Novel insight into evolutionary process from average genome size in marine bacterioplanktonic biota

Abstract

Genome evolution is an essential force that shapes biodiversity. Genome reduction has been demonstrated in numerous microorganisms and is interpreted as a means of decreasing material cost during DNA synthesis. However, most of the previous studies have been conducted under pure-culture or endosymbiotic conditions, which greatly restrict horizontal gene transfer between microbial species. In nature, microbes frequently interact with each other and gene exchanges are quite common within the microbiota. The evolutionary trend underlying genome size is still poorly understood. The ocean provides an ideal setting for examining the evolutionary process of microbial genomes under natural conditions. We retrieved bacterial compositional information for marine bacterioplanktonic biota from the Visualization and Analysis of Microbial Population Structures database and compared their average genome size along an ocean depth gradient. The results showed that marine bacterioplanktonic biotas tend to minimize their average genome size in the course of their evolutionary processes and that niche differentiation is probably the major force driving genome size reduction. These findings provide novel insight into the understanding of the evolutionary process underlying microbial genome size and mechanisms for maintaining microbial biodiversity in nature.