Handbook of Vanilla Science and Technology

The vanilloid orchids, Vanilloideae, have been recognized as a formal subfamily of Orchidaceae only during the past 10 years or so as DNA data have been used to re-evaluate relationships within the orchid family (Cameron et al. 1999; Cameron 2004, 2006). For a detailed review of this DNA-driven revolution in orchid taxonomy, see Cameron (2007). Current systems of orchid classification divide the family into 5 subfamilies (Chase et al. 2003). The largest, with approximately 650 genera and 18,000 species, is Epiden-droideae. This subfamily includes the vast majority of tropical epiphytic orchids and those most highly prized as ornamental plants. In contrast, the second subfamily, Orchidoideae, contains almost exclusively terrestrial species classified within approximately 200 genera. These 2 subfamilies, both characterized by flowers containing only a single fertile anther (monandrous), are known to have evolved from a common ancestor. All species within the third subfamily, Vanilloideae, also have flowers with just a single fertile anther, but this condition is considered to have evolved independently from the two other aforementioned monandrous subfamilies (Freudenstein et al. 2002). In other words, the reduction in stamen/ anther number from several to one occurred at least twice within Orchidaceae. The hypothetical ancestor of all orchids, as for most monocotyledons in general, probably possessed six stamens within a radially symmetrical flower similar to what we would see in a daylily, onion, or daffodil today. Through the process of evolution, orchid flowers are thought to have undergone significant structural modifications resulting in flowers with pronounced bilateral symmetry, loss of stamens, and fusion of the remaining stamen(s) with the pistil to form a central column. A snapshot of this evolutionary process, frozen in time, can be seen today within the fourth subfamily, Apostasioideae, which contains only two extant genera: Neuwiedia and Apostasia. Species of Neuwiedia possess flowers with three fertile anthers. These are only partially fused with the base of the pistil, and the perianth of the flower is only slightly bilateral in symmetry. These are the most “primitive” of all orchids, in that they show the fewest modifications from the basic blueprint of a hypothetical pre-orchid ancestor. Species of Apostasia are similar in floral morphology, but have lost an additional anther so that the mature flowers contain just two. A pair of fertile anthers also defines the fifth orchid subfamily, Cypripedioideae. This group of species, most commonly referred to as “lady’s slipper orchids”, are classified within just four genera. The labellum or lip of these flowers is always inflated into a sack or pouch. In terms of relative size, Cypripedioideae (ca. 120 species) is more diverse than Apostasioideae (15 species), but less diverse than Vanilloideae (ca. 200 species).

14.2 DIVERSITY WITHIN VANILLOIDEAE

Prior to being classified as their own subfamily of Orchidaceae, the vanilloid orchids were moved around from one orchid group to another. Robert Dressler’s (1993) comprehensive, well respected, pre-molecular system of orchid classification listed many of the vanilloid orchids under the category incertae cedis (meaning of uncertain status). It is a mix of presumably primitive and advanced floral features among vanilloid orchids that has caused confusion for orchid taxonomists as to how these species should be best classified. At one time, it was suggested that they might be best treated as a family of plants, Vanillaceae, closely related to but separate from Orchidaceae (Lindley 1835). Throughout most of the nineteenth and twentieth centuries, however, they have been placed among the lower branches of the orchid subfamily Epidendroideae (Dressler 1979). The controversy of their position among orchids was eventually laid to rest using comparisons of DNA sequence information. In fact, one of the most unexpected results of the first molecular systematic studies of orchids was the segregation of Vanilla and its relatives from the other orchids with a single fertile stamen, and their position near the base of the orchid family tree (Cameron et al. 1999). Recognition of the group as a subfamily, therefore, helped to solve one of the better known enigmas of orchid systematics.

14.2.1 Tribe pogonieae