On January 5, 2023, Brian Morton, professor of biological sciences, published a new paper in the journal Genes, titled “Do Noncoding and Coding Sites in Angiosperm Chloroplast DNA Have Different Mutation Processes?” The article examines the context-dependency of substitutions at fourfold degenerate sites in protein-coding regions and compares the pattern to what has been observed at intergenic sites. These two types of DNA sites have long been assumed to have the same mutation patterns.
In this paper, Morton observed a strong similarity between the two types of sites, but also noted intriguing differences. Furthermore, the data are not consistent with selective constraints being responsible for the difference in substitution patterns between intergenic and fourfold degenerate sites. Morton found that it suggested the difference may be a result of different epigenetic modifications that result in slightly different mutation patterns in coding and intergenic DNA.