Hybridization-facilitated genome merger and repeated chromosome fusion after 8million years.

Abstract

The small genus Ricotia (nine species, Brassicaceae) is confined to the eastern Mediterranean. By comparative chromosome painting and a dated multi-gene chloroplast phylogeny, we reconstructed the origin and subsequent evolution of Ricotia. The ancestral Ricotia genome originated through hybridization between two older genomes with n=7 and n=8 chromosomes, respectively, on the Turkish mainland during the Early Miocene (c.17.8million years ago, Ma). Since then, the allotetraploid (n=15) genome has been altered by two independent descending dysploidies (DD) to n=14 in Ricotia aucheri and the Tenuifolia clade (2spp.). By the Late Miocene (c.10Ma), the latter clade started to evolve in the most diverse Ricotia core clade (6spp.), the process preceded by a DD event to n=13. It is noteworthy that this dysploidy was mediated by a unique chromosomal rearrangement, merging together the same two chromosomes as were merged during the origin of a fusion chromosome within the paternal n=7 genome c.20Ma. This shows that within a time period of c.8Myr genome evolution can repeat itself and that structurally very similar chromosomes may originate repeatedly from the same ancestral chromosomes by different pathways (end-to-end translocation versus nested chromosome insertion).