title: Breeding systems, hybridization and continuing evolution in Avon Gorge Sorbus.
abstract: OBJECTIVE
  Interspecific hybridization and polyploidy are key processes in plant evolution and are responsible for ongoing genetic diversification in the genus Sorbus (Rosaceae). The Avon Gorge, Bristol, UK, is a world 'hotspot' for Sorbus diversity and home to diploid sexual species and polyploid apomictic species. This research investigated how mating system variation, hybridization and polyploidy interact to generate this biological diversity.
  
  
  METHODS
  Mating systems of diploid, triploid and tetraploid Sorbus taxa were analysed using pollen tube growth and seed set assays from controlled pollinations, and parent-offspring genotyping of progeny from open and manual pollinations.
  
  
  RESULTS
  Diploid Sorbus are outcrossing and self-incompatible (SI). Triploid taxa are pseudogamous apomicts and genetically invariable, but because they also display self-incompatibility, apomictic seed set requires pollen from other Sorbus taxa - a phenomenon which offers direct opportunities for hybridization. In contrast tetraploid taxa are pseudogamous but self-compatible, so do not have the same obligate requirement for intertaxon pollination.
  
  
  CONCLUSIONS
  The mating inter-relationships among Avon Gorge Sorbus taxa are complex and are the driving force for hybridization and ongoing genetic diversification. In particular, the presence of self-incompatibility in triploid pseudogamous apomicts imposes a requirement for interspecific cross-pollination, thereby facilitating continuing diversification and evolution through rare sexual hybridization events. This is the first report of naturally occurring pseudogamous apomictic SI plant populations, and we suggest that interspecific pollination, in combination with a relaxed endosperm balance requirement, is the most likely route to the persistence of these populations. We propose that Avon Gorge Sorbus represents a model system for studying the establishment and persistence of SI apomicts in natural populations.
authors: School of Biological Sciences, University of Bristol, Bristol, UK.
authors: Shanna
authors: S
authors: Ludwig
authors: Ashley
authors: A
authors: Robertson
authors: Timothy C G
authors: TC
authors: Rich
authors: Milena
authors: M
authors: Djordjevic
authors: Radosav
authors: R
authors: Cerovic
authors: Libby
authors: L
authors: Houston
authors: Stephen A
authors: SA
authors: Harris
authors: Simon J
authors: SJ
authors: Hiscock
mesh_terms: D005075:Biological Evolution; D004171:Diploidy; D006824:Hybridization, Genetic; D018895:Microsatellite Repeats; D053205:Pollen Tube / Q000235:genetics; D054817:Pollination / Q000235:genetics; D011123:Polyploidy; D012639:Seeds / Q000235:genetics; D057945:Self-Fertilization / Q000235:genetics; D031994:Sorbus / Q000235:genetics*; D006113:United Kingdom