Labellar anatomy of the Nervilia plicata complex (Orchidaceae: Epidendroideae) in tropical Asia

Abstract

Nervilia plicata is a morphologically variable terrestrial orchid with a wide geographic range in tropical Asia. Several forms have previously been recognised as distinct taxa due primarily to differences in the size, outline and colour of perianth parts. As a first step towards understanding the links between floral polymorphism, pollination ecology and genetic differentiation within the N. plicata complex, we sought to ascertain whether distinct colour morphs in 18 populations in Thailand and South China were correlated with discrete differentiation in macro- or micromorphological characters. Perianth morphology was studied in the field and under stereomicroscopy, and labellum anatomy was examined using light microscopy and scanning electron microscopy. Phenetic analyses comprising Principal Coordinate Analysis (PCoA) and Unweighted Pair Group Method with Arithmetic Mean (UPGMA) of 16 characters revealed four clusters representing distinct floral morphs — pink morph with small, entire labellum; purple morph with large, undulate labellum; white and yellow morph with large, entire labellum; and white and yellow morph with small, entire labellum — among which the size, shape, density and periclinal wall surface of dome-shaped papillae in the secretory zone of the labellar adaxial epidermis varied discontinuously. Our results provide evidence of micromorphological differentiation among N. plicata populations in parts of tropical Asia. Pollinator observations and analyses of genetic diversity are required to establish whether this variation is of any ecological, evolutionary or taxonomic significance.