TY - JOUR T1 - Hybrid origins and the earliest stages of diploidization in the highly successful recent polyploid <em>Capsella bursa-pastoris</em> JF - bioRxiv DO - 10.1101/006783 SP - 006783 AU - Gavin M. Douglas AU - Gesseca Gos AU - Kim A. Steige AU - Adriana Salcedo AU - Karl Holm AU - Emily B. Josephs AU - Ramesh Arunkumar AU - J. Arvid Ă…gren AU - Khaled M. Hazzouri AU - Wei Wang AU - Adrian E. Platts AU - Robert J. Williamson AU - Barbara Neuffer AU - Martin Lascoux AU - Tanja Slotte AU - Stephen I. Wright Y1 - 2015/01/01 UR - http://biorxiv.org/content/early/2015/01/20/006783.abstract N2 - Whole genome duplication events have occurred repeatedly during flowering plant evolution, and there is growing evidence for predictable patterns of gene retention and loss following polyploidization. Despite these important insights, the rate and processes governing the earliest stages of diploidization remain poorly understood, and the relative importance of genetic drift, positive selection and relaxed purifying selection in the process of gene degeneration and loss is unclear. Here, we conduct whole genome resequencing in Capsella bursa-pastoris, a recently formed tetraploid with one of the most widespread species distributions of any angiosperm. Whole genome data provide strong support for recent hybrid origins of the tetraploid species within the last 100-300,000 years from two diploid progenitors in the Capsella genus. Major-effect inactivating mutations are frequent, but many were inherited from the parental species and show no evidence of being fixed by positive selection. Despite a lack of large-scale gene loss, we observe a decrease in the efficacy of natural selection genome-wide, due to the combined effects of demography, selfing and genome redundancy from whole genome duplication. Our results suggest that the earliest stages of diploidization are associated with quantitative genome-wide decreases in the strength and efficacy of selection rather than rapid gene loss, and that non-functionalization can receive a 'head start' through a legacy of deleterious variants and differential expression originating in parental diploid populations. ER -