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Title: Analysis of Breeding Systems, Ploidy, and the Role of Hexaploids in Three Hypericum perforatum L. Populations

item Qu, Luping
item Widrlechner, Mark
item RIGBY, SHAWN - Iowa State University

Submitted to: Industrial Crops and Products
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 1/18/2010
Publication Date: 7/1/2010
Citation: Qu, L., Widrlechner, M.P., Rigby, S.M. 2010. Analysis of Breeding Systems, Ploidy, and the Role of Hexaploids in Three Hypericum perforatum L. Populations. Industrial Crops and Products. 32(1):1-6.

Interpretive Summary: St. John's wort (Hypericum perforatum) is both a medicinal plant and rangeland weed, with a complex reproductive system that involves seed production by both sexual and asexual means, called apomixis. Plants within the species and even within single populations can also have different ploidy levels (multiple sets of chromosomes). Information about the breeding system and the number of chromosome sets may valuable both for crop improvement and for developing weed-control strategies. This study was begun in order to study three populations of St. John's wort, using the laboratory tool of flow cytometry, which quantifies DNA content to determine the breeding and chromosome biology of these plants. Both seeds and seedlings were examined. In seeds, the DNA content of cell nuclei in embryos and endosperm tissue can be distinguished to help determine the reproductive system. All three populations were primarily, but not exclusively, producing seeds asexually, with tetraploids (a type containing four chromosome sets) predominant. Uncommon hexaploid types (with six chromosome sets) were examined more carefully and appear to be genetic 'dead ends', which likely play a very minor role in gene flow and the further evolution of H. perforatum. This information should be valuable to genetic resource managers for germplasm conservation, to plant breeders and medicinal-plant producers for crop improvement and the production of uniform products, and to weed scientists.

Technical Abstract: Hexaploid seeds are produced by predominantly tetraploid populations of Hypericum perforatum, but the fate of hexaploid seedlings and their reproductive behavior have not been closely examined. We used flow cytometry to analyze single seeds and individual plant samples of three accessions of H. perforatum to determine ploidy levels and reproductive pathways. Seed samples of all three accessions were facultative apomicts, with tetraploid cytotype predominant (85-91%) and a lower frequency of hexaploids (9-14%), with diploids (5%) detected in only one population. Seedling populations consisted of tetraploids (87-97%) and hexaploids (3-13%). Hexaploid embryos are most likely generated by a 2n gamete of the tetraploid and fertilized by a normal, reduced tetraploid male gamete. These hexaploids are expected to produce unbalanced gametes because they possess chromosome complements that include two triploid sets originally derived from two different species. The observation that some tetraploid seeds had endosperm with high cellular DNA content indicates that some unbalanced male gametes produced by hexaploids were evidently viable and could effect fertilization. Whether this mechanism is also true for egg cells or whether the hexaploids are capable of producing unreduced embryo sacs is unknown. Because of severe reproductive difficulties, hexaploid seedlings may play a very minor role in gene flow and the further evolution of H. perforatum. The likelihood that hexaploids will evolve to types with an increased frequency of bivalent paring in meiosis is relatively low.