Reproductive Synchrony as Hybridization Barrier Acknowledgments Author Contributions We would like to thank ‘‘Associac¸a˜o dos Amigos do Mar’’ (Viana do Castelo) and the staff for providing facilities for laboratory work, and Fernando Ca´novas for help in using R. Conceived and designed the experiments: CAM EAS GAP. Performed the experiments: CAM. Analyzed the data: CAM GAP. Contributed reagents/ materials/analysis tools: EAS GAP. Wrote the paper: CAM EAS GAP. References 1. Mayr E (1963) Animal species and evolution. Harvard University Press, Cambridge. 797 p. 2. Rieseberg LH, Willis JH (2007) Plant speciation. Science 317: 910–4. 3. Palumbi SR (1994) Genetic divergence, reproductive isolation, and marine speciation. Ann Rev Ecol Syst 25: 547–572. 4. Levitan DR, Fukami H, Jara J, Kline D, McGovern TM, et al. (2004) Mechanisms of reproductive isolation among sympatric broadcast-spawning corals of the Montastraea annularis species complex. Evolution 58: 308–23. 5. Martin NH, Willis JH (2007) Ecological divergence associated with mating system causes nearly complete reproductive isolation between sympatric Mimulus species. Evolution 61: 68–82. 6. Heyward AJ, Babcock RC (1986) Self- and cross-fertilization in scleractinian corals. Mar Biol 90: 191–195. 7. Denny M, Dairiki J, Distefano S (1992) Biological consequences of topography on wave-swept rocky shores: I. Enhancement of external fertilization. Biol Bull 183: 220–232. 8. Mead KS, Denny MW (1995) The effects of hydrodynamic shear stress on fertilization and early development of the purple sea urchin Strongylocentrotus purpuratus. Biol Bull 188: 46–56. 9. Berndt M, Callow J, Brawley S (2002) Gamete concentrations and timing and success of fertilization in a rocky shore seaweed. Mar Ecol Prog Ser 226: 273–285. 10. Brawley SH (1992) Fertilization in natural populations of the dioecious brown alga Fucus ceranoides and the importance of the polyspermy block. Mar Biol 113: 145–157. 11. Ladah L, Bermudez R, Pearson G, Serra˜o E (2003) Fertilization success and recruitment of dioecious and hermaphroditic fucoid seaweeds with contrasting distributions near their southern limit. Mar Ecol Prog Ser 262: 173–183. 12. Ladah LB, Feddersen F, Pearson GA, Serra˜o EA (2008) Egg release and settlement patterns of dioecious and hermaphroditic fucoid algae during the tidal cycle. Mar Biol 155: 583–591. 13. Marshall DJ (2002) In situ measures of spawning synchrony and fertilization success in an intertidal, free-spawning invertebrate. Mar Ecol Prog Ser 236: 113–119. 14. Marshall DJ, Semmens D, Cook C (2004) Consequences of spawning at low tide: limited gamete dispersal for a rockpool anemone. Mar Ecol Prog Ser 266: 135–142. 15. Pearson GA, Brawley SH (1996) Reproductive ecology of Fucus distichus (Phaeophyceae): an intertidal alga with successful external fertilization. Mar Ecol Progr Ser 143: 211–223. 16. Pearson GA, Brawley SH (1998) A model for signal transduction during gamete release in the fucoid alga Pelvetia compressa. Plant Physiol 118: 305–313. 17. Pearson GA, Serra˜o EA, Brawley SH (1998) Control of gamete release in fucoid algae: Sensing hydrodynamic conditions via carbon acquisition. Ecology 79: 1725–1739. 18. Pearson GA, Serra˜o EA (2006) Revisiting synchronous gamete release by fucoid algae in the intertidal zone: fertilization success and beyond? Integr Comp Biol 46: 587–597. 19. Serra˜o EA, Pearson G, Kautsky L, Brawley SH (1996) Successful external fertilization in turbulent environments. Proc Natl Acad Sci USA 93: 5286–5290. 20. Orton JH, Southward AJ, Dodd JM (1956) Studies on the biology of limpets. II. The breeding of Patella vulgate L. in Britain. J Mar Biol Ass UK 35: 149–176. 21. Serra˜o EA, Havenhand J (2009) Fertilization strategies. In: Wahl M, ed. Marine Hard Bottom Communities Springer-Verlag, Berlin, Heidelberg. pp 149–164. 22. Rundle HD, Whitlock MC (2001) A genetic interpretation of ecologically dependent isolation. Evolution 55: 198–201. 23. Engel CR, Daguin C, Serra˜o EA (2005) Genetic entities and mating system in hermaphroditic Fucus spiralis and its close dioecious relative F. vesiculosus (Fucaceae, Phaeophyceae). Mol Ecol 14: 2033–46. 24. Perrin C, Daguin C, Van De Vliet M, Engel CR, Pearson GA, et al. (2007) Implications of mating system for genetic diversity of sister algal species: Fucus spiralis and Fucus vesiculosus (Heterokontophyta, Phaeophyceae). Eur J Phycol 42: 219–230. 25. Billard E, Daguin C, Pearson G, Serra˜o E, Engel C, et al. (2005a) Genetic isolation between three closely related taxa: Fucus vesiculosus, F. spiralis, and F. ceranoides (Phaeophyceae). J Phycol 41: 900–905. 26. Zardi GI, Nicastro KR, Canovas F, Costa JF, Serra˜o EA, et al. (2011) Adaptive traits are maintained on steep selective gradients despite gene flow and hybridization in the intertidal zone. PloS ONE 6(6): e19402. 27. Lu¨ ning K (1990) Seaweeds. Their environment, biogeography, and ecophysi- ology. Wiley, New York. 28. Billard E, Serra˜o EA, Pearson GA, Engel CR, Destombe C, et al. (2005b) Analysis of sexual phenotype and prezygotic fertility in natural populations of Fucus spiralis, F. vesiculosus (Fucaceae, Phaeophyceae) and their putative hybrids. Eur J Phycol 40: 397–407. 29. Dudgeon S, Kubler JE, Wright WA, Vadas RL, Petraitis PS (2001) Natural variability in zygote dispersal of Ascophyllum nodosum at small spatial scales. Funct Ecol 15: 595–604. 30. Dudgeon S, Petraitis PS (2001) Scale-dependent recruitment and divergence of intertidal communities. Ecology 82: 991–1006. 31. Serra˜o EA, Kautsky L, Lifvergren T, Brawley SH (1997) Gamete dispersal and pre-recruitment mortality in Baltic Fucus vesiculosus. Phycologia 36: 101–102. 32. Coleman MA, Brawley SH (2005a) Spatial and temporal variability in dispersal and population genetic structure of a rockpool alga. Mar Ecol Prog Ser 300: 63–77. 33. Coleman MA, Brawley SH (2005b) Are life history characteristics good predictors of genetic diversity and structure? A case study of the intertidal alga Fucus spiralis (Heterikontophyta; Phaeophyceae). J Phycol 41: 753–762. 34. Billard E, Serra˜o E, Pearson G, Destombe C, Valero M (2010) Fucus vesiculosus and spiralis species complex: a nested model of local adaptation at the shore level. Mar Ecol Prog Ser 405: 163–174. 35. Moalic Y, Arnaud-Haond S, Perrin C, Gareth GA, Ester EA (2011) Travelling in time with networks: revealing present day hybridization versus ancestralpo- lymorphism between two species of brown algae, Fucus vesiculosus and F. spiralis. BMC Evol Biol 11: 33. 36. Harrison PL, Babcock RC, Bull GD, Oliver JK, Wallace CC, et al. (1984) Mass spawning in tropical reef corals. Science 223: 1186–1189. 37. Clifton KE (1997) Mass spawning by green algae on coral reefs. Science 275: 1116–1118. 38. Serra˜o EA (1996) Reproductive biology of Fucus vesiculosus L. in the Baltic Sea. Dissertation, University of Maine. 39. Yamahira K (2004) How do multiple environmental cycles in combination determine reproductive timing in marine organisms? A model and test. Funct Ecology 18: 4–15. 40. Schmidt PS, Serra˜o EA, Pearson GA, Riginos C, Rawson PD, et al. (2008) Ecological genetics in the north atlantic: environmental gradients and adaptation at specific loci. Ecology 89: S91–S107. 41. Coyne JA, Orr HA (2004) Speciation. Sinauer Associates, Sunderland. 545 p. 42. Knowlton N, Mate´ JL, Guzma´n HM, Rowan R, Jara J (1997) Direct evidence for reproductive isolation among the three species of the Montastraea annularis complex in Central America (Panama´ and Honduras). Mar Biol 127: 705–711. 43. Tomaiuolo M, Hansen TF, Levitan DR (2007) A theoretical investigation of sympatric evolution of temporal reproductive isolation as illustrated by marine broadcast spawners. Evolution 61: 2584–95. 44. Ca´novas F, Mota CF, Serra˜o EA, Pearson GA (2011) Driving south: A multi- gene phylogeny of the brown algal family Fucaceae reveals relationships and recent drivers of a marine radiation. BMC Evolutionary Biology 11: 371. 45. Johnson LE, Brawley SH (1998) Dispersal and recruitment of a canopy-forming intertidal alga: the relative roles of propagule availability and post-settlement processes. Oecologia 117: 517–526. 46. Forgarty ND, Steven VV, Levitan DR (2012) Weak prezygotic isolating mechanisms in threatened Caribbean Acropora Corals. PloS ONE 7(2): e30486. 47. Mu¨ ller DG, Seferiadis K (1977) Specificity of sexual chemotaxis in Fucus serratus and Fucus vesiculosus (Phaeophyceae). Z Pflanzenphysiol 84: 85–94. 48. Serra˜o EA, Alice LA, Brawley SH (1999) Evolution of the Fucaceae (Phaeophyceae) inferred from nrDNA-ITS. J Phycol 35: 382–394. 49. Stebbins GL (1957) Self fertilization and population variability in the higher plants. Am Nat 91: 337–354. 50. Takebayashi N, Morrell PL (2001) Is self-fertilization an evolutionary dead end? Revisiting an old hypothesis with genetic theories and a macroevolutionary approach. Am J Bot 88: 1143–1150. 51. Denny MW, Shibata MF (1989) Consequences of surf-zone turbulence for settlement and external fertilization. Am Nat 134: 859–889. 52. Brawley SH, Johnson LE, Pearson GA, Speransky V, Li R, et al. (1999) Gamete release at low tide in Fucoid algae: maladaptive or advantageous? Am Zool 39: 218–229. PLoS ONE | www.plosone.org 11 April 2012 | Volume 7 | Issue 4 | e35978