Very recently, a taxonomic synopsis for Vanilla was published posthumously based upon the work of the late Mexican botanist Miguel A. Soto Arenas (Soto Arenas and Cribb, 2010). Within this important preliminary work are presented keys to the species, information about geographic distribution, and lists of select specimens. It serves as a significant step toward updating the systematic treatment of the genus. The 15 Mexican and Central American species were treated more completely in a posthumously published work by Soto Arenas and Dressler (2010). Within this paper one will find detailed descriptions, illustrations, and information on the molecular characterization of the Mesoamerican species.
The current worldwide checklist of all orchid species today recognizes 110 species of Vanilla (Govaerts et al., 2008). Most of these (61 species) are Neotropical natives of South America, Central America, Caribbean islands, and southern Florida. Africa claims 23 native species, with at least five of these restricted to Madagascar. The remaining species of Vanilla are found on the Indian subcontinent and throughout tropical Southeast Asia. No species of Vanilla are native to Australia. Likewise, Polynesia and other oceanic islands of the Pacific lack native species of Vanilla. This is perplexing to some since “Tahitian Vanilla” is cultivated throughout the Pacific, and its scientific name, Vanilla tahitensis, implies that it is indigenous to the French Polynesian island of Tahiti. What was described more than 75 years ago (Moore, 1933) as a new “species” of Vanilla, however, has been proven recently by Lubinsky et al. (2008) to be nothing more than a primary hybrid between Neotropical V. planifolia (the maternal parent) and V. odorata (the paternal parent).
In terms of classification of species within the genus Vanilla, these were formally placed into one of two possible sections by Rolfe (1896). The first, Vanilla section Aphyllae, was erected to accommodate all of the leafless species in the genus (e.g., V. aphylla, V. barbellata, V. roscheri, and others). Species within this section grow on the African mainland, Madagascar, Southeast Asia, and also on islands in the Caribbean. Although some of these species produce fleshy fruits, there is no evidence that any of them are aromatic. Rolfe’s classification of these species together implies that they share a recent common ancestor, but molecular studies have demonstrated that this is not the case (Cameron, 2005). Instead, there appears to be at least three independent cases of probably leaf loss in Vanilla—once in Africa, once in the Caribbean, and at least once in Asia. The section, therefore, is not monophyl-etic, but an artificial grouping of species with shared vegetative morphology derived by convergent evolution. According to modern rules of natural classification, it should not be recognized formally.
For the remaining species not classified in Vanilla section Aphyllae, Rolfe created section Foliosae. As the name indicates, all of these are leafy. This is a large group of species, and so Portères (1954) further divided the section into subsections. Vanilla section Foliosae subsection Membranaceae is a small cluster of species characterized by thin stems, thin leaves, short aerial roots, and flowers in which the labellum is not fused with the column. The labellum also lacks the complex bristles, hairs, and scales characteristic of other Vanilla species, and the fruits tend to dry on the vines and split lengthwise. Vanilla mexicana exemplifies this section, and molecular systematic studies have demonstrated that the group is the most primitive of all Vanilla species. These plants are very difficult to cultivate, probably because they have close relationships with mycorrhizal fungi, and there is no evidence that the fruits produce aromatic vanillin.
The other remaining species of the genus, including V. planifolia and V. pompona, were classified into either Vanilla section Foliosae subsection Lamellosae or subsection Papillosae. The former group is so named because species within this section are characterized by flowers with flattened scale-like appendages (lamellae), hairs, bristles, and complex ornamentation on their labella, which is always fused to the column along its margins to form a floral tube. The latter subsection was proposed for those species characterized by fleshy leaves and flowers usually with thick trichomes positioned in the center of the labellum, but without lamellate scales. Species within this leafy section are pantropical in distribution, but recent molecular systematic studies have demonstrated that this group is also artificial. Instead, species of Vanilla cluster primarily by geographic origin, as can be seen in Figure 1.3. Specifically, all Old World species (from the African and Asian Paleotropics) share a common ancestor together with the leafless New World species. These were probably dispersed from Africa to the Caribbean at some point in the past. All remaining Neotropical species, including V. planifolia, share a different common ancestor. It is within this group that aromatic fruits producing significant levels of vanillin are found. As such, the group has informally been named the “Neotropical, fragrant, leafy species.” Note that molecular studies position V. tahitensis inside this group of Neotropical relatives, thereby confirming the hybrid origin of Old World Tahitian Vanilla, many individuals of which are tetraploid, from New World parents.
FIGURE 1.3 Phylogenetic relationships among select species of Vanilla. The cladogram is based on molecular sequence data from different genes including nuclear ribosomal ITS, plastid rbcL, matK, rpoC1, and others. The hybrid origin of V. tahitensis from a cross between V. odorata and V. planifolia is highlighted by the dashed lines. Informal clades and subclades are labeled on the branch representing the common ancestor of each major species group.
In their recent synopsis of the genus, Soto Arenas and Cribb (2010) classify 106 species and offered a new infrageneric classification of Vanilla based primarily on molecular phylogenetic reconstructions. The species with membranaceous leaves are classified as Vanilla subgenus Vanilla, which contains two informal “groups.” A second subgenus, Vanilla subgen. Xanatha was created for the remainder of the species. The name is based on the Mexican Totonac Indian name for Vanilla, “xanath.” This subgenus is further divided into a pair of sections: Xanatha and Tethya. The former corresponds to mostly leafy neotropical species and is divided into six informal groups (e.g., the V. palmarum group and V. pompona group). The latter is almost entirely paleotropical in distribution, except that it also includes the Caribbean leafless species. Those taxa are clustered into an informal unit (the V. barbellata group), along with 11 other groups that are included within the section (e.g., the V. phalaenopsis group and V. africana group).