant alleles that  bas-1  encodes an AADC enzyme. Expression of a reporter construct in transgenics suggests that the  bas-1  gene is expressed, as expected, in identified serotonergic and dopaminergic neurons. The  bas-1  gene is one of six AADC-like sequences in the  C. elegans  genome, including a duplicate that is immediately downstream of the  bas-1  gene. Some of the six AADC genes are quite similar to known serotonin- and dopamine-synthetic AADC's from other organisms whereas others are divergent, suggesting previously unidentified functions. In comparing the AADC genes of  C. elegans  with those of the congeneric  C. briggsae , we find only four orthologous AADC genes in  C. briggsae . Two  C. elegans  AADC genes – those most similar to  bas-1  – are missing from  C. briggsae . Phylogenetic analysis indicates that one or both of these  bas-1 -like genes were present in the common ancestor of  C. elegans  and  C. briggsae , and were retained in the  C. elegans  line, but lost in the  C. briggsae  line. Further analysis of the two  bas-1 -like genes in  C. elegans  suggests that they are unlikely to encode functional enzymes, and may be expressed pseudogenes.    Conclusions  The  bas-1  gene of  C. elegans  encodes a serotonin- and dopamine-synthetic AADC enzyme. Two  C. elegans  AADC-homologous genes that are closely related to  bas-1  are missing from the congeneric  C. briggsae ; one or more these genes was present in the common ancestor of  C. elegans  and  C. briggsae . Despite their persistence in  C. elegans , evidence suggests the  bas-1 -like genes do not encode functional AADC proteins. The presence of the genes in  C. elegans  raises questions about how many 'predicted genes' in sequenced genomes are functional, and how duplicate genes are retained or lost during evolution. This is another example of unexpected retention of duplicate genes in eukaryotic genomes.

Function and evolution of the serotonin-synthetic bas-1 gene and other aromatic amino acid decarboxylase genes in Caenorhabditis.