Systems defending microbes (#CRISPR, R-M, etc) from #phages and other mobile genetic elements (MGEs) are increasingly found in MGEs themselves (within cellular genomes). So, who's defending whom from what? An essay ecoevorxiv.org/zuh4c 1/5
Why are there so many defense systems in each genome? It's reasonable to think that they allow multi-layered diverse defenses from phages. But what if they are mostly the result of the multiplicity of MGEs in genomes? 2/5 
How do defense systems affect gene flow? Less transfer could be a costly by-product of defense systems. But MGEs have often more to lose than to gain from infection by other MGEs. Lowering gene flow may be a primary reason of selection for defense systems (by MGEs). 3/5
Are MGEs at the origin of “defense islands”? MGEs tend to accumulate in chromosome hotspots. When degraded, genes adaptive to the cell main remain. As time goes by, this may lead to cooption, ”defense islands”, even tight integration in terms of function & expression. 4/5
So, who are defense systems actually defending (and from whom)? Written with @dbikard (@GenGen_IP @Microbio_IP) hopefully soon in an OA journal. @EcoEvoRxiv Comments welcome (apologies for small citation list for such a large topic). 5/5
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