Chickpea diversity driven by transposon insertion polymorpism

   Chickpea is the second most important legume crop, which is used as a food by people in different parts of the world due to its high nutritive value. Omics technologies have revolutionized the characterization of chickpea genetic diversity by considering single-nucleotide polymorphisms, while structural variants and transposons have been overlooked. The specific contribution of transposons to the phenotypic diversification of crop species is still poorly documented, therefore its characterization is important. We focused on landraces collected before the “green revolution”, as they are a valuable source of species diversity and can be used to broaden the genetic base of modern cultivars. Analyzing 190 chickpea genomes, we found 42,324 new transposon insertion sites from 83 families and showed that such sites are highly polymorphic. Most insertions were caused by mobilization of retrotransposons (67 % of insertions); among DNA transposons, the highest number of insertions was found for the superfamilies MuDR, PIF, hAT, CMC, and TcMar. We also demonstrated an uneven distribution of insertion sites along chromosomes. Analysis of the localization of transposon insertion sites relative to genes and their structural elements has shown that the largest number of insertions in all transposon superfamilies falls on introns and the smallest, on exons. We also showed that transposon insertion sites, which until recently have been overlooked by population genomics, are an important factor that diversifies phenotypes and can be used in GWAS as markers replacing SNPs. Comparative analysis of landraces collected in different geographic regions showed that the Ethiopian accessions have many unique transposon insertion sites. Our results highlight the unique role of transposon mobilization in chickpea diversification and have important implications for breeding improved chickpea varieties adapted to global climate change.

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