Evolutionary Classification of CRISPR‐Cas Systems

CRISPR-Cas systems are bacterial and archaeal adaptive immune systems. The mechanisms and biology of CRISPR are discussed in many recent reviews. This chapter provides only bare essentials as the context for the discussion of CRISPR-Cas diversity, classification, and evolution. The CRISPR immune response includes three principal phases: adaptation, expression, and interference. Similar to other defense systems, archaeal and bacterial CRISPR systems show remarkable diversity of Cas protein sequences, gene composition, and architecture of the genomic loci. The chapter reviews the current state and future challenges of CRISPR-Cas classification systems, the emerging picture of the diverse routes of the origin and evolution of the different CRISPR variants, and the evolutionary connections between CRISPR and other defense and signal transduction systems of prokaryotes.