title: The evolution of floral nectaries in Disa (Orchidaceae: Disinae): recapitulation or diversifying innovation?
abstract: OBJECTIVE
  The Orchidaceae have a history of recurring convergent evolution in floral function as nectar production has evolved repeatedly from an ancestral nectarless state. However, orchids exhibit considerable diversity in nectary type, position and morphology, indicating that this convergence arose from alternative adaptive solutions. Using the genus Disa, this study asks whether repeated evolution of floral nectaries involved recapitulation of the same nectary type or diversifying innovation. Epidermis morphology of closely related nectar-producing and nectarless species is also compared in order to identify histological changes that accompanied the gain or loss of nectar production.
  
  
  METHODS
  The micromorphology of nectaries and positionally equivalent tissues in nectarless species was examined with light and scanning electron microscopy. This information was subjected to phylogenetic analyses to reconstruct nectary evolution and compare characteristics of nectar-producing and nectarless species.
  
  
  RESULTS
  Two nectary types evolved in Disa. Nectar exudation by modified stomata in floral spurs evolved twice, whereas exudation by a secretory epidermis evolved six times in different perianth segments. The spur epidermis of nectarless species exhibited considerable micromorphological variation, including strongly textured surfaces and non-secreting stomata in some species. Epidermis morphology of nectar-producing species did not differ consistently from that of rewardless species at the magnifications used in this study, suggesting that transitions from rewardlessness to nectar production are not necessarily accompanied by visible morphological changes but only require sub-cellular modification.
  
  
  CONCLUSIONS
  Independent nectary evolution in Disa involved both repeated recapitulation of secretory epidermis, which is present in the sister genus Brownleea, and innovation of stomatal nectaries. These contrasting nectary types and positional diversity within types imply weak genetic, developmental or physiological constraints in ancestral, nectarless Disa. Such functional convergence generated by morphologically diverse solutions probably also underlies the extensive diversity of nectary types and positions in the Orchidaceae.
authors.affiliation: Department of Biological Sciences, 2500 University Drive NW, University of Calgary, Calgary, Alberta, T2N 1N4, Canada.
authors.forename: Nina
authors.initials: N
authors.lastname: Hobbhahn
authors.forename: Steven D
authors.initials: SD
authors.lastname: Johnson
authors.forename: Benny
authors.initials: B
authors.lastname: Bytebier
authors.forename: Edward C
authors.initials: EC
authors.lastname: Yeung
authors.forename: Lawrence D
authors.initials: LD
authors.lastname: Harder
mesh_terms: D044822:Biodiversity; D005075:Biological Evolution; D035264:Flowers / Q000502:physiology* / Q000648:ultrastructure; D029595:Orchidaceae / Q000502:physiology* / Q000648:ultrastructure; D010802:Phylogeny; D057048:Plant Nectar / Q000502:physiology*; D054046:Plant Stomata / Q000502:physiology / Q000648:ultrastructure
chemical_list: D057048:Plant Nectar
keywords: Disa; Disinae; Orchidaceae; deceit pollination; evolution; functional convergence; modified stoma; nectar; nectary; orchid; reward; rewardless