An ancient association? Crickets disperse seeds of early-diverging
orchid Apostasia nipponica

Date:
August 10, 2020
Source:
Kobe University
Summary:
A professor presents evidence of the apparently unusual seed
dispersal system by crickets and camel crickets in Apostasia
nipponica (Apostasioideae), acknowledged as an early-diverging
lineage of Orchidaceae.



FULL STORY ========================================================================== Associate Professor SUETSUGU Kenji (Kobe University Graduate School of
Science) presents evidence of the apparently unusual seed dispersal system
by crickets and camel crickets in Apostasia nipponica (Apostasioideae), acknowledged as an early-diverging lineage of Orchidaceae. These findings
were published on August 11 in the online edition of Evolution Letters.


==========================================================================
Seed dispersal is a key evolutionary process and a central theme
in terrestrial plant ecology. Animal-mediated seed dispersal, most
frequently by birds and mammals, benefits seed plants by ensuring
efficient and directional transfer of seeds without relying on random
abiotic factors such as wind and water. Seed dispersal by animals is
generally a coevolved mutualistic relationship in which a plant surrounds
its seeds with an edible, nutritious fruit as a good food for animals that consume it . Birds and mammals are the most important seed dispersers,
but a wide variety of other animals, including turtles and fish, can
transport viable seeds. However, the importance of seed dispersal by invertebrates has received comparatively little attention. Therefore, discoveries of uncommon mechanisms of seed dispersal by invertebrates
such as wetas, beetles and slugs usually evoke public curiosity toward animal-plant mutualisms.

Unlike most plants, all of the >25,000 species of orchids are
heterotrophic in their early life history stages, obtaining resources
from fungi before the production of photosynthetic leaves. Orchid seeds, therefore, contain minimal energy reserves and are numerous and dust-like, which maximizes the chance of a successful encounter with fungi in the substrate. Despite considerable interest in the ways by which orchid
flowers are pollinated, little attention has been paid to how their
seeds are dispersed, owing to the dogma that wind dispersal is their predominant strategy. Orchid seeds are very small and extremely light,
and are produced in large numbers. These seeds do not possess an endosperm
but instead usually have large internal air spaces that allow them to
float in the air column. In addition, orchid seeds are usually winged or filiform, evolved to be potentially carried by air currents. Furthermore,
most orchid seeds have thin papery coats formed by a single layer of non-lignified dead cells. It has been thought that these fragile thin
seed coats cannot withstand the digestive fluids of animals, in contrast
to the thick seed coats in indehiscent fruits, which are considered an adaptation for endozoochory.

However, it is noteworthy that the subfamily Apostasioideae commonly has indehiscent fruits with hard, crustose black seed coats. Apostasioids
are the earliest-diverging subfamily of orchids and consist of only two
genera (Apostasia and Neuwiedia), with only ~20 species distributed in southeastern Asia, Japan, and northern Australia. All Apostasia and most Neuwiedia species investigated to date are known to possess berries with
hard seed coats.

Apostasioids are also well known for several unique traits, such as a non- resupinate flower with an actinomorphic perianth and pollen grains that do
not form pollinia. These have been considered ancestral characteristics
in orchids, given that they are similar to those found in the members of Hypoxidaceae (which is closely related to Orchidaceae) family. Similarly,
the presence of an indehiscent fruit with a thick seed coat, found in
most Apostasia and Neuwiedia species can be an ancestral trait in orchids.

Here Suetsugu has studied the Apostasia nipponica (Apostasioideae)
seed dispersal system in the forest understory of the warm-temperate
forests on Yakushima Island, Kagoshima Prefecture, Japan. Consequently, Suetsugu presents the evidence for seed dispersal by crickets and camel crickets in A. nipponica.

Similar results were obtained in different years, indicating that this interaction is likely stable, at least in the investigated site. It
probably constitutes a mutualism, wherein both partners benefit from the association - - orthopteran visitors obtain nutrients from the pulp and
A. nipponica achieves dispersal of seeds from the parent plants. The
seeds of A. nipponica are coated with lignified tissue that likely
protects the seeds as they pass through the digestive tract of crickets
and camel crickets. Although neither the cricket nor camel cricket
can fly, they potentially transport the seeds long distances owing to
their remarkable jumping abilities. Despite the traditional view that
the minute, dust-like, and wind-dispersed orchid seeds can travel long distances, both genetic and experimental research has indicated that
orchids have limited dispersal ability; orchid seeds often fall close
to the maternal plant (within a few meters), particularly in understory species. Given that A.

nipponica fruits are produced close to the ground in dark understory environments where the wind speed is low, seed dispersal by crickets is probably a successful strategy for this orchid.

Orchid seeds lack a definitive fossil record due to their extremely
minute size. Therefore, the interaction described here provides some
important clues as to the animals that may have participated in the
seed dispersal of the ancestors of orchids. Given that the origin
of crickets and camel crickets precedes the evolution of orchids,
they are among the candidates for seed dispersers of the ancestors of
extant orchids. Owing to many plesiomorphic characteristics and the earliest-diverging phylogenetic position, members of Apostasioideae have
been extensively studied to understand their floral structure, taxonomy, biogeography, and genome. However, there is still a lack of information regarding seed dispersal in the subfamily. Therefore, Suetsugu has
documented the animal-mediated seed dispersal of Apostasioideae members
for the first time. Whether seed dispersal by animals (and particularly
by orthopteran fruit feeders) is common in these orchids warrants
further investigation. It is possible that this method of dispersal is
an ancestral trait in Apostasioideae, given that indehiscent fruits with
a hard seed coat are common within the clade. Further research, such as
an ancestral character- state reconstruction analysis of more data on the
seed dispersal systems of other apostasioids, can provide deeper insights
into the early evolution of the seed dispersal system in Orchidaceae.


========================================================================== Story Source: Materials provided by Kobe_University. Note: Content may
be edited for style and length.


========================================================================== Journal Reference:
1. Kenji Suetsugu. A novel seed dispersal mode of Apostasia nipponica
could
provide some clues to the early evolution of the seed
dispersal system in Orchidaceae. Evolution Letters, 2020; DOI:
10.1002/evl3.188 ==========================================================================

Link to news story: https://www.sciencedaily.com/releases/2020/08/200810113202.htm

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