5.4
The Evolution of Floral Resin Glands Related to Pollination
Resin glands that serve in support of pollination, until now, have only been
documented for two orders of angiosperms, Alismatales in monocots and
Malpighiales in core eudicots. Evolutionary studies focusing on the floral resin
glands of Araceae (monocots) are lacking; however, Armbruster [273] suggested
some hypotheses about how this evolution occurred in Malpighiales. The first
hypothesis presumes that the resins in Dalechampia (Euphorbiaceae) and Clusia
(Clusiaceae) may have originated as a system of defense against insect attacks. That
author also hypothesizes that bees may have been attracted to resiniferous flowers (or
inflorescences) by chance and afterward, through coevolution, the flowers increased
the amount of resin secreted concomitantly with the raise of resin utilization by bees.
Finally, he also suggests that the resin has originated suddenly, without a previous
function.
Phylogenetic studies of Dalechampia have lead to the hypothesis that early
divergent species of the genus were pollinated by male euglossine bees, which
collected fragrances, or by bees that collected pollen [286]. The shift from a pollen
or fragrance reward to resin may have occurred because those species apparently
produced resin through both the sepals of the pistillate flower and the protective
bracts of the male inflorescence; thus the resin secretion in Dalechampia most
likely originated as a mechanism of defense and, secondarily, took on a reward
function [286]. In contrast, the presence of isoprenylated benzophenones in the resin
and the exudate present in the secretory ducts of all other vegetative organs of Clusia
suggests that this resin has the same composition or the composition is very similar
in the different organs and that both secretory ducts have defense functions in the
plant. Later, bees started to inflict injuries to the plant to collect the exudate for use in
the construction of their nests. Therefore, the reward feature of flowers to pollinators
may have arisen from wounded flowers and, consequently, leading to the reproductive success of some species [273, 287, 288].
6
Conclusions
Angiosperm flowers reached an extraordinary degree of morphological diversity
in the different plant lineages whose evolution may have occurred in synchrony
with the evolution of pollinators or may have been subject to adaptive factors
through time. In order to better understand the interrelationships of floral evolution,
one will need to consider the various morphological modifications and developmental constraints in the particular phylogenetic contexts in which flowers originated.
Regardless of the factors involved in this diversification, the mechanisms of attraction and reward of pollinators played a key role in ensuring the species’ perpetuation
and reproductive success in different environments. The floral glands are fundamental in these two processes. Although the types of floral glands are different depending
on the group, their structure, ontogenesis, and secretory activity are directly related
28 Diversity of Floral Glands and Their Secretions in Pollinator Attraction
739
The Evolution of Floral Resin Glands Related to Pollination
Resin glands that serve in support of pollination, until now, have only been
documented for two orders of angiosperms, Alismatales in monocots and
Malpighiales in core eudicots. Evolutionary studies focusing on the floral resin
glands of Araceae (monocots) are lacking; however, Armbruster [273] suggested
some hypotheses about how this evolution occurred in Malpighiales. The first
hypothesis presumes that the resins in Dalechampia (Euphorbiaceae) and Clusia
(Clusiaceae) may have originated as a system of defense against insect attacks. That
author also hypothesizes that bees may have been attracted to resiniferous flowers (or
inflorescences) by chance and afterward, through coevolution, the flowers increased
the amount of resin secreted concomitantly with the raise of resin utilization by bees.
Finally, he also suggests that the resin has originated suddenly, without a previous
function.
Phylogenetic studies of Dalechampia have lead to the hypothesis that early
divergent species of the genus were pollinated by male euglossine bees, which
collected fragrances, or by bees that collected pollen [286]. The shift from a pollen
or fragrance reward to resin may have occurred because those species apparently
produced resin through both the sepals of the pistillate flower and the protective
bracts of the male inflorescence; thus the resin secretion in Dalechampia most
likely originated as a mechanism of defense and, secondarily, took on a reward
function [286]. In contrast, the presence of isoprenylated benzophenones in the resin
and the exudate present in the secretory ducts of all other vegetative organs of Clusia
suggests that this resin has the same composition or the composition is very similar
in the different organs and that both secretory ducts have defense functions in the
plant. Later, bees started to inflict injuries to the plant to collect the exudate for use in
the construction of their nests. Therefore, the reward feature of flowers to pollinators
may have arisen from wounded flowers and, consequently, leading to the reproductive success of some species [273, 287, 288].
6
Conclusions
Angiosperm flowers reached an extraordinary degree of morphological diversity
in the different plant lineages whose evolution may have occurred in synchrony
with the evolution of pollinators or may have been subject to adaptive factors
through time. In order to better understand the interrelationships of floral evolution,
one will need to consider the various morphological modifications and developmental constraints in the particular phylogenetic contexts in which flowers originated.
Regardless of the factors involved in this diversification, the mechanisms of attraction and reward of pollinators played a key role in ensuring the species’ perpetuation
and reproductive success in different environments. The floral glands are fundamental in these two processes. Although the types of floral glands are different depending
on the group, their structure, ontogenesis, and secretory activity are directly related
28 Diversity of Floral Glands and Their Secretions in Pollinator Attraction
739