Genus Diversity within Vanilloideae: Tribe Vanilleae
Having examined higher-level relationships among subfamilies of Orchidaceae, and lower-level relationships among species within Vanilla, let us now consider relationships among the genera of Vanilloideae. Examples of these genera are shown in Figure 1.4. The subfamily is divided into two tribes, the first of which is Vanilleae. In addition to Vanilla itself, this tribe contains eight other tropical genera. Two of these, Eriaxis and Clematepistephium, are endemic to the isolated Pacific island of New Caledonia. Both genera are monotypic, meaning that they contain only a single species each. An unusual aspect of one of these two species is that Clematepistphium smilacifolium grows in the dense shade of the New Caledonian rainforests as a climbing vine. Unlike species of Vanilla, however, Clematepistphium vines produce no aerial roots. Instead they climb by twisting around the trunks of small trees. Its large, leathery leaves exhibit prominent venation patterns that are reticulate (net-like) rather than exclusively parallel as we see in most orchids and other monocotyledons (Cameron and Dickison, 1998).
The two New Caledonian endemics described above were once classified as species of the genus Epistephium, but that genus of 20 species is now considered to be exclusively South American in distribution. Most of these species are erect herbs native to open savanna habitats, and they are most commonly found in nutrient-poor areas of Brazil and Venezuela. Some have been described as scrambling loosely through surrounding vegetation, but none are true climbers. The leaves of Epistephium exhibit reticulate venation like their New Caledonian relatives, and the stunning flowers are mostly dark pink or violet. Like most vanilloid orchids, however, they are almost impossible to cultivate. The fruits of these orchids are capsules that dehisce to release distinctive seeds with circular wings, a feature in Orchidaceae found only among Vanilloideae (Cameron and Chase, 1998).
Winged seeds are also found in three other genera of vanilloid orchids: Pseudovanilla, Erythrorchis, and Galeola. These are all closely related, and are native to Southeast Asia, Northeast Australia, and a few Pacific islands. All three of these genera are leafless climbing vines, two of which (Erythrorchis and Galeola) completely lack chlorophyll. These nonphotosynthetic genera are exclusively parasitic on fungi, a lifestyle technically known as mycoheterotrophy. The leafless genus Pseudovanilla is similar to the other two in most aspects, but does eventually develop green pigment within its stems even if it may persist in a presumably nonphotosyn-thetic state during the juvenile stages of its life cycle. Recent studies have shown that these orchids are the closest living relatives of vanilla (Cameron and Molina, 2006). They climb by means of aerial roots produced at each node of the stem, just like vanilla, and their flowers are remarkably similar to those of Vanilla species. Their fruits, however, are designed to accommodate the winged seeds within and so are dry, dehiscent, and nonaromatic at maturity.
FIGURE 1.4. Representative genera of subfamily Vanilloideae, the “vanilloid orchids.” (a) Pogonia ophioglossoides from the United States; (b) Pseudovanilla foliata from Queensland, Australia; (c) Epistephium elatum from Ecuador; (d) Erythrorchis cassythoides from New South Wales, Australia; (e) Clematepistephium smilacifolium vine and leaf with reticulate venation from New Caledonia; and (f) Eriaxis rigida from New Caledonia.
There are two other genera of Vanilloideae that grow as nonphotosynthetic mycoheterotrophs: Cyrtosia and Lecanorchis. Both grow as erect herbs within forested areas of southeast Asia, and both share a number of floral features with Vanilla, which has made them difficult to be classified within the subfamily. For example, the fruits of Cyrtosia are like those of Vanilla in being fleshy and contain small, black, spherical, crustose seeds, but are typically bright red to attract bird or mammal dispersers (Nakamura and Hamada, 1978). The small flowers of Lecanorchis are similar in structure to many species of Vanilla in that the labellum is fused with the column along its margins to produce a floral tube. Also, like many species of Vanilla, the labellum of Lecanorchis is ornamented with characteristic bristles and hairs, but Lecanorchis fruits are dry capsules lacking odor and containing numerous dust-like seeds with long slender appendages. Further study of the natural history of all these genera is warranted.
Genus Diversity within Vanilloideae: Tribe Pogonieae
The second tribe within subfamily Vanilloideae is Pogonieae, which contains tropical members but also half a dozen temperate species as well. The tribe is divided into four or possibly five genera. Pogonia is one of the temperate genera, and is unusual in that its species are in disjunction between eastern North America (one species, P. ophioglossoides) and eastern Asia (3–5 species). These plants are found most commonly in acidic bogs, around the edges of lakes, and within wet savannas. Also native to North America, specifically the eastern United States, is the genus Isotria. There are two species in the genus, both of which are characteristic among orchids for having leaves arranged into a whorl of five or six. These plants are spring ephem-erals that emerge and reproduce quickly within their deciduous forest habitat before the tree canopy closes fully during the summer months. One other genus, Cleistes, has members in temperate North America, and this is the genus Cleistes. Most species of this genus (>30 species) are native to tropical South America where they are most commonly found in open savannas that experience seasonal periods of drought. They are equipped with underground tubers that presumably allow them to survive by entering an annual state of dormancy. However, one species of this genus, Cleistes divaricata, is native to the southeastern United States. Detailed systematic studies of Pogonieae and vanilloid orchids indicate that this species might be better treated as a separate genus (Cameron and Chase, 1999). The final genus of Pogonieae is Duckeella, which contains one or possibly two species indigenous to Venezuela and northern Brazil. The genus produces long linear leaves and bright yellow flowers that rise above wet grassland and savanna habitats. It may occasionally be found rooted in mats of floating vegetation.
Final Thoughts
The vanilloid orchids are a tremendously diverse group of flowering plants. Whereas the greatest amount of research has been focused on V. planifolia, it is important to realize and to appreciate that this is only one species of a lineage that has become adapted to a variety of habitats, lives in greater or lesser partnerships with fungi, exhibits a variety of growth habits, relies on different pollinators, and develops flowers of diverse form (see Figure 1.5). In other words, V. planifolia may be the only orchid species of significant agricultural value (out of more than 25,000 naturally occurring species), but it is not entirely unique in the family. Rather, it is just one of approximately 110 species in the genus Vanilla, all of which are similar to and yet different from one another. Furthermore, Vanilla is only one genus out of 15 genera that are classified within the orchid subfamily Vanilloideae (the “vanilloid orchids”), and some of these are remarkable like vanilla in terms of their growth patterns, floral structure, and fruit dispersal mechanisms. Unfortunately, these orchids are generally overlooked by biologists and those in the vanilla industry, who know only of V. planifolia. Many of the genera and species discussed in this chapter are rare and in great danger of extinction primarily due to habitat destruction. By further appreciating and studying their diversity, there is offered a hope of their survival and evolution for another 70 million years.