Figure 1. Formation of parallel spindles during meiosis II in atps1 gives unreduced spores. In atps1 formation of meiosis II, spindles in a plane parallel to that during meiosis I results in each half of the cell getting one of the two sister chromatids. Cytokinesis occurs only at the end of meiosis II, and the planes of division overlap, resulting in a dyad of diploid spores. doi:10.1371/journal.pgen.1000272.g001 References 1. Leitch AH, Leitch IJ (2008) Genomic plasticity and the diversity of polyploid plants. Science 320: 481–483. 2. Bretagnolle F, Thompson JD (1995) Gametes with the somatic chromosome number: Mecha- nisms of their formation and role in the evolution of autopolyploid plants. New Phytologist 129: 1–22. 3. Simillion C, Vandepoele K, Van Montagu M, Zabeau M, Van de Peer Y (2002) The hidden duplication past of Arabidopsis thaliana. Proc Natl Acad Sci U S A 99: 13627–13632. 4. Mittelsten Scheid O, Afsar K, Paszkowski J (2003) Formation of stable epialleles and their para- mutation-like interaction in tetraploid Arabidopsis thaliana. Nat Genet 34: 450–454. 5. Otto SP, Whitton J (2000) Polyploid incidence and evolution. Annu Rev Genet 34: 401–437. 6. d’Erfurth I, Jolivet S, Froger N, Catrice O, Novatchkova M, et al. (2008) Mutations in ATPS1 (Arabidopsis thaliana Parallel Spindle 1) lead to the production of diploid pollen grains. PLoS Genet 4: e1000274. doi:10.1371/journal.pgen.1000274. 7. Harlan JR, deWet JMJ (1975) On O. Winge and a Prayer: The origins of polyploidy. Bot Rev 41: 361–389. 8. Henry IM, Dilkes BP, Young K, Watson B, Wu H, et al. (2005) Aneuploidy and genetic variation in the Arabidopsis thaliana triploid re- sponse. Genetics 170: 1979–1988. 9. Ramanna MS, Jacobsen E (2003) Relevance of sexual polyploidization for crop improvement—A review. Euphytica 133: 3–18. 10. Fukuda Y (1927) Cytological studies on the development of pollen grains in different races of solanum. Bot Mag Tokyo 41: 459–474. 11. Peloquin SJ, Boiteux LS, Carputo D (1999) Meiotic mutants in potato. Valuable variants. Genetics 153: 1493–1499. 12. Mercier R, Vezon D, Bullier E, Motamayor JC, Sellier A, et al. (2001) SWITHC1 (SWI1): A novel gene required for the establishment of sister chromatid cohesion and bivalent formation at meiosis. Genes Dev 15: 1859–1871. 13. Ravi M, Marimuthu MP, Siddiqi I (2008) Gamete formation without meiosis in Arabidop- sis. Nature 451(7182): 1121–1124. 14. Li J, Lee GI, Van Doren SR, Walker JC (2000) The FHA domain mediates phosphoprotein interactions. J Cell Sci 113: 4143–4149. 15. Pereira G, Schiebel E (2001) The role of the yeast spindle pole body and the mammalian centro- some in regulating late mitotic events. Curr Opin Cell Biol 13: 762–769. 16. Job D, Valiron O, Oakley B (2003) Microtubule nucleation. Curr Opin Cell Biol 15: 111–117. 17. Paradez A, Wright A, Ehrhardt DW (2006) Microtubule cortical array organization and plant cell morphogenesis. Curr Opin Plant Biol 9: 571–578. 18. Ehrhardt DW, Shaw SL (2006) Microtubule dynamics and organization in the plant cortical array. Ann Rev Plant Biol 57: 859–875. PLoS Genetics | www.plosgenetics.org 2 November 2008 | Volume 4 | Issue 11 | e1000272