title: Reticulate evolution in North American black-fruited hawthorns (Crataegus section Douglasia; Rosaceae): evidence from nuclear ITS2 and plastid sequences.
abstract: OBJECTIVE
  The taxonomic complexity of Crataegus (hawthorn; Rosaceae, Maleae), especially in North America, has been attributed by some to hybridization in combination with gametophytic apomixis and polyploidization, whereas others have considered the roles of hybridization and apomixis to be minimal. Study of the chemical composition and therapeutic value of hawthorn extracts requires reproducible differentiation of entities that may be difficult to distinguish by morphology alone. This study sought to address this by using the nuclear ribosomal spacer region ITS2 as a supplementary DNA barcode; however, a lack of success prompted an investigation to discover why this locus gave unsatisfactory results.
  
  
  METHODS
  ITS2 was extensively cloned so as to document inter- and intraindividual variation in this locus, using hawthorns of western North America where the genus Crataegus is represented by only two widely divergent groups, the red-fruited section Coccineae and the black-fruited section Douglasia. Additional sequence data from selected loci on the plastid genome were obtained to enhance further the interpretation of the ITS2 results.
  
  
  RESULTS
  In the ITS2 gene tree, ribotypes from western North American hawthorns are found in two clades. Ribotypes from diploid members of section Douglasia occur in one clade (with representatives of the east-Asian section Sanguineae). The other clade comprises those from diploid and polyploid members of section Coccineae. Both clades contribute ribotypes to polyploid Douglasia. Data from four plastid-derived intergenic spacers demonstrate the maternal parentage of these allopolyploids.
  
  
  CONCLUSIONS
  Repeated hybridization between species of section Douglasia and western North American members of section Coccineae involving the fertilization of unreduced female gametes explains the observed distribution of ribotypes and accounts for the phenetic intermediacy of many members of section Douglasia.
authors.affiliation: Department of Ecology and Evolutionary Biology, University of Toronto, 25 Willcocks Street, Toronto ON, Canada M5S 3B2 Department of Natural History, Royal Ontario Museum, 100 Queen's Park, Toronto ON, Canada M5S 2C6 mehdi.zarrei@sickkids.ca.
authors.forename: M
authors.initials: M
authors.lastname: Zarrei
authors.affiliation: Department of Ecology and Evolutionary Biology, University of Toronto, 25 Willcocks Street, Toronto ON, Canada M5S 3B2 Department of Biology, University of Toronto Mississauga, 3359 Mississauga Rd, Mississauga ON, Canada L5L 1C6.
authors.forename: S
authors.initials: S
authors.lastname: Stefanović
authors.affiliation: Department of Ecology and Evolutionary Biology, University of Toronto, 25 Willcocks Street, Toronto ON, Canada M5S 3B2 Department of Natural History, Royal Ontario Museum, 100 Queen's Park, Toronto ON, Canada M5S 2C6.
authors.forename: T A
authors.initials: TA
authors.lastname: Dickinson
mesh_terms: D001483:Base Sequence; D005075:Biological Evolution; D002467:Cell Nucleus / Q000235:genetics*; D027825:Crataegus / Q000235:genetics*; D021903:DNA, Ribosomal Spacer / Q000235:genetics*; D005638:Fruit / Q000235:genetics*; D014644:Genetic Variation; D006824:Hybridization, Genetic; D008954:Models, Biological; D009656:North America; D009690:Nucleic Acid Conformation; D010802:Phylogeny; D018087:Plastids / Q000235:genetics*; D011123:Polyploidy; D013045:Species Specificity
chemical_list: D021903:DNA, Ribosomal Spacer
keywords: Coccineae; Crataegus; Douglasia; Reticulate evolution; Rosaceae; allopolyploid; autopolyploid; concerted evolution; cpDNA; gametophytic apomixis; hawthorn; hybrids; nrITS; taxonomy