Reproductive diversity and strategies for breeding in St. John's wort (Hypericum perforatum L.)

Abstract

St. John's wort (Hypericum perforatumL.) is a medicinal crop plant that reproduces by apomixis. To study the reproductive diversity within the initial breeding material, 656 plants from 92 accessions of the species were analysed by flow cytometric seed screening (FCSS) and compared to 66 plants of cv. ‘Topaz’ as a control. Among plants from the 92 accessions, 16 were obligate sexuals, nine were obligate apomicts and all remaining ones were facultative apomicts. Among controls, there were six obligate and 60 facultative apomicts. Tests of reproductive pathways indicated that, in the investigated collection, all apomictic plants were tetraploid and all sexual plants were diploid. A strategy for a cross breeding program in St. John's wort is illustrated by crossing one diploid obligately sexual mother plant with six tetraploid facultative apomicts. All 91 F₁ progeny obtained were triploid with 58% being obligate sexuals and 42% facultative apomicts. To quantify the different routes of seed formation within facultatively apomictic individuals, the reproductive pathways were reconstructed from about 30 single seeds from each of five tetraploid and two triploid plants using FCSS. The frequency of seed formed by apomixis ranged from 54% to 67% among four tetraploid genotypes. One triploid and two tetraploids formed 23% to 73% of the so-called B_III hybrids (fertilization of unreduced eggs) and one triploid was obligately sexual. The results show that while the mode of reproduction in H. perforatum is highly variable, tetraploid obligate apomicts as well as diploid and triploid obligate sexuals can be selected as starting material for breeding purposes.