Biology Reference
In-Depth Information
5.3
Contribution of amino acids to the taste of nectar
One unique aspect of the presence of amino acids in nectar is the potential
contribution of these compounds to its taste (Gardener & Gillman, 2002).
Amino acids have much more diverse chemical strucures than sugars and
their taste also varies with concentration (Birch & Kemp, 1989). It has been
well established that insects have several different classes of labellar chemo-
sensory receptors responding to water, sugar, and salts (Shiraishi &
Kuwabara, 1970; Hansen et al., 1998). The water cell is believed to control
drinking behaviour (Dethier, 1976). The sugar cell recognizes the sugars in
nectar and is believed to mediate the attraction of flies to sucrose-rich solu-
tions (Omand & Dethier, 1969). Stimulation of the salt cell in conjunction
with the water cell results in enhanced elicitation of feeding behaviour
(Hansen et al., 1998).
Examination of the effects of amino acids on insect chemoreceptors
(Shiraishi & Kuwabara, 1970; Hansen et al., 1998) has permitted the descrip-
tion of four taste classes of amino acids. Those in Class I (asparagine,
glutamine, alanine, cysteine, glycine, serine, threonine, tyrosine) have no ef-
fect on the chemoreceptors of two species of fly. Those in Class II (arginine,
aspartic acid, glutamic acid, histidine, lysine) are generally inhibitory to fly
chemoreceptors. Two amino acids in Class III (proline and hydroxyproline)
have the unique ability to stimulate the salt cell (Hansen et al., 1998; Wacht
et al., 2000). Class IV (isoleucine, leucine, methionine, phenylalanine, tryp-
tophan, valine) includes amino acids with the ability to stimulate the sugar cell.
Thus, amino acids in nectar have the potential to modify insect behaviour by
stimulating insect chemosensory receptors. Gardener and Gillman (2002)
have devised a graphical method to represent the composition of amino acids
in nectar, and Figure 2 uses this to show the possible “taste” of a number of
plant nectars (Carter et al., 2006). It should be stressed that this is based
on the chemosensory responses of flies and amino acids may taste differently
to vertebrate pollinators (see Birch & Kemp, 1989).
Among the amino acids found in nectar, proline is unique because it can
stimulate the salt cell, resulting in increased feeding behavior (Hansen et al.,
1998). Proline has also been identified at high levels in some nectars (Gardener
& Gillman, 2001a; Kaczorowski et al., 2005; Carter et al., 2006). In choice
tests, both cabbage white butterflies and honeybees have shown preferecnes
for sugar solutions enriched by amino acids, including proline (Inouye &
Waller, 1984; Alm et al., 1990; Carter et al., 2006). Proline is an especially
important amino acid for insects. It is by far the most abundant amino acid
in honeybee haemolymph, and is required for egg laying (Crailsheim &
