American Society of Naturalists

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“Transitions between the terrestrial and epiphytic habit drove the evolution of seed-aerodynamic traits in orchids”

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Xu-Li Fan, Guillaume Chomicki, Kai Hao, Qiang Liu, Ying-Ze Xiong, Susanne S. Renner, Jiang-Yun Gao, and Shuang-Quan Huang (Feb 2020)

Orchidaceae have repeatedly evolved large seed air spaces, and this correlates with being terrestrial, not epiphytic

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A selection of orchid seeds, including a seed of the terrestrial genus <i>Apostasia</i>, which is sister to all remaining orchids. SEM photos of the seeds of all 121 species included in the phylogenetic trait analyses are shown as online supporting material. <br />(Photos by Xu-Li Fan)
A selection of orchid seeds, including a seed of the terrestrial genus Apostasia, which is sister to all remaining orchids. SEM photos of the seeds of all 121 species included in the phylogenetic trait analyses are shown as online supporting material.
(Photos by Xu-Li Fan)

The height at which seeds are released is a critical parameter determining the efficiency of seed dispersal by wind. Does this apply to the minute seeds of orchids? We addressed this question with phylogenetically-controlled analyses of 20 seed traits in 121 terrestrial and epiphytic species (from 63 genera), spanning the orchid family. Close-up images of all studied seeds are included with our paper. It turns out that seed air space is closely correlated with the terrestrial or epiphytic habit, which each evolved several times, including returns from the epiphytic to the terrestrial habit. This suggests that aerodynamic traits are under strong selection to increase dispersal ability even in orchid seeds.


Abstract

Orchids are globally distributed, a feature often attributed to their tiny dust-like seeds. They were ancestrally terrestrial, but in the Eocene expanded into tree canopies, with some lineages later returning to the ground, providing an evolutionarily replicated system. Because seeds are released closer to the ground in terrestrial species than in epiphytic ones, seed traits in terrestrials may have been under selective pressure to increase seed dispersal efficiency. In this study, we test the expectations that (i) seed air space –a trait known to increase seed floatation time in the air– is larger in terrestrial lineages and (ii) has increased following secondary returns to a terrestrial habit. We quantified and scored 20 seed traits in 121 species and carried out phylogenetically-informed analyses. Results strongly support both expectations, suggesting that aerodynamics traits even in dust seeds are under selection to increase dispersal ability, following shifts in average release heights correlated with changes in habit.