Abstract
Helosis cayennensis (Balanophoraceae s.str.) is a holoparasite characterised by aberrant vegetative bodies and tiny, reduced unisexual flowers. Here, we analysed the development of female flowers to elucidate their morpho-anatomy and the historical controversy on embryo sac formation. We also studied the developmental origin of inflorescences and the ontogeny of fruits, embryo and endosperm and discussed in a phylogenetic framework. Inflorescences were analysed by optical, fluorescence and scanning electron microscopy. Inflorescences of H. cayennensis arise endogenously. Female flowers lack perianth organs, thus only consist of the ovary, two styles and stigmata. Ovules are undifferentiated; two megaspore mother cells develop inside a nucellar complex. The female gametophyte, named Helosis-type, is a bisporic four-celled embryo sac, provided with a typical egg apparatus and a uni-nucleated central cell. Fertilization was not observed, yet a few-celled embryo and cellular endosperm developed. In sum, results confirm that, among Santalales holoparasites, Helosis is intermediate in the reduction series of its floral organs. Although perianth absence best supports the Balanophoraceae s.str. clade, our literature survey on female flower developmental data across Balanophoraceae s.l. highlights the many gaps that need to be filled to really understand these features in the light of new phylogenetic relationships.
Highlights
Balanophoraceae is a relatively small family of 17 genera of root holoparasitic geophytes characterised by an aberrant vegetative and subterranean body, without leaves, stems or roots, called tuber, which may have rhizome-like ramifications [1,2,3,4,5]
The inflorescence has a differentiated axis on a fragile peduncle that raises the oval apical portion with deciduous scales and unisexual flowers embedded in a dense stratum of trichomes called
The mature embryo sac is composed of a typical egg apparatus and an enormous uni-nucleated central cell
Summary
Balanophoraceae is a relatively small family of 17 genera of root holoparasitic geophytes characterised by an aberrant vegetative and subterranean body, without leaves, stems or roots, called tuber, which may have rhizome-like ramifications [1,2,3,4,5] These parasitic plants are attached to the root of shrub and tree host species from dark, tropical forests. A peculiarity of the inflorescences is their endogenous origin (in relation to their own tissues), a unique feature in angiosperms [4] Their flowers are tiny and a wide range of extreme reductions can be observed among genera, especially in female flowers [1,4] and in their seeds, with embryos formed by few cells [2,4].
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