Vanilla can therefore be considered as a TCG, a “Taxonomic Complex Group” sensu Ennos et al. (Ennos et al., 2005). Indeed, it exhibits (1) a uniparental reproduction mode (vegetative reproduction), (2) interspecific hybridization in sympatric areas, and (3) polyploidy. These mechanisms have profound effects on the organization of the biological diversity and have been described as being responsible for the difficulty to define discrete, stable and coherent taxa in such TCGs (Ennos et al., 2005). TCGs are widespread in plants and uniparental reproduction can produce a complex mixture of sexual outcrossing and uniparental lineages that can be at different ploidy levels and the whole complex can be involved in reticulate evolution generating novel uniparental lineages by hybridization (Ennos et al., 2005). This has great implications on the way conservation programs should be conducted. In such TCGs, as it is often not possible to classify biodiversity into discrete and unambiguous species, traditional species-based conservation programs are not appropriate. As recommended, in situ conservation should focus on the evolutionary processes that generate taxonomic diversity rather than on the poorly defined taxa resulting from this evolution (Ennos et al., 2005). This includes concentrating on species that might be widespread (and thus not concerned by classical conservation efforts) but responsible for the generation of taxonomic diversity (through hybridization, introgression, or polyploidization).
Therefore, not only the mechanisms described in this chapter provide a better understanding of the Vanilla genus evolution, but they also are of major importance with regard to future genetic resources management and conservation (Crandall et al., 2000; Moritz, 2002).
These mechanisms are also of major interest with regard to the future improvement of V. planifolia. Interspecific hybridizations between V. planifolia and other aromatic species have already proved successful. In Madagascar, the production of V. planifolia × V. tahitensis hybrid variety “Manitra ampotony” led to an increased vanillin content (6.7% vanillin versus 2.5% in V. planifolia) and the (V. planifolia × V. pompona) × V. planifolia “Tsy taitry” shows increased resistance to different diseases (Dequaire, 1976; FOFIFA, 1990; Nany, 1996). In India, V. planifolia × V. aphylla hybrids were produced to increase Fusarium resistance (Minoo et al., 2006). At the intraspecific level, self-pollination could also be used to increase diversity in this genetically uniform crop (Bory et al., 2008d; Minoo et al., 2006). Furthermore, the agronomic characterization (vigor, resistance, vanillin production) of autotetraploid types is currently being performed in Reunion Island as a first step for testing for the potentialities of polyploidy breeding strategies in V. planifolia.
Unraveling the evolution and acquisition of traits of agronomic interest in the genus will also be of major importance. These traits include fragrance of fruits, which despite its considerable breeding interest has received limited attention particularly with regards to its evolution. Fragrant fruit species are almost exclusively restricted to America (Soto Arenas, 2003), and this character could have been selected as favoring fruit dispersion by bats (Soto Arenas, 1999) or sticky-seed dispersion by bees through fruit fragrance collection as observed in V. grandifl ora (Lubinsky et al., 2006). This matter was recently addressed by surveying intron length variations in the COMT gene family (Besse et al., 2009), encoding key enzymes in the phenylpropanoid pathway putatively involved in the biosynthesis of vanillin. Further work is also needed to understand the evolution, mechanisms and genetic determinism of spontaneous self-pollination in the genus (V. palmarum) — a highly desirable character that would considerably reduce bean production costs. Finally, elucidating how the aphyllous species of the genus have evolved and differentiated might be of great interest in our understanding of adaptation to dry conditions, given the predicted future of great climatic changes.
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