dispersers. This would parallel the evolutionary pathway of floral scent, which has in
many cases evolved secondarily out of VOC-based defense mechanisms [151].
For example, limonene, a monoterpene which dominates the scent of oranges and
has been considered to play a defensive role, is in fact an attractant of vertebrate
and invertebrate antagonists [152]. This may apply to many compounds that are
considered to be defensive solely based on their broad chemical characteristics.
4.2
Developmental and Phylogenetic Constraints
Fruit scent composition may be affected by both phylogenetic and developmental
constraints. Often defined in different ways, we refer to developmental constraints as
the tendency of fruits to emit VOCs that are present in unripe fruits or other plant
parts, not because of their fitness benefit in the fruits but because it is developmentally impossible, or too costly, for the plant to change the VOC profile of only ripe
fruits. We refer to phylogenetic constraints as the tendency of a species to possess
a trait not because of its fitness benefits to its own ecological circumstances but
because it was inherited from an ancestor. The two are inherently connected, as, for
example, developmental constraints would slow down adaptive change and hence
render closely related taxa more similar.
4.2.1 Developmental Constraints
While hardly studied, the role of developmental constraints in determining ripe fruit
scent is probably marginal. Nonvolatile ripe fruit secondary metabolites are independent of other plant parts and robust to changes in the abiotic environment, thus
indicating that their presence is adaptive rather than a by-product of regulatory or
biochemical processes originating outside the fruit [6, 7].
The strongest argument against a significant role of developmental constraints on
ripe fruit scent is the significant change, qualitatively and quantitatively, in the scent
of fruits as they mature [37, 39, 40, 51]. Since ripe fruits develop from unripe fruits,
the fact that a countless number of species changes their scent profile drastically
indicates that selection can effectively alter ripe fruit scent. Nonetheless, it should
be remembered that different scents – i.e., different unique mixtures of VOCs – can
originate from the same biochemical pathways [153]. For example, species such as
Micronychia macrophylla, a lemur-dispersed species from Madagascar, change the
scent of ripe fruits qualitatively and quantitatively, but do so primarily using
terpenoids [51], which tend to originate from few biochemical pathways [34].
The tendency to emit chemically similar odorants may be the result of some
constraints. But this is not universal: in the same system, Ficus tiliifolia, another
lemur-dispersed species, changes its scent profile from a terpene-dominated VOC
bouquet in unripe fruits to an aliphatic acid-dominated scent in ripe fruits [51].
Finally, another factor that could constrain the amount of scent emitted by a fruit
is simply its size. Fruit size is one of the factors most malleable to selection by
frugivores: in bird-dispersed species, there is an upper cap determined by bird gape
width [154, 155], and in species that rely on larger and more energy-demanding
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many cases evolved secondarily out of VOC-based defense mechanisms [151].
For example, limonene, a monoterpene which dominates the scent of oranges and
has been considered to play a defensive role, is in fact an attractant of vertebrate
and invertebrate antagonists [152]. This may apply to many compounds that are
considered to be defensive solely based on their broad chemical characteristics.
4.2
Developmental and Phylogenetic Constraints
Fruit scent composition may be affected by both phylogenetic and developmental
constraints. Often defined in different ways, we refer to developmental constraints as
the tendency of fruits to emit VOCs that are present in unripe fruits or other plant
parts, not because of their fitness benefit in the fruits but because it is developmentally impossible, or too costly, for the plant to change the VOC profile of only ripe
fruits. We refer to phylogenetic constraints as the tendency of a species to possess
a trait not because of its fitness benefits to its own ecological circumstances but
because it was inherited from an ancestor. The two are inherently connected, as, for
example, developmental constraints would slow down adaptive change and hence
render closely related taxa more similar.
4.2.1 Developmental Constraints
While hardly studied, the role of developmental constraints in determining ripe fruit
scent is probably marginal. Nonvolatile ripe fruit secondary metabolites are independent of other plant parts and robust to changes in the abiotic environment, thus
indicating that their presence is adaptive rather than a by-product of regulatory or
biochemical processes originating outside the fruit [6, 7].
The strongest argument against a significant role of developmental constraints on
ripe fruit scent is the significant change, qualitatively and quantitatively, in the scent
of fruits as they mature [37, 39, 40, 51]. Since ripe fruits develop from unripe fruits,
the fact that a countless number of species changes their scent profile drastically
indicates that selection can effectively alter ripe fruit scent. Nonetheless, it should
be remembered that different scents – i.e., different unique mixtures of VOCs – can
originate from the same biochemical pathways [153]. For example, species such as
Micronychia macrophylla, a lemur-dispersed species from Madagascar, change the
scent of ripe fruits qualitatively and quantitatively, but do so primarily using
terpenoids [51], which tend to originate from few biochemical pathways [34].
The tendency to emit chemically similar odorants may be the result of some
constraints. But this is not universal: in the same system, Ficus tiliifolia, another
lemur-dispersed species, changes its scent profile from a terpene-dominated VOC
bouquet in unripe fruits to an aliphatic acid-dominated scent in ripe fruits [51].
Finally, another factor that could constrain the amount of scent emitted by a fruit
is simply its size. Fruit size is one of the factors most malleable to selection by
frugivores: in bird-dispersed species, there is an upper cap determined by bird gape
width [154, 155], and in species that rely on larger and more energy-demanding
416
O. Nevo and M. Ayasse