Genome evolution of wild barley (Hordeum spontaneum) by BARE-1 retrotransposon dynamics in response to sharp microclimatic divergence

The replicative spread of retrotransposons in the genome creates new insert ional polymorphisms, increasing retrotransposon numbers and potentially bot h their share of the genome and genome sire. The BARE-1 retrotransposon con stitutes a major, dispersed, active component of Hordeum genomes, and BARE- 1 number is positively correlated with genome size. We have examined genome size and BARE-1 insertion patterns and number in wild barley, Hordeum spon taneum, in Evolution Canyon, Lower Nahal Oren. Mount Carmel. Israel, along a transect presenting sharply differing microclimates. BARE-1 has been suff iciently active for its insertional pattern to resolve individuals in a way consonant with their ecogeographical distribution in the canyon and to dis tinguish them from provenances outside the canyon, On both slopes, but espe cially on the drier south-facing slope, a simultaneous increase in the BARE -1 copy number and a decrease in the relative number lost through recombina tion, as measured by the abundance of solo long terminal repeats, appear to have driven the BARE-1 share of the genome upward with the height and dryn ess of the slope. The lower recombinational loss would favor maintenance of more full-length copies, enhancing the ability of the BARE-1 family to con tribute to genome size growth. These local data are consistent with regiona l trends for BARE-1 in H, spontaneum across Israel and therefore may reflec t adaptive selection for increasing genome size through retrotransposon act ivity.