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Persistent nuclear ribosomal DNA sequence polymorphism in the Amelanchier agamic complex (Rosaceae)
C S Campbell1, M F Wojciechowski, B G Baldwin
1Department of Plant Biology and Pathology, University of Maine, Orono 04469-5722, USA. campbell@maine.maine.edu
Molecular Biology and Evolution
|January 1, 1997
Summary
Polymorphic DNA sequences in Amelanchier plants suggest ancient gene flow between North American clades. This genetic diversity, unusual for its age, may be maintained by polyploidy and asexual reproduction in these species.
Area of Science:
- Plant genetics
- Molecular evolution
- Phylogenetics
Background:
- Nuclear ribosomal DNA (nrDNA) internal transcribed spacers (ITS) in Amelanchier exhibit unusual, ancient polymorphism.
- This genetic variation has resisted homogenization by concerted evolution, suggesting unique evolutionary mechanisms at play.
Purpose of the Study:
- To investigate the origins and evolutionary mechanisms of ancient ITS sequence polymorphism in Amelanchier.
- To determine the role of gene flow between major North American clades in generating this genetic diversity.
Main Methods:
- Phylogenetic analysis of ITS sequences to resolve major Amelanchier clades.
- Sequencing and analysis of cloned ITS1-5.8S gene-ITS2 regions from polymorphic individuals.
Main Results:
- Two major North American Amelanchier clades (A and B) were identified based on ITS sequences.
- Five eastern North American taxa show extensive polymorphism, indicating hybrid origins involving clade A (A. humilis or A. sanguinea) and clade B.
- Sequence analysis of A. "erecta" revealed ITS repeats identical to A. humilis or clade B, or recombinant forms.
Conclusions:
- Ancient ITS polymorphism in Amelanchier is likely driven by historical gene flow between distinct clades.
- Hybridization events, possibly involving A. humilis or A. sanguinea, have generated polymorphic taxa.
- Persistence of these polymorphisms may be facilitated by polyploidy and agamospermy, common in the genus.