Most species engage in broad interactions, in which they interact with multiple partner species. The evolutionary processes that generate such generalized interactions remain unknown, partly due to the difficulty in comparing their breadth. We argue that the interaction specificity of species involved in broad interactions evolves in three ways: (a) assemblage specialization, in which a species specializes on particular host species that contribute unique resources, yielding specialization on the entire host assemblage; (b) apparent generalism, in which a species specializes on one or few host species that contribute unique resources, but also associates with other host species that contribute functionally redundant resources; and (c) true generalism, in which a species associates with multiple hosts that overlap functionally, and that are geographically interchangeable based on opportunity for encounter, leading to frequent host switching. We performed a phylogenetically controlled analysis of data on mycorrhizal fungal associations for approximately 25% of the orchid subfamily Cypripedioideae to determine whether these plants have specialized on their mycorrhizal fungal communities, or whether they are true generalists. We also assessed the impact of environmental factors on these associations. Our results suggested strong support for apparent generalism, suggesting strong specialization on particular, dominant fungi and weak specialization on others. Large orchid clades associated with dominant fungal species, notably Tulasnella cystidiophora for genus Cypripedium, and T. cystidiophora and T. calospora for genus Paphiopedilum. Significant phylogenetic signal in fungal species richness per plant species, but not in the fungal phylogenetic diversity per plant species nor in the composition of fungal assemblages across orchid species suggested that plant phylogeny is an important determinant of fungal association. Mixed linear models showed that environment influenced specificity across plant species, and that observed differences were strongly driven by differences in sampling effort. Synthesis. We found evidence of specialization of plant species on dominant fungal species, and to a lesser extent on their close relatives. The strong dominance of particular fungal species in these associations suggests important ecological roles for them, while environmental gradients in specificity suggest strong environmental filtering of these interactions.
Orchidaceae (with >28,000 orchid species) are one of the two largest plant families, with economically and ecologically important species, and occupy global and diverse niches with primary distribution in rainforests. Among orchids, 70% grow on other plants as epiphytes; epiphytes contribute up to ~50% of the plant diversity in rainforests and provide food and shelter for diverse animals and microbes, thereby contributing to the health of these ecosystems. Orchids account for over two‐thirds of vascular epiphytes and provide an excellent model for studying evolution of epiphytism. Extensive phylogenetic studies of Orchidaceae and subgroups have ;been crucial for understanding relationships among many orchid lineages, although some uncertainties remain. For example, in the largest subfamily Epidendroideae with nearly all epiphytic orchids, relationships among some tribes and many subtribes are still controversial, hampering evolutionary analyses of epiphytism. Here we obtained 1,450 low‐copy nuclear genes from 610 orchid species, including 431 with newly generated transcriptomes, and used them for the reconstruction of robust Orchidaceae phylogenetic trees with highly supported placements of tribes and subtribes. We also provide generally well‐supported phylogenetic placements of 131 genera and 437 species that were not sampled by previous plastid and nuclear phylogenomic studies. Molecular clock analyses estimated the Orchidaceae origin at ~132 million years ago (Ma) and divergences of most subtribes from 52 to 29 Ma. Character reconstruction supports at least 14 parallel origins of epiphytism; one such origin was placed at the most recent common ancestor of ~95% of epiphytic orchids and linked to modern rainforests. Ten occurrences of rapid increase in the diversification rate were detected within Epidendroideae near and after the K‐Pg boundary, contributing to ~80% of the Orchidaceae diversity. This study provides a robust and the largest family‐wide Orchidaceae nuclear phylogenetic tree thus far and new insights into the evolution of epiphytism in vascular plants.
Premise The effective ex situ conservation of exceptional plants, whether in living collections or cryo‐collections, requires more resources than the conservation of other species. Because of their expertise with rare plants, botanical gardens are well positioned to lead this effort, but a well‐developed strategy requires a clear understanding of the resources needed. Methods Grant funding was obtained from the Institute of Museum and Library Services to support a three‐year project on cryobanking, and to provide smaller grants to 10 other botanical gardens for one‐year projects on either (1) seed behavior studies or (2) the development of protocols for in vitro propagation or cryopreservation. Results Nine of the partner gardens worked on 19 species (one was unable to continue due to the COVID‐19 pandemic), while the larger project focused on 14 species. A point system was developed for tasks accomplished, and the average costs per point of the larger and smaller projects were similar. Labor accounted for half the costs. Projects focused on species in the Asteraceae and Orchidaceae had lower costs per point than other species. Discussion Both large and small projects can contribute to a strategy for exceptional plant conservation for similar costs. Prioritizing species with lower costs could help advance the field while allowing time for work on more difficult species to develop.
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