campestris are miRNAs ( Shahid et al., 2018) argues against the shotgun hypothesis in this case. However, the fact that the trans-species sRNAs delivered to hosts from the parasitic plant C. The plant response to Phytophthora may make use of this strategy ( Hou et al., 2019). How could such a system be stable over evolutionary time and/or be useful against multiple species? One suggestion is a ‘shotgun’ strategy, in which a very diverse set of trans-species sRNAs is deployed to hit target mRNAs randomly. This implies that target sites are not under purifying selection to maintain complementarity to trans-species sRNAs. Trans-species silencing is expected to benefit the source organism while being detrimental to the target organism in parasitic/pathogenic relationships. Silencing by plant trans-species sRNAs relies on extensive complementarity between the sRNA and target mRNA, similar to normal endogenous plant miRNAs ( Liu et al., 2014). The parasitic plant Cuscuta campestris produces trans-species microRNAs (miRNAs) which silence mRNAs in multiple host plants ( Shahid et al., 2018). Fungal plant pathogens produce sRNAs with complementarity to host mRNAs ( Weiberg et al., 2013) and host plants produce trans-species sRNAs that silence mRNAs in both pathogenic fungi ( Zhang et al., 2016 Cai et al., 2018) and oomycetes ( Hou et al., 2019). These trans-species sRNAs seem especially prominent in plant/pathogen and plant/parasite interactions. Small regulatory RNAs (sRNAs) produced in one organism can sometimes function to silence mRNAs in another organism.
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