Evolution and Phylogeny of the Pelexia Alliance (Orchidaceae: Spiranthoideae: Spiranthinae)

Abstract

The Pelexia alliance is distinguished by a blade-like, truncate rostellum, a viscidium with a sUrface that must be ruptured, and an apical extension on the anther covering the immature viscidium. Pelexia, Sarcoglottis, and Cyclopogon and their sections are treated cladistically. The unique position of the stylar canal entrance above the stigmatic surface in Pelexia and Sarcoglottis is considered specialized. Transfer of Pelexia sect. Potosia is confirmed. Various elements of the alliance appear specialized for different habitats and pollinators. The Pelexia alliance seems to have diverged early in the history of the subtribe. Pelexia Poit. ex Lindley is a member of Spiranthinae (Orchidaceae), a largely terrestrial group characterized by a dorsal erect anther, fascicled roots, inconspicuous staminodia, and resupinate flowers. In Schlechter's (1920) revision of the subtribe, characteristics of the column apex, such as the rostellum and viscidium, were used to segregate the various members into four complexes; these are termed the Spiranthes, Brachystele, Pelexia, and Stenorrhynchos alliances (Balogh 1982). As redefined by Balogh (1982), the Brachystele alliance has a very reduced rostellum that is usually inseparable from the apical viscidium portion; the Pelexia alliance has an elongate, broad, laminar rostellum with an apical viscidium portion; the Spiranthes alliance has a broad laminar rostellum with a centrally located viscidium portion; and the Stenorrhynchos alliance has a subulate, bristlelike rostellum that is ensheathed by the viscidium portion. As distinguished by Schlechter (1920), the Pelexia alliance contained Pelexia, Sarcoglottis Presl, Cyclopogon Presl, Deiregyne Schltr., Gamosepalum Schltr., Schiedeella Schltr., and Eurystyles Wawra. In Balogh (1982) Deiregyne and Gamosepalum were combined and transferred to a separate alliance, Eurystyles was transferred to a separate alliance, and Schiedeella was transferred to the Stenorrhynchos alliance. Evidence cited below favors combining the remaining elements into three genera, Pelexia, Sarcoglottis, and Cyclopogon (including Beadlea Small and Manniella americana C. Schweinf. & Garay). The distinctive rostellum that remains more or less intact after removal of the viscidium portion has proven to be correlated with other character states such as the rupturable viscidium and the presence of an apical extension on the anther that protects the immature viscidium. These structural specializations along with others discussed below provide sufficient basis for an attempt to analyze the group cladistically. MATERIALS AND METHODS Materials used in this study were taken from fluid-preserved flowers on deposit at AMO and in the personal collection of Burns-Balogh. It is important in a study based on column structure to use fluid-preserved or fresh flowers because pressing and drying causes loss in detail of the column structures. For an outgroup, the Pelexia alliance is compared with the typical element of the subtribe, the Spiranthes alliance, as represented by Spiranthes Rich., a genus common in temperate North America and comparatively unspecialized or plesiomorphic in many significant character states. Polarity of characters was based on outgroup comparison (Watrous and Wheeler 1981) in which more general character states are plesiomorphic. The cladogram (fig. 1) was constructed in the most parsimonious manner attempting to minimize the amount of homoplasy. During the present study, the various character states have been found to follow along the generic lines established by Balogh (1982) confirming results that were based on traditional taxonomic methods. In Balogh (1982) no attempt was made to go into detail of each genus or alliance or the evolution of each group. The purpose of this paper is to apply cladistic methods to the Pelexia alliance in order to understand the evolution of the group.