After googleing for a while and not finding any hint, I would like to ask the experts here for their opinion. I have a very strange result for the distribution of the depth of coverage (see picture). It seems as there were two curves. Some samples more than others, some (not show. In total I have 20 samples, they are bee samples and were sequenced on Illumina HiSeq3000 (whole genome). I have been working with bee sequence data before and never saw this behaviour. Normally always more or less a smooth distribution. I wonder if somebody of you came across something similar or has an idea why there is this irregular distribution? I cannot think of something to explain this..