1 | INTRODUCTION
Various seed and fruit traits such as size and color tend to be
correlated, forming what are known as “dispersal syndromes” that may
have arisen to attract particular dispersers (Brodie, 2017; Valenta and
Nevo, 2020) or may facilitate transport from their parent plants and
move by wind or currents (Nilsson et al., 1991; Bullock & Claeke, 2000;
Ohnishi et al., 2008; Nathan et al., 2011). Among seed dispersal by
animals, dispersal depending on ants is specifically called
myrmecochory. Myrmecochory is widespread among angiosperms and
ecologically important; over 11,000 species of myrmecochorous plants in
77 families and 334 genera, participate in myrmecochory across various
ecosystems (Lengyel et al., 2010). One of the evolutionary significance
of myrmecochory is considered as directed seed dispersal. The advantages
of the directed ant-dispersal hypothesis are that ant nests maintain
moist conditions and the seeds carried to the nest can escape
desiccation and have a higher seedling survival (Levey & Byrne, 1993),
and the ants bury the seeds at depths where humidity and temperature are
suitable for germination (Gibson, 1993).
Seeds of many myrmecochorous plants possess appendages, such as
elaiosomes containing various fatty acids and proteins. These are likely
to be nutritional rewards for ants who disperse the seeds (Brew et al.,
1989; Lanza et al., 1992). If the
seeds are carried to an ant nest, the elaiosomes will likely be consumed
inside the nest before the seeds are discarded (Culver & Beattie,
1978). Some granivorous ants, such as species of genera Messor,Pheidole, and Tetramorium , lose or abandon their seeds
during transportation and thus also disperse seeds without elaiosomes
(Retana et al., 2004; Kobayashi, 2009). Various studies have shown that
nutritional contents, such as lipids, amino acids, and proteins, in the
elaiosome are important for inducing the seed dispersal. In contrast,
how seeds without elaiosomes enable efficient dispersal by ants is less
investigated, although some chemical signaling might be involved
(Youngsteadt et al., 2008). It is also unclear whether the seeds are
discarded on suitable sites in a directed manner in species without
elaiosomes. In this study, we investigated seed dispersal without
specialized nutritional rewards to ants. We also tested the possibility
of directed seed dispersal by ants by examining whether abandonment of
dummy seeds is directionally biased towards suitable location using an
achlorophyllous plant whose seeds do not have elaiosomes.
We used the plant Monotropastrum humile (D. Don) H. Hara
(Ericaceae) and the ant Nylanderia flavipes Smith (Formicidae) to
test the hypothesis. M. humile is an achlorophyllous and
myco-heterotrophic plant that inhabits forest floors in the temperate
regions of Asia (Ohashi et al. 2016). The plant is about 10 cm height
and does not perform photosynthesis and parasitizes the fungi ofRussula and Lactarius species of Russulaceae (Bidartondo
and Bruns, 2001). M. humile is a full
myco-heterotrophic species
(Merckx, 2013). Seeds are smaller than 0.5 mm in length (Uehara &
Sugiura, 2017) and require their host fungi for germination, thus, seed
destination is a determinant of survival. Their host, the mycorrhizal
fungi of Russula species, inhabit the shallow areas of the forest
floor (Rachel, 2004; Courty et al., 2008), and Imamura and Kurogi (2003)
reported that M. humile roots inhabit the forest floor to a depth
of 5–10 cm. For seed dispersal, M. humile uses the insects
wandering on the ground when their berries fall to the ground during the
fruiting season. Cockroaches (Uehara & Sugiura, 2017) and camel
crickets (Suetsugu, 2017) have been reported as their seed dispersers.
According to these studies, the M. humile berries fall to the
ground during the fruiting season, the wandering insects are presumed to
consume pulpy flesh around the seeds and consequently contribute to the
dispersal of seeds involved in their foraging behavior.
Although M. humile seeds have no elaiosomes and seem to be
non-myrmecocohorous plants (Suetsugu, 2017; Uehara & Sugiura, 2017),
Suetsugu (2017) and Uehara and Sugiura (2017) reported that several ant
species, including N . flavipes , frequently visited theM. humile fruits. In addition, we have observed, by using a
fixed-point camera, that N. flavipes ants were attracted to the
fruits and seeds of M. humile and that a part of the seeds,
although not many, were certainly carried by them. Since the
seeds are coated with a phlegmatic organic layer, it is possible thatM. humile attract the ants with the fruit- and seed odors and
that their seed are carried by the ants. The seeds might be abandoned
after the outer components are consumed. Consequently, the seed could be
dispersed to the suitable, host-inhabited sites on the forest floor.
To validate directed seed dispersal by ants, it is necessary to track
where the seeds are carried to and discarded. However, the seeds of
myrmecochorous plants are small and difficult to track in the field.
Thus, at first, we performed the bioassay using intact and odorless
seeds to verify whether the seed odors of M. humile are key
signals for dispersal by ants. Afterwards, bioassays were performed
using small pieces of the paper coated with the odor component ofM. humile seeds as dummy seeds. We tested whether the dummy seeds
were carried into the nests and were carried out of the nests by the
ants.