EVOLUTION OF NEUROTRANSMITTER SYNTHETIC ENZYME GENES AND THE PATTERNING OF SEROTONERGIC NEURONS IN CAENORHABDITIS AND OTHER NEMATODES
C. Loer
Dept Biology, Univ San Diego, San Diego, CA, USA
The biogenic amine neurotransmitters serotonin and dopamine are synthesized via two enzymatic steps by an aromatic amino acid hydroxylase (AAH) and aromatic amino acid decarboxylase (AADC). The AADC enzyme, which has a broad substrate specificity, is shared by both synthetic pathways in all animals, including the vertebrates, insects and nematodes. AADC belongs to a larger family of pyridoxal phosphate-dependent decarboxylases (PLP-DC) that includes histidine DC (histamine sythesis), tyrosine DC (octopamine synthesis in animals, alkaloid synthesis in plants) and the more distantly related glutamate decarboxylase (GABA synthesis). We have previously identified the serotonin- and dopamine synthetic AADC in the nematode C. elegans as encoded by the bas-1 gene. We have found that C.e. genome encodes five additional PLP-DC genes, some of which have no obvious homolog in other animals. In addition, two close relatives of bas-1 found in C.e. are missing in the congeneric C. briggsae genome. Phylogenetic analysis reveals that the bas-1-like genes (or a common ancestor) were originally present in the C.b. lineage, but were lost. In C. elegans, the bas-1-like genes may be expressed pseudogenes. Ongoing sequencing of other Caenorhabditis genomes should provide additional insight into questions about the evolutionary fates of duplicate genes such as these.
We have also recently begun an examination of the patterning of serotonergic neurons in free-living nematode relatives of C. elegans. We have found considerable variability in the numbers and patterns of serotonergic neurons, especially those regulating reproductive behaviors. We have also examined how the expression of serotonin in some central body neurons is regulated by the Antennapedia class homeotic complex gene lin-39 in C. elegans and other nematodes.