High‐density mutation tracks are associated with proton‐beam irradiation patterns in Sorghum bicolor

Abstract

Induced mutagenesis is a cornerstone of crop functional genomics, yet the extent to which distinct radiation sources reshape the spatial distribution of mutations remains difficult to evaluate in reduced‐representation datasets. Here, we analyze a published genotyping‐by‐sequencing (GBS) panel (192,040 loci) to compare proton‐beam and gamma‐ray mutagenesis in Sorghum bicolor . Because GBS sampling is nonuniform, all analyses were conducted within an explicitly defined GBS‐callable sequence space. Within this callable space, 96‐channel trinucleotide spectra were broadly similar between radiation types, whereas spatial summaries differed. Macroscale analysis using the Gini coefficient indicated that proton‐treated lines exhibit a highly unequal, spike‐like distribution of mutations, whereas gamma‐treated lines show a more diffuse window‐level distribution. Microscale spatial statistics were consistent with clustering patterns that were more prominent in proton‐treated lines, including an aggregation scale of ∼500 kb, with a substantial fraction of the mutational burden falling into high‐density tracks. Within the callable locus set, coding‐ and promoter‐proximal categories were not depleted of induced mutation events (single‐nucleotide variants) across treatments. Furthermore, we did not detect a negative association between total mutational load and the coding‐region mutation fraction in this dataset. These findings suggest that, within this dataset, proton mutagenesis is characterized not by unique chemical signatures but by a distinct spatial geometry that concentrates detectable mutation events. Because proton irradiation was represented by a single dose, whether proton treatment produces stronger clustering than gamma irradiation at equal mutational burden remains to be directly tested.