The cytohistological basis of apospory in Hypericum perforatum L.

St. John's wort (Hypericum perforatum L., 2n = 4x = 32) is a medicinal plant that produces pharmaceutically important metabolites with antidepressive, anticancer and antiviral activities. It is also regarded as a serious weed in many countries. H. perforatum is furthermore an attractive model system for the study of apomixis. Natural populations of H. perforatum are predominantly composed of tetraploid individuals, although diploids and hexaploids are known to occur. It has been demonstrated that while diploids are sexual, polyploids are facultative apomictic whereby a single individual can produce both sexual and apomictic seeds. Despite our increasing understanding of gamete formation in sexually reproducing species, relatively little is known regarding the cytological basis of reproduction in H. perforatum. Here, we have studied embryo sac formation and the genetic constitution of seeds by means of staining-clearing of ovules/ovaries, DIC microscopy and flow cytometric seed screening (FCSS) of embryo and endosperm DNA contents. Comparisons of female sporogenesis and gametogenesis between sexual and apomictic accessions have enabled the identification of major phenotypic differences in embryo sac formation, in addition to complex fertilization scenarios entailing reduced and unreduced male and female gametes. These data provide new insights into the production of aposporous seeds in H. perforatum, and complement ongoing population genetic, genomic and transcriptomic studies.