Quantitation of potentially linked gene cassettes using QPCR. In this series of panels, the left hand vertical axis represents abundance as measured by QPCR. The horizontal axis indicates the different environmental DNA samples in which the abundance of the target gene cassettes was measured. The right hand vertical axis shows the ratio of the abundances of the two gene cassettes considered, demonstrating a stoichiometric relationship through a constant ratio between DNA samples. Graph A indicates the 429/482 bp co-assorting pair, Graph B indicates the 258/347 bp co-assorting pair and Graph C shows the 253/325 bp non co-assorting pair as discussed in the text.