Gene family sizes in genomes undergoing reductive evolution compared to a phylogenetically related larger sequenced genome. (a)Mycobacterium leprae (reductive) vs Mycobacterium tuberculosis H37Rv; (b)Shigella flexneri (reductive) vs Escherichia coli K12. Orthologous genes in the genome pairs (identified by amino-acid sequence similarity) are displayed in arbitrary order and plotted against the number of homologs in their own genome (that is, paralogs). Only protein-coding genes are included. IS elements from S. flexneri 2a are excluded.