Discordance Down Under: Combining phylogenomics & fungal symbioses to detangle difficult nodes in a diverse tribe of Australian terrestrial orchids

Orchid mycorrhizal fungi (OMF) associations in the Orchidaceae are thought to have been a major driver of diversification in the family. In the terrestrial orchid tribe Diurideae, it has long been hypothesised that OMF symbiont associations may reflect evolutionary relationships among orchid hosts. Given that recent phylogenomic efforts have been unable to fully resolve relationships among subtribes in the Diurideae, we sought to ascertain whether orchid OMF preferences may lend support to certain phylogenetic hypotheses. First, we used phylogenomic methods and Bayesian divergence time estimation to produce a genus-level tree for the Diurideae. Next, we synthesised decades of published fungal sequences and morphological/germination data to identify dominant fungal partners at the genus scale and perform ancestral state reconstruction to estimate the evolutionary trajectory of fungal symbiont shifts. Across the tribe, we found phylogenomic discordance stemming from incomplete lineage sorting. However, our results also revealed unprecedented phylogenetic niche conservatism of fungal symbionts within the tribe: entire genera, subtribes, and even groups of related subtribes associate with only a single fungal family, suggesting that fungal symbiont preferences in the Diurideae do indeed reflect phylogenetic relationships among orchid hosts. Moreover, we show that these relationships have evolved directionally from generalist associations with multiple fungal families towards more specific partnerships with only one fungal family. Orchid symbiont preferences here provide new insights into the placement of several groups with longstanding phylogenetic uncertainty. In spite of complex evolutionary histories, host-symbiont relationships can be used to help detangle alternative phylogenetic hypotheses.