Abstract
The pantropical genus Vanilla is a member of the Orchidaceae family, one of the largest and most diverse families of angiosperms in the world. It originated in America and differentiated in America, Africa, and Asia. About 132 species of Vanilla have now been accepted since the discovery of the genus. Within the Vanilla genus, leaflessness has appeared at least three independent times, in Africa, Asia, and America as an adaptation to drought conditions. The South-West Indian Ocean (SWIO) region, a biodiversity hotspot for orchids, is home to seven leafless species. Chloroplast markers failed to differentiate species within this recent SWIO clade. Morphological traits used for description are very similar between species and do not provide reliable identification. Moreover, some of the species have overlapping flowering periods and geographic distribution in Madagascar, increasing the probability of finding some sympatric species and, therefore, hybrids. Leafless Vanilla species from the SWIO islands are thus an excellent model for understanding the evolution of orchids. For their conservation, and to confirm the identity of these leafless species, an integrated approach with classical taxonomy using a large number of samples, intense fieldwork on biology and ecology, and molecular studies using variable markers is necessary.
Highlights
With more than 24,000 species and 700 genera worldwide, Orchidaceae is one of the largest families of flowering plants [1,2], and originated around 83 Mya (Million years ago) [3,4]
In Madagascar, we observed that the number of opened flowers per inflorescence per day can go up to three in the leafless white-flower species (V. madagascariensis, V. decaryana, V. bosseri), whereas this number is on average one flower and rarely two in the yellow-flower populations of Montagne des Français (V. humblotii)
V. penicillata which is the only species included in section Xanata [9]. This chloroplast phylogeny showed the surprising result that the Caribbean leafless species originated from an African ancestor, via transoceanic dispersion to America, rather than from an American leafy species [3]. This molecular data contradicts previous suggestions made by Portères in 1954, that leafless Vanilla species from the three geographical distributions (SWIO, Asia and Caribbean) originated from leafy species from neighboring continents [10]
Summary
With more than 24,000 species and 700 genera worldwide, Orchidaceae is one of the largest families of flowering plants [1,2], and originated around 83 Mya (Million years ago) [3,4]. Diversity 2020, 12, 443 there by wild, as well as cultivated, genetic resources This region ranks first in terms of leafless Vanilla species richness [9,10]. It is renown worldwide for its Vanilla fruit production, with the islands of Madagascar, Comoros and La Réunion being amongst the first world producers [11]. This paper provides an updated bibliographical review about the Vanilla genus. It is enriched by our own observations, and is focused on leafless species, those indigenous and endemic to the SWIO islands (Madagascar, Comoros, Mayotte, Seychelles) and the East coast of Southern Africa. Some elements about the complexity of their taxonomy will be discussed
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