Evolution of Vanilloid Orchids
An unsubstantiated hypothesis has persisted among biologists that the orchid family is only recently evolved relative to other flowering plants. To support this opinion, botanists cite the relatively low levels of genetic diversity among orchid genera and species, many of which can be hybridized easily with one another. They provide evidence in the fact that the geologically young Andes of South America and Highlands of New Guinea are centers of greatest orchid diversity. The close relationship between orchids and social bees, which are thought to have evolved much later than other insects, is also given as proof, and the fact that most orchid genera are found in either the Paleotropics or the Neotropics, but rarely are pantropical, indicates to some that Orchidaceae evolved only recently and certainly long after the separation of today’s continents.
Molecular phylogenetic studies of Vanilloideae challenge the notion that the entire orchid family is recently evolved, however, and new perspectives on the systematics of Orchidaceae downplay or even contradict some of the facts mentioned above. For example, Vanilla is one of a few orchid genera with a transoceanic distribution that may not be due entirely to long-distance dispersal. Extant species are native to North America, South America, Africa, and Asia (see Figure 1.2). The fact that vanilloid orchids survive in the Guyana Shield region of South America, tropical Australia and Africa, Madagascar, and on the island of New Caledonia (a nonvolcanic Pacific island with a peculiar ancient flora that separated from Gondwana around 65 million years ago) may also provide evidence of their considerable age and possible status as ancient relicts (Cameron, 1999, 2000).
FIGURE 1.2 Paleotropical distribution of Vanilla, and estimates of species diversity within each geographic region.
Furthermore, subfamily Vanilloideae is positioned near the base of the orchid family tree, and Orchidaceae is the basal family within the large monocot order Asparagales (including onions, agaves, hyacinths, and the iris family, among others). Molecular clock estimates of the evolutionary age of these plants have calculated that Orchidacaeae may trace their origins back at least 76–119 million years (Janssen and Bremer, 2004; Ramirez et al., 2007). Vanilloid orchids, in turn, are at least 62 million years old. Molecular clocks can only provide minimum ages, so these plants are probably even older. Critical to this approach is the use of a calibration point for the “clock,” which, in the case of Orchidaceae, has been provided by a 15–20-million-year-old fossil specimen of orchid pollen attached to an extinct bee preserved in amber (Ramirez et al., 2007).
Subfamily Vanilloideae among Orchids
As mentioned already, the vanilloid orchids, Vanilloideae, have been recognized as a subfamily of Orchidaceae only in the past decade, as DNA data have been used to reevaluate relationships among all orchids. Cameron (2007) has provided a detailed review of this DNA-driven revolution in orchid taxonomy from 1997 to 2007. The current systems of orchid classification (e.g., Chase et al., 2003) divide Orchidaceae into five subfamilies. The largest, with approximately 650 genera and 18,000 species, is Epidendroideae, which is dominated by tropical epiphytes and those orchids most highly prized as ornamentals. Orchidoideae, the second largest subfamily, is made up almost exclusively of terrestrial species classified within approximately 200 genera. Both subfamilies are characterized by monandrous flowers (meaning they have only one anther). All species within the subfamily Vanilloideae also possess flowers with just a single fertile anther, but this condition is considered to have evolved independently from Orchidoideae and Epidendroideae, and is the result of a unique mode of floral development (Freudenstein et al., 2002). In other words, the reduction in stamen/anther number from several (probably from six down to three and eventually down to one) occurred at least two times within Orchidaceae. 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 clue to explain the beginnings of this hypothetical evolutionary continuum can be found today by examining living members of the fourth orchid subfamily, Apostasioideae, which contains two genera: Neuwiedia and Apostasia. Species of Neuwiedia are triandrous, possessing 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. Apostasioid orchids in many ways may be viewed as the most “primitive” of all orchids in that they show the least number of modifications from the basic blueprint of a hypothetical pre-orchid monocot ancestor. Diandrous flowers (i.e., with two fertile anthers) define the fifth orchid subfamily, Cypripedioideae. This group of about 120 species is commonly called “lady’s slipper orchids.” In terms of relative size, Cypripedioideae is more diverse than Apostasio-ideae (15 species), but less diverse than Vanilloideae (200 species), which will be considered further below.
Before they were classified as their own subfamily of Orchidaceae, most of the vanilloid orchids were considered to be primitive members of the monandrous subfamily Epidendroideae, but somewhat unconvincingly so. In fact, Dressler’s (1993) pre-molecular system of orchid classification listed many of the vanilloid orchids under the category insertae sedis (meaning “of uncertain status”). At one time, it was even suggested that they might be best treated as a separate family all their own, Vanillaceae, closely related to, but separate from, Orchidaceae (Lindley, 1835). Why the uncertainty? A mix of what are assumed to be both primitive and advanced floral features among vanilloid orchids can be claimed to be the source of greatest confusion. Their precise position among orchids was eventually laid to rest using comparisons of DNA sequence information, and among the most unexpected results of the first molecular phylogenetic studies of orchids was the relocation of vanilla and its relatives from a position among the other orchids with a single fertile anther to a placement near the base of the orchid family tree (Cameron et al., 1999). Recognition of Vanilloideae as a monophyletic subfamily helped in solving one of the more perplexing enigmas of orchid systematics.
Species Diversity within Vanilla
Within Vanilloideae are no fewer than 15 genera, but Vanilla is the most diverse of these. There is yet to be published a formal monograph of the genus, but there does exist a taxonomic treatment of Vanilla that considered all the species known at the time. Unfortunately, this treatment was written more than 50 years ago (Portères, 1954).