title: Transposable element evolution in the allotetraploid Capsella bursa-pastoris.
abstract: Shifts in ploidy affect the evolutionary dynamics of genomes in a myriad of ways. Population genetic theory predicts that transposable element (TE) proliferation may follow because the genomewide efficacy of selection should be reduced and the increase in gene copies may mask the deleterious effects of TE insertions. Moreover, in allopolyploids, TEs may further accumulate because of hybrid breakdown of TE silencing. However, to date the evidence of TE proliferation following an increase in ploidy is mixed, and the relative importance of relaxed selection vs. silencing breakdown remains unclear.
  
  
  
  We used high-coverage whole-genome sequence data to evaluate the abundance, genomic distribution, and population frequencies of TEs in the self-fertilizing recent allotetraploid Capsella bursa-pastoris (Brassicaceae). We then compared the C. bursa-pastoris TE profile with that of its two parental diploid species, outcrossing C. grandiflora and self-fertilizing C. orientalis.
  
  
  
  We found no evidence that C. bursa-pastoris has experienced a large genomewide proliferation of TEs relative to its parental species. However, when centromeric regions are excluded, we found evidence of significantly higher abundance of retrotransposons in C. bursa-pastoris along the gene-rich chromosome arms compared with C. grandiflora and C. orientalis.
  
  
  
  The lack of a genomewide effect of allopolyploidy on TE abundance, combined with the increases TE abundance in gene-rich regions, suggests that relaxed selection rather than hybrid breakdown of host silencing explains the TE accumulation in C. bursa-pastoris.
authors.affiliation: Department of Ecology and Evolutionary Biology, University of Toronto, Toronto, ON, Canada.
authors.forename: J Arvid
authors.initials: JA
authors.lastname: Ågren
authors.affiliation: Key Laboratory of Plant Resources Conservation and Sustainable Utilization, South China Botanical Garden, the Chinese Academy of Sciences, China.
authors.forename: Hui-Run
authors.initials: HR
authors.lastname: Huang
authors.affiliation: Department of Ecology and Evolutionary Biology, University of Toronto, Toronto, ON, Canada stephen.wright@utoronto.ca.
authors.forename: Stephen I
authors.initials: SI
authors.lastname: Wright
mesh_terms: D005075:Biological Evolution; D029687:Capsella / Q000235:genetics*; D004171:Diploidy; D005787:Gene Frequency; D005828:Genetics, Population; D059646:Genome Size; D054817:Pollination; D011123:Polyploidy; D057945:Self-Fertilization
keywords: Brassicaceae; C-value; Capsella bursa-pastoris; genome size; mating system; polyploidy