Embryology and carpology of Stelis kolomeitsevae (Pleurothallidinae, Orchidaceae) reveal a new type of orchid embryogenesis

Abstract The development and structure of the ovule and fruit, megasporogenesis, embryo sac formation, and stages of embryogenesis were investigated in detail for the first time in a representative of the genus Stelis Sw. The obtained data allow recognition of a new type of embryogenesis, the Stelis type, which differs from the previously described Pleurothallis-type characteristic of Pleurothallidinae. For the first time in this subtribe, four new features of the female gametophyte were identified: (i) formation of a megaspore tetrad instead of a triad; (ii) oblique division of the micropylar cell of the megaspore dyad; (iii) a modified Polygonum-type embryo sac containing five to seven nuclei, most commonly six; and (iv) localization of four nuclei in the chalazal half of the embryo sac. The embryogenesis of S. kolomeitsevae is distinguished by the following features: (i) documented division of the basal cell, confirming the participation of its derivatives in the formation of a multicellular suspensor; (ii) a zigzag configuration of the four-celled linear embryo, caused by lateral displacement of one of the suspensor derivatives; (iii) development of two- to three-tubular suspensor cells, one of which extends into the micropylar cavity within the inner integument, whereas the others remain inside the embryo sac and are oriented towards the chalazal zone; (iv) formation of distinct apical and intercalary growth zones in the suspensor; and (v) occurrence of four reproducible morphotypes during embryogenesis, differing in the number of tubular and axial suspensor cells. In addition, it was established that the inferior syncarpous fruits of S. kolomeitsevae represent capsules of the Lilium type, characterized by lignification of the endocarp while the dehiscence zones remain non-lignified. Lignification of the endocarp begins at fertilization and continues into the early stages of embryogenesis, suggesting possible coordination between fruit-wall differentiation and embryo development.