METHYLATION OF SOMATIC AND SPERM DNA IN THE HOMOSPOROUS FERN CERATOPTERIS-RICHARDII

Plants, in general, have a high proportion of their CpG and CpNpG nucl eotide motifs modified with 5-methylcytosine (5mC). Developmental chan ges in the proportion of 5mC are evident in mammals, particularly duri ng gametogenesis and embryogenesis, but little information is availabl e from flowering plants due to the intimate association of gametes wit h sporophytic tissues. Tn ferns, sperm are uninucleate and free-swimmi ng and thus are easily isolated. We have examined 5mC in DNA isolated from fern sperm and other tissues with methylation-sensitive and -inse nsitive restriction enzyme isoschizomers, Southern blots probed with c hloroplast and nuclear ribosomal RNA genes and end-labeled restriction fragments. We conclude that fern sperm DNA is methylated to a similar or greater degree than DNA isolated from either sporophytes or gameto phytes.