Journal
ANNALS OF BOTANY
Volume 114, Issue 2, Pages 253-269Publisher
OXFORD UNIV PRESS
DOI: 10.1093/aob/mcu116
Keywords
Reticulate evolution; nrITS; cpDNA; Crataegus; Douglasia; Coccineae; Rosaceae; hawthorn; gametophytic apomixis; allopolyploid; autopolyploid; concerted evolution; hybrids; taxonomy
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Funding
- Natural Sciences and Engineering Research Council of Canada Strategic Research Project Grant [381073]
- Naturally Grown Herb and Spice Growers Co-operative
- Canadensys initiative
- NSERCC Discovery Grants [A3430, 326439]
- Louise Hawley Stone Charitable Trust
- ROM Green Plant Herbarium
- Department of Natural History of the Royal Ontario Museum
- Royal Ontario Museum Foundation
- ROM Reproductions Acquisitions and Research Fund
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Background and Aims 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. Key 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.
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