The Expression of Genes in Amphioxus Suggests a Role in Tissue Proliferation Rather than in Neurotransmission.

Serine hydroxymethyltransferases (SHMTs) are key enzymes in one-carbon metabolism, with vertebrates possessing two paralogs, cytosolic SHMT1 and mitochondrial SHMT2, implicated in nucleotide biosynthesis and glycine metabolism. In this study, we investigate the evolutionary history of animal genes and analyze the expression patterns of genes in developing amphioxus (). Phylogenetic analyses indicate the presence of and orthologs in deuterostomes, spiralians and placozoans, which is consistent with an ancient gene duplication event predating bilaterian diversification. Gene expression analyses in developing amphioxus show that expression is confined to the somites and absent from neural tissues. In contrast, is broadly expressed across germ layers, but its transcription is restricted to tissues characterized by strong cell proliferation. Notably, expression in the nervous system does not match the distribution of glycinergic neuron populations, implying a negligible role in glycine neurotransmitter synthesis. Instead, the spatial correlation of expression with mitotically active domains suggests a primary function in nucleotide biosynthesis via one-carbon metabolism. These findings indicate that SHMTs predominantly support cell proliferation rather than neurotransmission in amphioxus.