THE ROLE OF DNA METHYLATION IN INVERTEBRATES - DEVELOPMENTAL REGULATION OR GENOME DEFENSE

Cytosine methylation is widely distributed in multicellular organisms. We present a comprehensive survey of the existing data on the phyloge netic distribution of DNA methylation in invertebrates, together with new data for the crustacean Penaeus semisulcatus, the annelid Aporrect odea caliginosa trapezoides, and the parasitic platyhelminth Schistoso ma mansoni. Two alternative hypotheses addressing the function of cyto sine methylation in invertebrates are evaluated: (1) cytosine methylat ion is an ancient regulatory mechanism which was lost in species with low rates of cell turnover, and (2) cytosine methylation is primarily a defense mechanism against genomic parasites and is expected to be pr esent in all species with large genomes. We discuss the role of DNA me thylation in the evolution of development in light of these hypotheses and conclude that gene control and cell memory are important and prim itive functions of DNA methylation.