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 phylogenetic distribution of DNA methylation in invertebrates, together with new data for the crustacean Penaeus semisulcatus, the annelid Aporrectodea caliginosa trapezoides, and the parasitic platyhelminth Schistosoma mansoni. Two alternative hypotheses addressing the function of cytosine methylation in invertebrates are evaluated: (1) cytosine methylation 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 present in all species with large genomes. We discuss the role of DNA methylation in the evolution of development in light of these hypotheses and conclude that gene control and cell memory are important and primitive functions of DNA methylation.