Histogram showing the frequency of all annotated Apis genes with CpG [o/e] frequencies between 0 and 2. For comparison, a similar analysis was performed on the nematode (C. elegans) genome that is not methylated due to the lack of the genes encoding DNA methyl-transferases. The honey bee CpG-deficient genes (CpG [o/e] <0.7) expected to be methylated are labelled in black. We used the honey bee official set of 10,742 genes available at BeeBase http://www.beebsae.org. The y-axis depicts the number of genes with the specific CpG [o/e] values given on the x-axis.