This structural study compares the development of somatic embryos in tissue culture in Pelargonium x hortorum to that in three putative ancestors, P. alchemilloides, P. inquinans, and P. zonale. Previous studies showed that P. x hortorum and P. zonale produced similar numbers of embryos in response to various cytokinin treatments. Here we question whether embryos of P. x hortorum and P. zonale produced in response to 1 mM thidiazuron share similar pathways of development. We also compare somatic embryogenesis in these two genotypes to the developmental responses in P. inquinans and P. alchemilloides under the same cytokinin treatment. Histological sections show that both Pelargonium x hortorum and P. zonale exhibit direct somatic embryogenesis from single epidermal cells and may show indirect embryogenesis from callus produced at later stages. Early stages of somatic embryogenesis are characterized by small cells with dense cytoplasm and highly stained nuclei. As embryos mature, they develop a well-defined protoderm, and have no vascular connection to the explant. Developing embryos do not possess a suspensor but are broadly attached to the explant. Cotyledon development is initially atypical, although somatic embryos ultimately develop into normal plants. Both P. x hortorum and P. zonale may also exhibit shoot organogenesis from calllus, indicating that different developmental pathways may be initiated in the same explant at different times. Under the same cytokinin treatment, P. inquinans produces only callus while shoot organogenesis also occurs in P. alchemilloides. The developmental similarities in P. zonale and P. x hortorum support our hypothesis that factors influencing the ability to form somatic embryos in P. x hortorum were most likely inherited from the ancestral species P. zonale during hybrid evolution.

Key words: cytokinin, development, Pelargonium, shoot organogenesis, somatic embryogenesis, thidiazuron