Response to Casey et al.'s sensitivity of detecting environmental DNA comment

Abstract

Casey et al. correctly identified a need for clarification regarding the detection limits we reported for two PCR assays (Jerde et al. 2011). Serial dilutions of purified PCR amplicon DNA (not total genomic DNA) were used to evaluate sensitivity. One microliter of purified amplicon DNA from each assay was added to 25 μL reactions and the lowest detectable concentrations were approximately 207 copies/μL of DNA (3.30 × 10−8 ng/μL) for bighead carp, and 7 copies/μL of DNA (7.25 × 10−10 ng/μL) for silver carp. We determined the maximum sensitivity of our assays under ideal laboratory detection conditions because the detection limits of any PCR assay for bulk environmental samples is contingent on the complexity of the assayed DNA solution and water quality characteristics (Thompson et al. 2006). This clarification about the type of DNA used in the assay, that is amplicon DNA rather than total genomic DNA to determine maximum sensitivity, eliminates the concern of Casey et al. regarding credibility of our eDNA assays. No conclusions or observations reported by Jerde et al. (2011) are affected by the concerns raised by Casey et al. including: the sensitivity of eDNA surveillance exceeding that of traditional monitoring tools (Figure 3; Jerde et al. 2011); the presence of Asian carp above the electric barrier where eDNA first detected them (Figure S1; Jerde et al. 2011); and the eDNA approach, which has been applied by the U.S. Army Corps of Engineers using the protocols and procedures from our study resulting in similar detection patterns (http://www.lrc.usace.army.mil/AsianCarp/eDNA.htm). Finally, a number of recent studies demonstrate that eDNA is a reliable index of the occurrence and abundance of rare aquatic macrofauna (Dejean et al. 2011; Goldberg et al. 2011; Minamoto et al. 2011; Thomsen et al. 2012).