Inverse symmetry in complete genomes

The prevalence of three symmetries, reverse, complement, and inverse (or reverse-complement), in about 800 complete genomic sequences extant in GenBank is studied. Each symmetry is measured by an index, χ, where χ=1 indicates lack of symmetry and χ=0, perfect symmetry. All random sequences have χ∼1. For reverse and complement symmetries it is found that χ∼1 in all complete sequences. In sharp contrast, χ∼0.05 for the inverse symmetry in all cases. The results suggests that a variety of modes of inverse segmental duplications, including multiple random small events and singular genome-size events, played prominent roles in genome growth.