Stephen Wright, Detlef Weigel and colleagues report the whole-genome sequence of Capsella rubella, a highly selfing crucifer found throughout much of southern and western Europe. They compare mixed-stage flower bud transcriptomes from C. rubella and C. grandiflora, finding a shift in expression of genes associated with flowering phenotypes and providing insights into the transition to selfing. The shift from outcrossing to selfing is common in flowering plants1,2, but the genomic consequences and the speed at which they emerge remain poorly understood. An excellent model for understanding the evolution of self fertilization is provided by Capsella rubella, which became self compatible <200,000 years ago. We report a C. rubella reference genome sequence and compare RNA expression and polymorphism patterns between C. rubella and its outcrossing progenitor Capsella grandiflora. We found a clear shift in the expression of genes associated with flowering phenotypes, similar to that seen in Arabidopsis, in which self fertilization evolved about 1 million years ago. Comparisons of the two Capsella species showed evidence of rapid genome-wide relaxation of purifying selection in C. rubella without a concomitant change in transposable element abundance. Overall we document that the transition to selfing may be typified by parallel shifts in gene expression, along with a measurable reduction of purifying selection.

The Capsella rubella genome and the genomic consequences of rapid mating system evolution