Adenine DNA methylation associated to transcription is widespread across eukaryotes

DNA methylation in the form of 5-methylcytosine (5mC) is widespread in eukaryotes, while the presence of N6-methyladenine (6mA) has sparked considerable debate. Methodological disparities in quantifying and mapping 6mA in genomic DNA have fueled this controversy. Yet, the distantly related early branching fungi, ciliates and the algae Chlamydomonas reinhardtii exhibit robust 6mA methylation patterns, but the origin and evolution of 6mA remain unknown. Here we use Oxford Nanopore modified base calling to profile 6mA at base pair resolution in 18 unicellular eukaryotes spanning all major eukaryotic supergroups. Our results reveal that only species encoding the adenine methyltransferase AMT1 display robust genomic 6mA patterns. Notably, 6mA consistently accumulates downstream of transcriptional start sites, aligning with H3K4me3-enriched nucleosomes, suggesting a conserved role in placing transcriptionally permissive nucleosomes. Intriguingly, the recurrent loss of the 6mA pathway across eukaryotes, particularly in major multicellular lineages, implies a convergent alteration in the dual methylation system of the Last Eukaryotic Common Ancestor, which featured transcription-associated 6mA and repression-associated 5mC.