Oil-offering flowers evolved independently at least 28 times in 11 families of
angiosperms [221]. In most of these families, floral lipids occur in single lineages,
while in Orchidaceae and Iridaceae, elaiophores developed independently several
times during evolution [221]. In Orchidaceae, Renner and Schaefer [221] recorded
12 independent transitions to elaiophores, suggesting that there could be many more
such transitions in that family. In Iridaceae, there is indication of four independent
transitions to elaiophores and many reversions [221, 233, 263, 264]. Reversions
to an absence of elaiophores have been identified in various families and may
have occurred 36–40 times in angiosperms [221], exceptions being Stilbaceae and
Malpighiaceae and the genus Krameria (Krameriaceae) and Calceolaria
(Calceolariaceae) where the presence of elaiophores is probably common to all
members [223, 239, 265].
Oil-collecting bees display specific behavior and morphological adaptations, and this
evolved independently at least six times: in Centridini, Tapinostapidini, Tetrapediini,
Ctenoplectra, Macropis, and Rediviva [221, 262, 266]. However, it is difficult to
estimate the number of total transitions because some genera are nonmonophyletic
and estimations based on the behavior of oil-collecting bees indicate that gains and
losses are probably equal, depending on the adopted scenario [221, 267]. Clearly,
besides the behavioral feature of collecting floral oils, there are morphological relationships between bees and angiosperms. For example, species of Epicharis and some
species of Centris have specialized morphological adaptations for collecting floral oil
from Malpighiaceae [216, 260].
The interactions between plants and pollinators are at the origin of many events
of speciation and coevolution. These interactions are probably responsible for
most of the diversity within angiosperms. On the other hand, very specific interactions, characterized by a high degree of reciprocal evolution, are rare in most
ecosystems [254]. Thus, the availability of such highly specialized floral resources,
as lipids, can drastically reduce the diversity of floral visitors and pollinators
and consequently have a negative effect on the plants’ ability to expand and
diversify. Bees, however, rarely collect rewards exclusively from a single species
and normally obtain sources from a broad variety of different species [218, 220].
In addition to the specialized morphological adaptations needed to collect this type
of resource, specific behaviors are also necessary that generate adaptive costs. This
could explain why this specialized relationship between bees and flowers with floral
oils is fairly rare and could be the reason for the many reversals [221].
5
Resin Glands Related to Pollination
5.1
Resins Produced by Flowers and Inflorescences
Resins are complex mixtures of substances composed of liposoluble volatiles and
nonvolatile terpenoids and/or other secondary metabolites, such as phenolic compounds, that are secreted by specialized structures inside or on the surface of the
plant [268, 269]. They are commonly found in vegetative organs but have rarely
28 Diversity of Floral Glands and Their Secretions in Pollinator Attraction
735
angiosperms [221]. In most of these families, floral lipids occur in single lineages,
while in Orchidaceae and Iridaceae, elaiophores developed independently several
times during evolution [221]. In Orchidaceae, Renner and Schaefer [221] recorded
12 independent transitions to elaiophores, suggesting that there could be many more
such transitions in that family. In Iridaceae, there is indication of four independent
transitions to elaiophores and many reversions [221, 233, 263, 264]. Reversions
to an absence of elaiophores have been identified in various families and may
have occurred 36–40 times in angiosperms [221], exceptions being Stilbaceae and
Malpighiaceae and the genus Krameria (Krameriaceae) and Calceolaria
(Calceolariaceae) where the presence of elaiophores is probably common to all
members [223, 239, 265].
Oil-collecting bees display specific behavior and morphological adaptations, and this
evolved independently at least six times: in Centridini, Tapinostapidini, Tetrapediini,
Ctenoplectra, Macropis, and Rediviva [221, 262, 266]. However, it is difficult to
estimate the number of total transitions because some genera are nonmonophyletic
and estimations based on the behavior of oil-collecting bees indicate that gains and
losses are probably equal, depending on the adopted scenario [221, 267]. Clearly,
besides the behavioral feature of collecting floral oils, there are morphological relationships between bees and angiosperms. For example, species of Epicharis and some
species of Centris have specialized morphological adaptations for collecting floral oil
from Malpighiaceae [216, 260].
The interactions between plants and pollinators are at the origin of many events
of speciation and coevolution. These interactions are probably responsible for
most of the diversity within angiosperms. On the other hand, very specific interactions, characterized by a high degree of reciprocal evolution, are rare in most
ecosystems [254]. Thus, the availability of such highly specialized floral resources,
as lipids, can drastically reduce the diversity of floral visitors and pollinators
and consequently have a negative effect on the plants’ ability to expand and
diversify. Bees, however, rarely collect rewards exclusively from a single species
and normally obtain sources from a broad variety of different species [218, 220].
In addition to the specialized morphological adaptations needed to collect this type
of resource, specific behaviors are also necessary that generate adaptive costs. This
could explain why this specialized relationship between bees and flowers with floral
oils is fairly rare and could be the reason for the many reversals [221].
5
Resin Glands Related to Pollination
5.1
Resins Produced by Flowers and Inflorescences
Resins are complex mixtures of substances composed of liposoluble volatiles and
nonvolatile terpenoids and/or other secondary metabolites, such as phenolic compounds, that are secreted by specialized structures inside or on the surface of the
plant [268, 269]. They are commonly found in vegetative organs but have rarely
28 Diversity of Floral Glands and Their Secretions in Pollinator Attraction
735