Handbook of Vanilla Science and Technology

As discussed above, interspecific hybridization between V. planifolia and V. odorata is considered to be the origin of the commercially important species V. tahitensis (Lubinsky et al. 2008). Since the hybridization event is believed to predate the documented understanding of how to hand pollinate Vanilla (Lubinsky et al. 2008), it is likely to have occurred naturally. Efforts have been made to improve cultivated V. planifolia through intentional interspecific hybridization. A breeding program in Madagascar produced V. planifolia × V. tahitensis and V. planifolia × V. pompona hybrids (Bory et al. 2008a). The cultivar Tsy Taitry mentioned above was developed by backcrossing a V. planifolia × V. pompona hybrid to V. planifolia (described by Bory et al. 2008a). An interspecific hybridization program was also carried out in Puerto Rico, where hybrids between V. planifolia, V. phaentha, and V. pompona were developed (Childers et al. 1988; Knudson 1950; Theis and Jimenez 1957). More recently, production of interspecific hybrids between V. planifolia and V. aphylla, a leafless species, was reported (Divakaran et al. 2006). The trait of particular interest that was driving these projects is resistance to Fusarium, since losses to the disease can be devastating. V. planifolia is susceptible to the disease, whereas V aphylla, V. pompona, and V phaentha are reported to be tolerant to the disease (Divakaran et al. 2006; Weiss 2002; Childers et al. 1988). Introgression of disease resistance into a crop species from related species is a commonly used approach in plant breeding. As described above, the cultivars Tsy Taitry and Vaitsy are reported to have better tolerance to Fusarium than does V. planifolia.

The sequences of the nuclear 5.8S ribosomal DNA (rDNA) gene and the flanking internal transcribed spacers (ITS) are frequently used in plant phylogenetic studies to deduce the evolutionary relationships of different species. Although the rDNA genes are present in high copy within the genome, in diploid species concerted evolution has been found to result in homogenization of the sequences, such that within a species a single sequence often predominates, although some sequence variants can be maintained (Baldwin et al. 1995; Alvarez and Wendel 2003; Bailey et al. 2003; Small et al. 2004). As already discussed, past natural interspecific hybridization is the origin of many plant species. Such allopolyploid species may have distinct ITS sequence variants originating from their ancestral parental species. Phylogenetic placement of such divergent sequences into different clades can be used to infer the identities of the subgenome donors (Gaut et al. 2000; Sang et al. 1995). This was the approach used to identify the parental species of V. tahitensis (Lubinsky et al. 2008). However, in some polyploid species, over evolutionary timescales, concerted evolution has also occurred between homoeologous loci and has resulted in homogenization of the ITS regions to one of the parental types (Wendel et al. 1995; Wang et al. 2000; Fortune et al. 2008, Rotter et al. 2010).