Genome Minimization and Microbiome Connectivity: Lessons from Studying Genome Evolution in Expanding Low Productivity Ocean Regions

"Streamlining" refers to selection that favors minimization of cell size and complexity, but most often its used to describe small genome sizes (1-2 mb) in microbial cells that are not ostensibly symbionts. The essence of streamlining theory is that selection is most efficient in organisms that have large effective population sizes, and, in nutrient-limited systems, favors cell architecture that minimizes resources required for replication. Planktonic microbiomes in nutrient-poor ocean regions, which are expanding as a consequence of ocean warming, often have small genomes and multiple genomic signatures of streamlining selection: high coding density, low percentage of regulatory genes, high AT content, and low proteome N composition. Small genomes have been found in most microbiomes, raising questions about alternate evolutionary pathways that lead to small genome size, which can include symbioses, higher network connectivity as a consequence of reliance on the community for functions, or the occupation of carbon nutrition niches that have simpler coding requirements.  This lecture explores this topic with examples from the cell biology and evolution of highly streamlined planktonic microbes.