Transposable Elements in the A and B Genomes of Peanut

Cultivated peanut (Arachis hypogaea L.) is an allotetraploid with an AABB type genome. Its most probable diploid ancestors are A. duranensis and A. ipaënsis. These species diverged about 3 million years ago and have high DNA identity especially in genic regions. The Peanut Genome Consortium (www.peanutbase.org) has sequenced the genomes of two exemplars of these most probable ancestral species. Here we report on an investigation of their transposon component and its importance in genome structure and evolution. The most abundant transposons are LTR Ty3-gypsy retrotransposons that accumulate disproportionally in the pericentromeric regions, followed by DNA transposons and surprisingly abundant LINE elements. Ty1-copia elements are accumulated mostly in the euchromatic arms. A. ipaensis has a larger genome and higher overall transposon content than A. duranensis. Most transposon families are shared between the two genomes, however, their relative abundance and positions differ. Macro-scale differences in transposable element distribution suggest the basis for the strongly condensed centromeric bands which are present in the A genome, but much weaker in the B. Overall it is evident that the transposon components of the genomes have played a major role in their evolutionary divergence.