The diversity-generating benefits of a prokaryotic adaptive immune system

Published onlineJOURNAL ARTICLEProkaryotic CRISPR-Cas adaptive immune systems insert spacers derived from viruses and other parasitic DNA elements into CRISPR loci to provide sequence-specific immunity. This frequently results in high within-population spacer diversity, but it is unclear if and why this is important. Here we show that, as a result of this spacer diversity, viruses can no longer evolve to overcome CRISPR-Cas by point mutation, which results in rapid virus extinction. This effect arises from synergy between spacer diversity and the high specificity of infection, which greatly increases overall population resistance. We propose that the resulting short-lived nature of CRISPR-dependent bacteria-virus coevolution has provided strong selection for the evolution of sophisticated virus-encoded anti-CRISPR mechanisms.S.v.H. has received funding from the European Union’s Horizon 2020 research and innovation programme under the Marie Skłodowska-Curie grant agreement number 660039. E.R.W. received funding from the People Programme (Marie Curie Actions) of the European Union’s Seventh Framework Programme (FP7/2007-2013) under Research Executive Agency grant agreement number 327606. E.R.W., A.B. and M.B. also acknowledge the Natural Environment Research Council, the Biotechnology and Biological Sciences Research Council, the Royal Society, the Leverhulme Trust, the Wellcome Trust and the AXA research fund for funding. J.M.B.-D. was supported by the University of California San Francisco Program for Breakthrough in Biomedical Research, the Sandler Foundation, and a National Institutes of Health Director’s Early Independence Award (DP5-OD021344). H.C. was funded by the Erasmus+ programme (European Union), the Explora’Sup programme (Région Rhône-Alpes) and the Centre Régional des Œuvres Universitaires et Scolaires (CROUS; French State)

A global analysis of terrestrial plant litter dynamics in non-perennial waterways

Perennial rivers and streams make a disproportionate contribution to global carbon (C) cycling. However, the contribution of intermittent rivers and ephemeral streams (IRES), which sometimes cease to flow and can dry completely, is largely ignored although they represent over half the global river network. Substantial amounts of terrestrial plant litter (TPL) accumulate in dry riverbeds and, upon rewetting, this material can undergo rapid microbial processing. We present the results of a global research collaboration that collected and analysed TPL from 212 dry riverbeds across major environmental gradients and climate zones. We assessed litter decomposability by quantifying the litter carbon-to-nitrogen ratio and oxygen (O2) consumption in standardized assays and estimated the potential short-term CO2 emissions during rewetting events. Aridity, cover of riparian vegetation, channel width and dry-phase duration explained most variability in the quantity and decomposability of plant litter in IRES. Our estimates indicate that a single pulse of CO2 emission upon litter rewetting contributes up to 10% of the daily CO2 emission from perennial rivers and stream, particularly in temperate climates. This indicates that the contributions of IRES should be included in global C-cycling assessments