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Insects, not just wind, offer an ancient mechanism of orchid seed dispersal

This orchid enlists an unusual ally to spread its seeds. Ancient orchids may have done the same. Image credit:  Kenji Suetsugu

This orchid enlists an unusual ally to spread its seeds. Ancient orchids may have done the same. Image credit: Kenji Suetsugu

On Yakushima Island, at the southern tip of Japan, an orchid employs a very unusual strategy to disperse its seeds. Crickets visit the orchid at night, eat its fruits, and defecate the seeds in the vicinity. The discovery, reported recently in Evolution Letters, is a rare case of seed dispersal by orthopteran insects. More surprising, the finding is in orchids, which almost always spread their seeds on the wind, notes author Kenji Suetsugu, an evolutionary ecologist at Kobe University in Hyogo Prefecture, Japan.

“It had previously been assumed that wind dispersal was the norm for most, if not all, of the orchids,” says ecologist Jeff Ollerton at the University of Northampton in the United Kingdom, who was not involved in the recent work. He adds that the finding should inspire researchers to “rethink our understanding” of orchid seed dispersal.

The majority of living orchids today are epiphytes that cling to trees and vines. Their seeds are tiny, fragile, and dust-like. A light breeze can carry them aloft, from the interior fluff of a burst seed pod onto surrounding tree trunks and branches.

However, there are exceptions. One is the subject of the recent study: Apostasia nipponica or “Yakushima-ran” in Japanese. It belongs to an ancient orchid subfamily, the Apostasioideae, which evolved before epiphytic orchids with their telltale flower symmetry. Compared to those epiphytic orchids sold at hardware and grocery stores, A. nipponica looks more like a weed, with small, yellow star-shaped flowers. It grows on the ground and sets fleshy fruits that researchers previously thought were spread by rain and decay. But some anecdotal reports had hinted that animals ate the fruit of A. nipponica on Yakushima Island. If that were true, then perhaps ancestral orchids used a similar animal-mediated seed dispersal strategy, Suetsugu thought.

Beginning in 2015, he traveled to Yakushima to see what animals visited fruiting A. nipponica during the day. Finding little, Suetsugu concluded that any fruit consumption must be happening at night. He returned to the island in the summer of 2019 with a digital camera and filmed nocturnal activity at a study site of 10 fruiting orchids. This time Suetsugu observed both the cricket, Eulandrevus ivani, and the camel cricket, Diestrammena yakumontana, eating orchid fruit. He collected a few of the insects and counted the number of intact seeds in their droppings under a microscope. A chemical solution revealed ongoing cellular respiration in the intact seeds, confirming their viability even after a voyage through the gut. Suetsugu did not observe any decaying fruit, excluding the possibility of water dispersal, he adds.

Orchid seeds are so small and fragile that they haven’t been well-preserved in the fossil record. Hence, to the frustration of botanists, orchid evolutionary history remains a mystery, says plant ecologist Shumpei Kitamura, at Ishikawa Prefectural University in Nonoichi, Japan. Kitamura, who was not involved in the recent study, says there are only a few examples of animals dispersing orchid seeds, including birds and camel crickets. Both examples come from orchids thought to have evolved from wind-dispersed ancestors, he adds. But this latest example from the basalmost orchid subfamily hints that animal-mediated dispersal could actually be older in orchids, or at least among the oldest mechanisms.

However, this one study is probably not enough to confirm that animals are the original orchid seed spreaders, says Guillaume Chomicki, an evolutionary biologist at Durham University in England who studies the aerodynamics of seed dispersal in terrestrial versus epiphytic orchids.

“It’s impossible to say,” he notes, because this study only examined one orchid species. Chomicki, who lauds the recent work for offering a rare window into insect-mediated seed dispersal, would like to see future studies compare seed dispersal strategies across the Apostasioideae and evolutionarily younger orchids.

“It has long been thought that the orchids were wind dispersed, including ancestrally,” Chomicki notes. Discovering another strategy is actually older, he says, “would be quite something.”

Categories: Ecology | Evolution and tagged | | | | | | |
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