Homoplasy in the embryonic development of terrestrial and epiphytic orchids from the subtribe Malaxidinae (Orchidaceae).

Abstract

Main conclusion: During evolution, similar vectors of adaptive radiation may have evolved in the subtribe Malaxidinae. This was manifested in homologous series of variability in suspensor shape and seed coat ultrasculpture in clades of terrestrial and epiphytic orchids. The present study examines the variability of embryonic traits across clades and subclades of subtribe Malaxidinae (Orchidaceae), previously identified by molecular genetic data. Ovules and seeds from fruits of orchids of the genera Crepidium, Liparis (sections Cestichis and Blepharoglossum), Dienia, and Oberonia were examined by confocal laser microscopy with fluorescent dye staining. The branched or rounded suspensor in the studied species was unicellular and originated from the nondividing basal cell cb. The elongated or flattened transmission structure of one or two cells was located at the junction of the suspensor and the embryo proper. Two species (Oberonia gammiei and Liparis elliptica) were found to have unitegmal ovules. Three morphological groups of seeds were identified based on the shape and sculpture of the periclinal cell wall. A comparative analysis of the embryological characters in Malaxidinae species reveals that the lobed suspensor is a homoplasy present in different subclades of terrestrial and epiphytic orchids. The flat transmission cell is an apomorphy in the Cestichis subclade. Similarly, the independent formation of the unitegmal ovule occurred in two subclades of epiphytic orchids. The results of our study suggest that similar adaptive radiation vectors may have evolved in the subtribe Malaxidinae in the orchids we studied.