Mycorrhizal fungi play a key role in the life history of orchid species. This is because, in general, orchid seeds lack endosperm and have therefore no stored energy resources for germination. As a consequence, they rely on mycorrhizal symbioses as a source of energy and nutrients during at least the early phases of growth. However, at present little is known about the actual distribution of orchid mycorrhizal fungi and about the specificity of orchid-fungus associations. Given that the availability of suitable mycorrhizal fungi is crucial to orchid establishment, some authors have suggested that the enormous diversity of orchid species – the total number of species has been estimated at 19,500  – might be partly driven by irregular fungal distributions, combined with high mycorrhizal specificities. One approach to determine the role of mycorrhizal fungi in the diversification of orchid species consists of studying hybrid zones as they provide the natural laboratories in which to analyze the evolutionary processes that influence the interactions between divergent populations. Hybridization occurs frequently within the orchid family whenever two species occur in sympatry.The general aim of this research is to determine the identity and specificity of the mycorrhizae associated to different orchid species and to investigate whether mycorrhizal fungi affect the formation of stable hybrid zones. More specific aims are to:develop the DNA array technique for the detection and identification of orchid mycorrhizal fungi; use this technique to determine mycorrhizal specificity of orchid species; characterize hybrid zones of several orchid species using DNA markers and morphological measurements; identify the mycorrhizae associated to hybrid zones using the DNA array technique; and use specific seed-sowing techniques to determine whether infection of orchid seeds obtained from experimental crossings is related to crossing