Embryogenesis, endospermogenesis and fruit development in Lophophytum (Balanophoraceae): Focus on endosperm and embryo initiation

- Parthenogenesis is proposed for the Argentine species of Lophophytum.
- The formation of the endosperm is performed without any fertilization.
- The zygote begins its division due to the influence of the endosperm.
- The mature seed of Lophophytum is formed of endosperm and undifferentiated embryo.

Studies on the development of the endosperm, embryo and fruit are scarce in Balanophoraceae, since a particular pattern for the family has not been established. Moreover, discrepancies between the reported cases are being observed by various authors. The aims in this study are to describe the processes of endospermogenesis, embryogenesis and fruit development for the Argentine Lophophytum species. Pistillate flowers and fruits at different stages of development were analysed using conventional optical and scanning electronic microscopy. Endospermogenesis without fertilization takes place in three stages: firstly, the formation of a coenocyte of 2-12 nuclei which originate from polar nuclei. The second stage is the fusion of the endosperm nuclei, plus both nuclear and cytoplasmic material from cells of nucella. Once inside the coenocytes, fusion of all the nuclei results in a single giant nucleus reaching dimensions of 120 × 60 μm. The third stage is the endosperm formation sequence with rounds of coordinated mitosis, giving rise to nuclei of equal dimensions; the simultaneous cytokinesis gives rise to the early endosperm cells. The zygote, influenced by the endosperm, undergoes three or four rounds of mitotic division, resulting in a mass of very small cells. The mature seed is formed only of endosperm and undifferentiated embryo. The mature embryo is undifferentiated, globular, consisting of between 24 and 32 cells and it completely lacks any radicle, cotyledons or stem. The mature ovary becomes a tiny achene in both species. We proposed the existence of parthenogenesis for the Argentine species of Lophophytum, which is justified by the formation of the endosperm without any fertilization, which is the start of the development of the autonomous endosperm. The zygote begins its division due to the influence of the endosperm.