Biology Reference
In-Depth Information
Relationships between Ornamentation Type and Pollination
The reason why Angiosperms display such a large diversity of pollen ornamenta-
tion remains to date rather unclear. Several studies addressing this question have
been produced, often leading to conflicting, or at least different, conclusions.
Most studies however examine the hypothesis of a link between pollen ornamen-
tation and the pollination system. The underlying idea is that since pollen grains
need a vector (biotic or abiotic) to reach the female parts, the pollen surface may
play a role in the efficiency of the interaction either with the pollination agent or
with the receptive area of the female organs. A relationship between abiotic pol-
lination (wind and water) and smooth (or nearly) pollen grains has been proposed
in many studies [1,22] but concerning biotic pollination, the results are more con-
troversial. [2,3,23,24]. According to previously published studies [3,20,24], our
results suggest that the relationship between pollen ornamentation and pollina-
tors may actually depend on the taxon. The association between psilate (=smooth)
pollen and beetles seems to be rather specific to the Araceae, since entomogamous
species are generally thought to produce pollen grains with a deeply sculptured
exine [1,23]. The idea is that the sculptures would enhance the adherence of the
pollen grains to the insect body by allowing a better storage of the pollenkitt. This
sticky substance, of which functions are not yet quite understood, is produced
by entomogamous species and stored on the surface of the pollen wall [25]. The
pollenkitt would enable pollen grains to adhere on the hairs of insects or on the
feathers of birds in case of ornithophily [23,24]. In the Araceae however, pollen
grains were depicted as poor in pollenkitt [26] and it was suggested that sticky
secretions on the stigma and/or the inner spathe surface may play the same role
as the pollenkitt [27], accounting for the lack of pollen ornamentation in beetle
pollinated species of Araceae.
The fact that no correlations could be detected in the palm family may be due
to various reasons. First, it has to be stressed that the sampling was sparser for
palms than for the Araceae. In particular, there was a poor overlap at the species
level between the pollen and the pollinator datasets. This led us to combine in-
formation from different species for the ornamentation type, which we are aware
may be questionable considering the high lability of the character even at the
intraspecific level. We tried to overcome this problem by attributing polymorphic
character states whenever intrageneric diversity was recorded, and by applying
two different treatments to polymorphic species in the comparative analyses but
we cannot exclude that the choices made here (due to the scarcity of data concern-
ing pollination systems in palms) may have biased the results. However, there is
a possibility that our results indeed reflect the reality and that pollen ornamen-
tation is not involved in the pollination syndrome in palms. In this family, the
traits linked to pollinator identity still remain almost unknown. Palm flowers are
