Diversity of Tulasnellaceae mycorrhizal associations of Australian terrestrial orchids
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2021
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Arifin, Arild
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Orchid mycorrhizal fungi (OMF) is essential for orchid seed germination and survival due to the tiny size and lack of endosperm of orchid seeds. Studying the fungal relationships in orchids are important as they provide insights into understanding fungal biodiversity and ecology. My first chapter is a study of the OMF associations in the Australian Cryptostylis orchids, which are sexually deceptive with several unusual features in relation to pollinator sharing and a mix of evergreen and leafless species. This chapter investigates the diversity of Tulasnella in Cryptostylis, finding that the five Australian Cryptostylis species associate with nine Tulasnella operational taxonomic units (OTUs)/species and an additional three Asiatic Cryptostylis associate with four Tulasnella OTUs. Interestingly, all the Tulasnella fungi are closely related despite the geographical distance among the orchid hosts. Furthermore, a small number of endophytic and ectomycorrhizal fungi also associate with Cryptostylis species and is not restricted to the putative mycoheterotrophic species, meaning the phenomena of also associating with ectomycorrhizal or endophytic fungi is not related with the orchid's trophic status and unusual in orchids.
My second chapter is exploring whether closely related orchids also have closely related mycorrhizal fungi. To address this, we studied the mycorrhizal associations of sexually-deceptive orchids in the subtribes Drakaeinae and Cryptostylidinae. Drakaeinae and Cryptostylidinae orchids associate with 20 closely related fungal OTUs/species, with four of them were shared between two orchid subtribes. Cophylogenetic analysis between Drakaeinae orchids and Tulasnella fungi shows both phylogenies are congruent, but no congruency between Cryptostylis and Tulasnella. The significant congruency between Drakaeinae and Tulasnella symbionts suggests a pattern of phylogenetic niche conservatism rather than coevolution since fungi can grow independently of orchids. Rapid diversification in Chiloglottis and Drakaea but not in Cryptostylis may reveal the different patterns of congruency in both subtribes.
While exploring Tulasnella from Cryptostylidinae as well as Drakaeinae, several undescribed species of Tulasnella were discovered. In Chapter Three we use multiple sequence locus phylogenetic analyses combined with morphological characteristics to delimit and describe six new Tulasnella species associated with orchids from the subtribes Cryptostylidinae and Drakaeinae. Five of the new species, Tulasnella australiensis, T. occidentalis, T. punctata, T. densa and T. concentrica, all associate with Cryptostylidinae whereas T. rosea associates with Spiculaea ciliata (Drakaeinae). Despite the orchid hosts being distantly related they share phylogenetically closely related Tulasnella species with similar macro and micromorphological features, with one of the most distinctive features of having binucleate hyphal compartments.
In Chapter Four, I describe a further two Tulasnella species from Rimacola elliptica and Pyrorchis nigricans, both orchids are members of subtribe Megastylidinae. These two fungal species belong to a two distantly related Tulasnella clades, which are also distantly related to those from Drakaeinae and Cryptostylidinae orchids as described in Chapter Three. One new Tulasnella species from Megastylidinae is characterised by multinucleate cell characteristics. This is the first report of a multinucleate Tulasnella, with only binucleate species reported, therefore this multinucleate species may harbour genetically diverse nuclei, making it more adaptable to e.g. environmental conditions, a subject worth further exploration. By delimiting and formally describing these Tulasnella species in Chapters Three and Four, it significantly contributes to the documentation of Tulasnella diversity and provides names and delimitations to underpin further research on the fungi and their relationships with orchids.
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