title: On the persistence of reproductive barriers in Eucalyptus: the bridging of mechanical barriers to zygote formation by F1 hybrids is counteracted by intrinsic post-zygotic incompatibilities.
abstract: Many previous studies conclude that pre-zygotic barriers such as mechanical isolation account for most reproductive isolation between pairs of taxa. However, the inheritance and persistence of barriers such as these after the first generation of hybridization is rarely quantified, even though it is a vital consideration in understanding gene flow potential. There is an asymmetrical pre-zygotic mechanical barrier to hybridization between Eucalyptus nitens and Eucalyptus globulus, which completely prevents small-flowered E. nitens pollen from mating with large E. globulus flowers, while the reverse cross is possible. We aimed to determine the relative importance of pre- and post-zygotic barriers in preventing gene flow following secondary contact between E. nitens and E. globulus, including the inheritance of barriers in advanced-generation hybrids.
  
  
  
  Experimental crossing was used to produce outcrossed E. nitens, E. globulus and their F1, F2, BCg and BCn hybrids. The strength and inheritance of a suite of pre- and post-zygotic barriers were assessed, including 20-year survival, growth and reproductive capacity.
  
  
  
  The mechanical barrier to hybridization was lost or greatly reduced in the F1 hybrid. In contrast, intrinsic post-zygotic barriers were strong and persistent. Line-cross analysis indicated that the outbreeding depression in the hybrids was best explained by epistatic loss.
  
  
  
  The removal of strong mechanical barriers between E. nitens and E. globulus allows F1 hybrids to act as a bridge for bi-directional gene flow between these species. However, strong and persistent post-zygotic barriers exist, meaning that wherever F1 hybridization does occur, intrinsic post-zygotic barriers will be responsible for most reproductive isolation in this system. This potential transient nature of mechanical barriers to zygote formation due to additive inheritance in hybrids appears under-appreciated, and highlights the often important role that intrinsic post-mating barriers play in maintaining species boundaries at zones of secondary contact.
authors.affiliation: Department of Botany, University of Otago, PO Box 56, Dunedin 9011, New Zealand matt.larcombe@otago.ac.nz.
authors.forename: Matthew J
authors.initials: MJ
authors.lastname: Larcombe
authors.affiliation: Centro de Estudos Florestais, Instituto Superior de Agronomia, Universidade de Lisboa, Tapada da Ajuda, 1349-017 Lisboa, Portugal.
authors.forename: João
authors.initials: J
authors.lastname: Costa E Silva
authors.affiliation: School of Biological Sciences, and ARC Centre for Forest Value, 10 University of Tasmania, Private Bag 55, Hobart, Tasmania 7001, Australia.
authors.forename: Paul
authors.initials: P
authors.lastname: Tilyard
authors.affiliation: seedEnergy Pty Ltd, 2 Derwent Avenue, Margate, Tasmania 7054, Australia.
authors.forename: Peter
authors.initials: P
authors.lastname: Gore
authors.affiliation: School of Biological Sciences, and ARC Centre for Forest Value, 10 University of Tasmania, Private Bag 55, Hobart, Tasmania 7001, Australia.
authors.forename: Brad M
authors.initials: BM
authors.lastname: Potts
mesh_terms: D005052:Eucalyptus / Q000235:genetics* / Q000502:physiology; D035264:Flowers / Q000235:genetics / Q000502:physiology; D051456:Gene Flow; D006824:Hybridization, Genetic; D011058:Pollen / Q000235:genetics / Q000502:physiology; D012098:Reproduction; D060047:Reproductive Isolation; D015053:Zygote
keywords: Eucalyptus globulus; Eucalyptus nitens; Reproductive isolation; Tasmania; exotic gene flow; experimental crossing; hybridization; post-zygotic barriers; pre-zygotic barriers; speciation