Discussion
Our investigations demonstrated that the species H. mutabilisshowed a mixed mating system, and its sexual reproduction depended on
pollinators. The floral reward (nectar) of H. mutabilis was
consumed by various visitors, including four bee species and one moth
species. The pollination efficiency of the moth species was relatively
lower than all bee species. The body length of five pollinator species
increased the amount of transferred pollen, and larger bees tended to
remove more pollen from anthers and deposit more pollen on stigmas,
resulting in higher pollination efficiency. However, there was no
correlation between the intertegular distance and pollination efficiency
of visitors in H.
mutabilis . Body length may be a better index of body size than
intertegular distance.
The nectar ofH.
mutabilis was not displayed at the base of the petals but at the base
of the ovary. So the flower structure of H. mutabilis shielded
visitors with shorter proboscis.
The best match between the length of the
proboscis and the circumference
of the nectar secretion at the base of the flower is the M.
pyrrhosticta , which suggests the M. pyrrhosticta should be the
most important pollinator in its native habitat (Fig. 5A). However, the
investigation results found that bees were still important pollinators
of H. mutabilis . Although the proboscis length does not match the
circumference of the basal nectar, they can suck in nectar from the gap
between the front petals. Different proboscis lengths of bees could
absorb different amounts of nectar, which can improve the chances of
pollen removal and pollen deposition.
Although the moth speciesM. pyrrhosticta had the
longest body length among the five visitor species, its
pollination
efficiency was relatively lower than all bees. The interpretation of the
relationship between body size and pollination efficiency may depend on
the influence of other morphological traits of M. pyrrhosticta .
Unlike bees, the moth without hairiness may carry a few pollen grains.
Moreover, the moth has a long proboscis so that they can get nectar
easily without entering deep into the corolla of flowers. In this case,
the moth may have little contact with floral sex organs. In terms of
four bee species, the body length of X. appendiculata was 1.8×
longer than that of honey bee A. mellifera (Fig. 5B). Moreover,
the pollen removal and pollen deposition of X. appendiculata were
1.9× and 2.4× more than that of A. mellifera(Fig. 6A and B), and the pollination
efficiency of X. appendiculata was 1.2× more than that ofA. mellifera (Fig. 6C). These results demonstrated that larger
bees removed and deposited more pollen than smaller bees in a single
visit, and larger bees were more
efficient pollinators in H. mutabilis . The reason was that larger
bees were more likely to come into contact with floral sex organs. This
finding is partly consistent with a previous study, which indicated that
larger bee species deposited more pollen grains onto stigmas than
smaller bees in Brassica napus (Chang et al . 2023), but
they did not consider the effect of body size on pollen removal and
pollination efficiency. Besides, we found that the intertegular distance
was not an effective index of body size, because there was no
correlation between the intertegular distance and pollination efficiency
of visitors in H. mutabilis . Only the moth species was excluded,
the intertegular distance of four bee species was significantly and
positively correlated to pollen removal and pollen deposition. However,
there were always significant positive correlations between the body
length of four or five pollinator species and pollen removal, pollen
deposition, and pollination efficiency. We concluded that body length
may be a better index of body size than intertegular distance.
The visitation rate of bees is another important behavioral parameter.
Honey bee A. mellifera was the most frequent visitor to flowers
of H. mutabilis . The visitation rate of A. mellifera was
5.1× more than that of X. appendiculata , even though the
pollination efficiency of A. mellifera was relatively lower. The
high visitation rate of bees to flowers may increase the pollinator’s
interaction with the stigma or pollen transfer, so A. melliferawas also a very efficient pollinator in H. mutabilis . Although
the pollination efficiency of X. appendiculata was higher than
honey bees, they visited fewer flowers than honey bees. Therefore, it is
difficult to comprehensively assess which insect species contributes
more pollination service to flowers in H. mutabilis , and further
study is needed to investigate the pollination service of various
pollinators to flowers throughout the flowering period of plants.
The flowers of H. mutabilis had a generalist pollination system
and attracted various pollinator species by providing plenty of nectar
rewards. Although H. mutabilis was highly self-compatible, there
was pollen limitation under natural conditions. The sexual reproduction
of H. mutabilis depended on pollinators. The visitation rate and
pollination efficiency of various pollinator species were greatly
different. The species H. mutabilis with a mixed mating system
may be an adaptation to variable pollinator visiting.