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Orchid mycorrhizal fungi are essential for orchid seed germination and survival due to the lack of endosperm in dust-like orchid seeds. I investigated the mycorrhizal associations in Cryptostylis and Drakaeinae orchids. Cryptostylis is an unusual orchid genus in Australia since species can be evergreen or leafless and all are sexually deceptive. Furthermore, all Australian Cryptostylis species use the same single pollinator, Lissopimpla excelsa. Leafless orchids, also known as mycoheterotrophic orchids, have a reduced capacity to photosynthesize and therefore rely on ectomycorrhizal fungi for carbon and nutrients. As a result, autotrophic and mycoheterotrophic orchids associate with a completely different suite of fungi. Surprisingly, the mycoheterotrophic Cryptostylis species examined, used the same Tulasnella fungi as the autotrophic species. Only a few cases of ectomycorrhizal fungi were found in the mycoheterotrophic species, but these were also found in the autotrophic species – a highly unusual phenomenon for autotrophic orchids. My second chapter explores whether closely-related orchids associate with closely-related mycorrhizal fungi in two orchid subtribes Drakaeinae and Cryptostylidinae. Cophylogenetic analyses between Drakaeinae orchids and Tulasnella fungi showed both phylogenies are congruent, suggesting coevolution of orchid mycorrhizal fungi and orchids. However, no significant congruency was detected in Cryptostylidinae and their Tulasnella fungi. Coevolutionary relationships are often encountered in e.g. host-parasite relationships driven by antagonistic interactions which lead to reciprocal adaptations. However, coevolution is an unexpected finding for orchid-fungal relationships as fungi are not dependent on the orchid, perhaps explaining the lack of cophylogeny between Cryptostylidinae and their orchids. The observed cophylogeny between the Drakaeinae and their fungi is surprising but perhaps because of a recent and rapid pollinator-driven diversification of certain Drakaeinae genera, entire genera associate with only one or two fungal species, resulting in a coevolutionary signal. Lastly, this study unearthed several undescribed Tulasnella fungi from Australian orchids, of which I am formally describing eight species using molecular phylogenetic analyses combined with morphological characteristics.