Discussion
Surprisingly, in our study heterospecific pollen interference did not
affect seed set nor seed number, i.e. it did not affect whether a flower
would produce or not at least one seed nor the amount of seeds produced.
Rare species had a tendency to produce more seeds, but this trend was
not significant. Breeding system did affect seed set, but not in
relationship with conspecific or heterospecific pollen treatments, with
self-incompatible species less likely to set seed compared to
self-compatible species. Lastly, we could show that for rare recipients
treated with pollen from rare donors, more distantly related
recipient-donor species pairs had a lower reduction in seed number
compared to closely related recipient-donor species pairs. Hereafter we
discuss these results as well as potential ecological and evolutionary
implication.
In a co-flowering community, we can expect common species to receive
pollen from a rare species unfrequently, thus an adaptation to that type
of pollen is unlikely. On the other hand, a rare species is likely to
receive frequently pollen from common species, thus making an adaptation
to heterospecific pollen receipt more likely, like predicted by the
tolerance-hypothesis (Hao et al., 2023). Indeed, in a study by
Arceo-Gomez and colleagues (Arceo-Gómez et al., 2016) the authors showed
how HP tolerance for a Clarkia species did depend on previous exposure
of the population to HP, but rather than acting on the recipient
individual, would act on the donor individual, by improving CP
performance. On the other hand, such adaptation was not observed for a
congeneric Clarkia species, suggesting that adaptation is context- and
species-specific. Adaptation could explain the low effect of HP overall
in our study species. On the other hand, despite our study species do
co-occur and co-flower in nature, the seed-material did not consistently
originate from populations co-occurring at local scale, but co-occurring
only at regional scale, thus missing potential adaptations at the
population level. Further, the (co-occurrence) history of the
populations from which the seed material was collected is unknown.
Another factor we analyzed is the evolutionary relatedness, measured as
phylogenetic distance, between recipient and donor species in
interaction with recipient and donor status. While overall no pattern
emerged, we could show a decrease of HPI for more distantly related
recipient-donor pairs when both were rare. The likely reason for the
absence of these patterns for common recipients and rare recipients with
common donors could be the lack of close relatives for these groups in
our study species set. Indeed, the range for these groups included only
phylogenetic distances larger than 189 * 106 years.
For closely related recipient-donor pairs, a stronger HPI could be
caused by similar recognition systems between recipient stigma and donor
pollen grains. For a better understanding of these patterns, a study
species set with a broader range should be used.
In this study we looked only at pairwise HP interactions, while in a
plant community it is likely to have multi-species mixes of HP that are
transferred between flowers. Arceo-Gomez (T. Ashman & Arceo-Gómez,
2011) performed HP hand-pollinations with mixes up to three species and
showed that HPI increased with the number of heterospecific pollen
donors. Further, the strength depended on specific species composition.
Indeed, in our study, HPI varied considerably among different
species-pairs. Some species are known to produce strongly allelopathic
pollen (Kanchan & Chandra, 1980), but in our study it did not look like
a specific species had a consistent negative effect on all other species
in terms of HPI (see Figure S 9).
For our species, self-incompatible species showed an overall lower seed
set, while breeding system did not affect seed number or HPI in any way.
Self-compatibility might play a role especially in a natural community,
where HP can act through the mentor effect (de Nettancourt, 1997), and
allowing for self-fertilization even in self-incompatible flowers, with
consequent ovule abortion (Lynn, Sullivan, & Galen, 2022). In our study
we showed how self-incompatible species are less likely to produce seeds
even when enough pollen is present. We emasculated our recipient species
whenever possible prior to treatment, but due to the small flower size,
three out of eight species were left with their anthers to avoid
complete flower abortion. These three species (Bupleurum
rotundifolium , Fallopia convolvulus and Myosotis
arvensis ) are all self-compatible. Thus, one explanation could be that
seed set induced by selfing is more secure compared to seed set from
outcrossed pollen, despite the genetic advantages of outbreeding.
Specific flower morphology and in particular a smaller stigma size in
restrictive flowers (i.e. flowers with a reduced access to the flower
interior) have been shown to reduce HP deposition, while at the same
time increasing CP deposition (Montgomery & Rathcke, 2012). In our
study, we did not analyze the effect of flower morphology or flower
traits, since due to our small sample size in species number (eight
species in total), species and trait would be confounded. In a natural
community, flower morphology would also play an important role in terms
of pollinator sharing and flower constancy (the tendency of pollinators
to forage on the same flower type (Waser, 1986)) since some flowers are
adapted to specific groups of pollinators and thus sharing among these
species is more likely. For example, both Ajuga chamaepitys andFallopia convolvulus , being lip flowers, rely on bumblebees as
their most common pollinators (Kuehn et al., 2004).
While in our study we did not find any strong effect of HP on seed set
and seed number, HPI remains an important aspect of co-flowering
communities (T. L. Ashman & Arceo-Gómez, 2013), since it allows species
to affect other species without direct competition and at a distance
above the direct interactions. In a co-flowering community; we can
expect a variable and complex pollination landscape that could promote
evolution of flower morphology and avoidance mechanism also due to HPI.
It seems that while mechanisms as HPI and adaptations to it do play a
role in shaping plant communities, this patterns are highly variable
depending on the context and on the species observed. We conclude that
heterospecific pollen interference plays a minor role for rare plant
species. Rather, other factors, like pollen limitation mediated by low
pollinator visitation rates, are likely to affect rare plant species at
the level of interactions. Adaptation and species-specific interactions
may explain the low overall effect of HPI in our study. The complexity
of multi-species interactions and the specific composition of
heterospecific pollen mixes may further influence the strength of HPI.
Additional research is needed to explore these factors and their
implications for both in-sit and ex-situ conservation strategies.