Title: High Diversity Due to Balancing Selection in the Promoter Region of the Medea Gene in Arabidopsis lyrata
Abstract: Molecular imprinting is the differential expression and/or silencing of alleles according to their parent of origin [1Hurst L.D. McVean G.T. Do we understand the evolution of genomic imprinting?.Curr. Opin. Genet. Dev. 1998; 8: 701-708Crossref PubMed Scopus (71) Google Scholar, 2Spencer G.H. Population genetics and evolution of genomic imprinting.Annu. Rev. Genet. 2000; 34: 457-477Crossref PubMed Scopus (36) Google Scholar]. Conflicts between parents, or parents and offspring, should cause "arms races," with accelerated evolution of the genes involved in imprinting. This should be detectable in the evolution of imprinting genes' protein sequences and in the promoter regions of imprinted genes. Previous studies, however, found no evidence of more amino acid substitutions in imprinting genes [1Hurst L.D. McVean G.T. Do we understand the evolution of genomic imprinting?.Curr. Opin. Genet. Dev. 1998; 8: 701-708Crossref PubMed Scopus (71) Google Scholar, 3McVean G.T. Hurst L.D. Molecular evolution of imprinted genes: No evidence for antagonistic coevolution.Proc. R. Soc. Lond. Ser. B Biol. Sci. 1997; 264: 739-746Crossref PubMed Scopus (42) Google Scholar]. We have analyzed sequence diversity of the Arabidopsis lyrata Medea (MEA) gene and divergence from the A. thaliana sequence, including the first study of the promoter region. In A. thaliana, MEA is imprinted, with paternal alleles silenced in endosperm cells [4Grossniklaus U. Vielle-Calzada J.-P. Hoeppner M.A. Gagliano W.B. Maternal control of embryogenesis by MEDEA, a polycomb group gene in Arabidopsis.Science. 1998; 280: 446-450Crossref PubMed Scopus (667) Google Scholar, 5Kinoshita T. Yadegari R. Harada J.J. Goldberg R.B. Fischer R.L. Imprinting of the MEDEA Polycomb gene in the Arabidopsis endosperm.Plant Cell. 1999; 11: 1945-1952PubMed Google Scholar], and also functions in the imprinting machinery [4Grossniklaus U. Vielle-Calzada J.-P. Hoeppner M.A. Gagliano W.B. Maternal control of embryogenesis by MEDEA, a polycomb group gene in Arabidopsis.Science. 1998; 280: 446-450Crossref PubMed Scopus (667) Google Scholar, 6Grossniklaus U. Spillane C. Page D.R. Kohler C. Genomic imprinting and seed development: Endosperm formation with and without sex.Curr. Opin. Plant Biol. 2001; 4: 21-27Crossref PubMed Scopus (109) Google Scholar]; MEA protein binding at the MEA promoter region indicates self-regulated imprinting [7Baroux C. Gagliardini V. Page D.R. Grossniklaus U. Dynamic regulatory interaction of polycomb group genes: MEDEA autoregulation is required for imprinted gene expression in Arabidopsis.Genes Dev. 2006; 20: 1081-1086Crossref PubMed Scopus (118) Google Scholar, 8Gehring M. Huh J.H. Hsieh T.-F. Penterman J. Choi Y. Harada J.J. Goldberg R.B. Fischer R.L. DEMETER DNA glycosylase establishes MEDEA polycomb gene self-imprinting by allele-specific demethylation.Cell. 2006; 124: 495-506Abstract Full Text Full Text PDF PubMed Scopus (493) Google Scholar, 9Jullien P.E. Katz A. Oliva M. Ohad N. Berger F. Polycomb group complexes self-regulate imprinting of the polycomb group gene MEDEA in Arabidopsis.Curr. Biol. 2006; 16: 486-492Abstract Full Text Full Text PDF PubMed Scopus (174) Google Scholar]. We find the same paternal MEA allele silencing in A. lyrata endosperm but no evidence for adaptive evolution in the coding region, whereas the 5′ flanking region displays high diversity, with distinct haplotypes, suggesting balancing selection in the promoter region.