Handbook of Vanilla Science and Technology

Studies of Vanilloideae challenge the opinion that all orchid lineages are recently evolved, however, and new information concerning the systematics of Orchidaceae contradict some of the facts mentioned above. For example, Vanilla is one of a few orchid genera with a transoceanic distribution. Species are native to North America, South America, Africa, and Asia. Of course, this pattern could be explained by recent long distance dispersal of seeds (from Africa to the Caribbean, for example), but may also be indicative of a much older origin prior to the complete separation of Gondwana. The fact that vanilloid orchids survive as relicts in the Guyana Shield of South America, tropical Australia and Africa, Madagascar, and on the island of New Caledonia (a non-volcanic island that separated from Gondwana around 65 million years ago) may also provide evidence of their considerable age. Furthermore, the vanilloid orchids show high rates of molecular divergence between genera as well as between species in some cases.

Vanilloideae are positioned near the base of the orchid family tree, and Orchidaceae is the basal family within the large monocot order Asparagales (which includes the onion, iris, agave, and hyacinth families among others). Recent estimates of the evolutionary age of these plants have calculated that Orchidcaeae may trace their origins back at least 76 million years (Ramirez et al. 2007) or even as much as 119 million years (Janssen and Bremer 2004). Vanilloid orchids, in turn, are at least 62 million years old, and probably older. These dates are based on a “molecular clock” approach to determining divergence times among branches within an evolutionary tree constructed with DNA data. 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- to 20-million-year-old fossil specimen of orchid pollen attached to an extinct species of bee preserved in amber (Ramirez et al. 2007).

14.4 DIVERSITY WITHIN VANILLA

Of the 15 genera now classified within Vanilloideae and already discussed, Vanilla is the most diverse. A formal monograph of the genus has never been published, and the last treatment considering all the species was written over 50 years ago by Porteres (1954), who based his work on a revision by Rolfe (1896). A comparison of alternative systems of classification is presented in Table 14.1. The current worldwide checklist of all orchid species today recognizes 110 species of Vanilla (Govaerts et al. 2008). Half of these (ca. 61 species) are native to tropical South America, Central America, subtropical North America, and the Caribbean (i.e. they are Neotropical plants). Africa claims ca. 23 native species, with at least 5 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 or to the Pacific Islands. Vanilla tahitensis is grown throughout French Polynesia, but this “species” recently was shown to be a hybrid between Neotropical V. planifolia (the maternal parent) and V. odorata (the paternal parent). It is not a natural native of Polynesia (Lubinsky et al. 2008).

Table 14.1 Comparison of infrageneric classification systems for Vanilla, including an informal system based on recent molecular phylogenetic studies. Number of species classified by Rolfe (1896) and Porteres (1954) within each group are given

Rolfe (1896) | Porteres (1954) | DNA-based phylogeny (2009) | Representative species

Section Aphyllae (13 spp.) | Section Aphyllae (18 spp.)

Section Foliosae (37 spp.) | Section Foliosae

― | Subsection Membranaceous (15 spp.) | Membranaceae clade | V. mexicana

― | Subsection Papillosae (28 spp.) | ― | ―

― | Subsection Lamellosae (49 spp.)

― | ― | Neotropical, leafy, fragrant clade | V. planifolia

― | ― | Paleotropical clade

― | ― | ― V. africana subclade | V. africana

― | ― | ― African subclade | V. roscheri

― | ― | ― Asian subclade | V. aphylla

― | ― | ― Caribbean leafless subclade | V. barbellata