Elaiophore secretions are released to the surface of the gland by two main
pathways: granulocrine and eccrine [256]. The observation that vesicles and multivesicular bodies are mainly located close to the plasma membrane and the presence
of a large periplasmic space indicate that the secretion is likely released by the
granulocrine mechanism [256]. Recent studies also have shown that lipid release
occurs via secretory vesicles in most species [226, 227]. Although some studies have
reported the possible occurrence of eccrine secretion [256, 257], it is difficult to
prove the occurrence of this type of mechanism. Upon reaching the periplasmic
space of the cell, lipids cannot be transported by facilitated diffusion through the cell
wall, and the most likely hypothesis is that the protoplast exerts pressure on the
hydrophobic material accumulated in the periplasmic space and thereby forcing it to
cross the cell wall [23].
The accumulation of lipid secretions under the subcuticular space is commonly
observed both in trichomatous and in epidermal elaiophores [216, 230, 258]. In some
species, lipids are released only through cuticle rupture caused by contact with
pollinators [216, 231, 237]. In others, the secretion may permeate the cuticle through
microchannels, not forming subcuticular accumulation [231].
4.3
Lipids as a Specialized Reward to Pollinators
Floral oils are made available to highly specialized pollinators by only a
few angiosperms. Only bees collect floral oils, and of these only about 500 species,
i.e., less than 2% of global bee species, actively collect floral oils [216, 218, 259].
Oil-collecting bees belong to the tribes Ctenoplectrini, Centridini, Tapinostapidini,
and Tetrapediini (Apidae), and Macropidini and Redivivini (Melittidae) [259]. In
Tapinostapidini there are about 12 genera with 95 species collecting floral oils;
the largest group of oil-collecting bees (about 230 species) is in the genus Centris
(Centridini) [216, 221]. Oil-collecting bees have specific morphological adaptations
and behavior, for example, in Centris, where the oil-collecting apparatus is located
on the front pair of legs and the animals have soft and absorbent hairs [217, 260].
Floral lipids with their high nutritional value, as compared to the carbohydrates
of nectar [216–218, 259, 261], are mainly used as larval food by bees in combination
with pollen [216, 217] and possibly also in adult nutrition [218]; in addition, they
are used for the construction of nests [261].
4.4
Evolution of Oil-Offering Flowers
Studies on oil-offering flowers have mainly focused on collector observations, on
their morphology and on the distribution of elaiophores in flowers. Integrated studies
have shown that there are many independent interrelational transitions regarding the
presence of elaiophores in angiosperms and bee collector behavior [128, 216–221,
262]. Both the production of floral oils by angiosperms and the behavior of oil
collection by bees are polyphyletic [221, 262].
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E. D. Tölke et al.
pathways: granulocrine and eccrine [256]. The observation that vesicles and multivesicular bodies are mainly located close to the plasma membrane and the presence
of a large periplasmic space indicate that the secretion is likely released by the
granulocrine mechanism [256]. Recent studies also have shown that lipid release
occurs via secretory vesicles in most species [226, 227]. Although some studies have
reported the possible occurrence of eccrine secretion [256, 257], it is difficult to
prove the occurrence of this type of mechanism. Upon reaching the periplasmic
space of the cell, lipids cannot be transported by facilitated diffusion through the cell
wall, and the most likely hypothesis is that the protoplast exerts pressure on the
hydrophobic material accumulated in the periplasmic space and thereby forcing it to
cross the cell wall [23].
The accumulation of lipid secretions under the subcuticular space is commonly
observed both in trichomatous and in epidermal elaiophores [216, 230, 258]. In some
species, lipids are released only through cuticle rupture caused by contact with
pollinators [216, 231, 237]. In others, the secretion may permeate the cuticle through
microchannels, not forming subcuticular accumulation [231].
4.3
Lipids as a Specialized Reward to Pollinators
Floral oils are made available to highly specialized pollinators by only a
few angiosperms. Only bees collect floral oils, and of these only about 500 species,
i.e., less than 2% of global bee species, actively collect floral oils [216, 218, 259].
Oil-collecting bees belong to the tribes Ctenoplectrini, Centridini, Tapinostapidini,
and Tetrapediini (Apidae), and Macropidini and Redivivini (Melittidae) [259]. In
Tapinostapidini there are about 12 genera with 95 species collecting floral oils;
the largest group of oil-collecting bees (about 230 species) is in the genus Centris
(Centridini) [216, 221]. Oil-collecting bees have specific morphological adaptations
and behavior, for example, in Centris, where the oil-collecting apparatus is located
on the front pair of legs and the animals have soft and absorbent hairs [217, 260].
Floral lipids with their high nutritional value, as compared to the carbohydrates
of nectar [216–218, 259, 261], are mainly used as larval food by bees in combination
with pollen [216, 217] and possibly also in adult nutrition [218]; in addition, they
are used for the construction of nests [261].
4.4
Evolution of Oil-Offering Flowers
Studies on oil-offering flowers have mainly focused on collector observations, on
their morphology and on the distribution of elaiophores in flowers. Integrated studies
have shown that there are many independent interrelational transitions regarding the
presence of elaiophores in angiosperms and bee collector behavior [128, 216–221,
262]. Both the production of floral oils by angiosperms and the behavior of oil
collection by bees are polyphyletic [221, 262].
734
E. D. Tölke et al.