A Mechanism for Genome Size Reduction Following Chromosomal Rearrangements

A genomic inversion occurs when a segment of chromosome detaches by breakage and reinserts or reattaches in a reversed orientation. Several mechanisms can introduce inversions in a genome. Studies show that inversions can result in speciation and X chromosome evolution, probably through suppression of recombination between incompatible karyotypes. Previous research on Arabidopsis thaliana and Arabidopsis lyrata indicates that genome rearrangements are associated with genome shrinkage in A. thaliana, where rearranged regions are on average shorter in A. thaliana than are collinear but unrearranged (ancestral) regions in A. lyrata. We compare the genomes of two wild ancestors of peanut, Arachis duranensis and Arachis ipaensis, whose genomes merged several thousand years ago in a rare genetic event. These two ancestral diploids separated from each other about 3 million years ago, which makes these interesting models of genomic evolution. Several large inversions occurred in the A. duranensis genome, and lead to the contraction of genome through net removal of transposons and locally duplicated genes in inverted regions. This is evident when comparing size ratios of syntenic blocks in inverted and un-inverted regions, which have smaller syntenic block size in those inverted regions in A. duranensis. Our research proposes a possible mechanism of genome size changes, which needs to be tested with other species to confirm our model and to uncover the details in this mechanism.