NECTAR CONSUMERS - Nectaries and Nectar - page 326

and field trials with hybrid plants to show that an allele that increases nectar
production doubled hummingbird visitation. However, a single mutation
from violet to red flower colour was later shown to be sufficient for the shift
from bee pollination to hummingbird pollination, regardless of differences in
nectar production (Bradshaw & Schemske, 2003); see also Beardsley et al.
(2003).

More genetic tools are available for the genus Petunia (Solanaceae), which
has distinct bee, hawkmoth, and bird pollination syndromes (Galliot et al.,
2006) . Using separate crosses of hawkmoth-pollinated P. axillaris and bee-
pollinated P. integrifolia in a defined genetic background, Stuurman et al.
(2004) analysed phenotypic and genetic differences in colour, shape, nectar
reward, and scent. There are striking differences in nectar volume and con-
centration, showing that part of the volume difference is in relative water
contributions to the nectar, and in sugar composition. Nectar volume is con-
trolled by two QTLs, one that affects volume pleiotropically by altering
flower size and another that affects nectary physiology. Their additive ef-
fects account for almost the entire difference in nectar volume between the
two species. No significant QTL was detected for nectar concentration, but a
single QTL decreased the proportion of sucrose, which is consistent with the
activity of an invertase. In Petunia , volume is thus not genetically correlated
with sugar composition. The gene Nec1 is highly expressed in the nectaries
of Petunia hybrida during active secretion of nectar (Ge et al., 2000), and
future molecular analyses should provide insights into the genetic basis of
nectar secretion (see also Thornburg, 2007, Chapter 6 in this volume).

6 CONCLUSION

The sugar composition of nectar may seldom be physiologically important to
nectar consumers, since almost all animals are able to digest sucrose. Excep-
tions include certain ants feeding on extrafloral nectar of Acacia species
(Heil et al., 2005) and frugivorous birds of the sturnid-muscicapid lineage
(Martínez del Rio et al., 1992; Lotz & Schondube, 2006); both groups lack
gut sucrase and thus prefer hexose nectars. In terms of nectar concentration,
the concentrations leading to the highest rates of sugar intake (Table 3) are
actually very similar for various suction feeders (moths, butterflies, and or-
chid bees), as a result of common biophysical mechanisms (Kingsolver &
Daniel, 1995). In animals that lick nectar, the measured optimal concentra-
tions are higher for bees and bats, although less so for nectar-feeding ants or
birds. For all taxa, it is a paradox that these concentrations are consistently
higher than those of the nectars that the animals normally consume. The