Statistical physics of (meta)genomes

Despite the contingency of evolution, the gene content of microbes shows remarkable scaling relationships. For example, the number of transcription factors scales quadratically with the total number of genes, and similar scaling laws emerge for groups of gene sharing evolutionary history or function. On an ecological scale, microbial communities are also characterized by strong regularities in their species composition. At both the genomic and the ecological scales, statistical physics approaches help us understand the emergence of such regularities from simple ingredients, such as birth-death of individuals, duplication, transfer, and loss of genes. However, we are only in the early days of a unified description. Metagenome sequencing gives access to both the gene content and the taxonomic composition of microbial communities. Here we show how the combination of within-genomes laws and ecological patterns leads to regularities in the gene content of whole microbial communities. Exploiting one of these regularities, we introduce a robust method to estimate the average genome size of the community.