Characterization of leafless Vanilla species endemic to Madagascar for their conservation

Abstract

Biological diversity is unevenly distributed on the planet, being mostly concentrated in a few regions, called "biodiversity hotspots". The South West Indian ocean (SWIO) region is among the priority regions for biodiversity and orchid conservation. In addition to being a leader in Vanilla planifolia pods production, SWIO also presents an important specific richness in endemic leafless plants belonging to the genus Vanilla Plum. ex. Miller (Orchidaceae), mainly distributed in Madagascar. Because of their ability to adapt to drought and their medicinal properties, leafless Vanilla species are of major economic and scientific interest. They form a monophyletic group recently diverged from an African leafy ancestor. They cannot be differentiated by morphological traits and chloroplast genes, making their taxonomic status questionable. In Madagascar, leafless Vanilla species are distributed mainly in the dry forests along the West coast, classified among the most threatened ecosystems of the island. In order to contribute to their conservation, field surveys followed by sampling were carried out in several localities of Madagascar. Then, an integrative approach combining genetics, biology and ecology was carried out to solve the taxonomy of these species and determine their conservation status. According to genetic clustering using microsatellite loci and morphological analyses, the seven genetic groups identified in Madagascar differentiated likely due to several factors, including geographical barriers (rivers), isolation by environmental factors (IBE) (temperature, elevation, soil pH) strongly correlated to geographical isolation (IBD), and isolation by adaptation (IBA) of floral traits (resulting either from IBE or from divergent selection by pollinators). The integrative taxonomic strategy (ITS phylogeny, population-based microsatellite and morphological analyses) divides SOOI leafless Vanilla species into two major clades: white-flower versus yellow