Is the distribution of plasmid lengths bimodal?

Abstract

The length of a plasmid is a key property which contributes to many aspects of plasmid biology. When distributions of plasmid lengths are shown in the literature, they are usually plotted with length on a logarithmic scale. However, a quantity and its logarithm have distinct distributions which may differ considerably in shape. Mistaking the distribution of log-lengths for the distribution of lengths can therefore lead to distorted conclusions about the distribution; in particular, the distribution of log-lengths may be bimodal when the distribution of lengths is only unimodal. This particular confusion has arisen in the literature where a bimodal log-length distribution is suggested to support a distinction between large and small classes of plasmids. While the length distribution is indeed bimodal within many bacterial families, it is not across the ensemble of all plasmids. We suggest that authors should be careful to show the plasmid length distribution, or to distinguish the two distributions, to avoid misleading inferences.HighlightsThe distributions of lengths and log-lengths of plasmids are different, and have considerably different shapes.The typical practice of using a logarithmic scale for plasmid lengths leads to confusion between the two distributions.In particular, the distribution of log-lengths can be bimodal when the distribution of lengths is not.The length distribution within bacterial families is often bimodal, but across all plasmids, it is unimodal.Clearly distinguishing between the distributions will ensure that biological conclusions drawn from them are robust.