Biology Reference
In-Depth Information
Chapter 5 in this volume) (but note that in this case the shift is from hexose
to sucrose nectars). In the Canary Islands, sunbirds are not present but ap-
parently ornithophilous flowers are now visited by several opportunistic
nectar-feeding passerine birds such as warblers. Here Dupont et al. (2004)
found that phylogenetically related plants with different pollinators differed
in sugar composition, with hexose nectars being associated with bird visita-
tion. Sugars may also have evolved readily in a few passerine-pollinated
species of
Salvia
(Lamiaceae) in Africa. This genus, known for its classic
“bee” flowers, has a worldwide distribution, and about a quarter of the ap-
proximately 900 species have become ornithophilous (Wester & Claßen-
Bockhoff, 2006). Most of these are Neotropical and have shifted to hum-
mingbird pollination, while retaining high sucrose nectars; Schwerdtfeger
(1996) recorded 69.9% nectar sugar as sucrose in 19 bee-pollinated species
of
Salvia
, and 77.7% sucrose in seven hummingbird-pollinated species.
South Africa has 23 species of
Salvia
, and bird pollination has evolved in
three species with large reddish flowers and dilute nectars (Wester &
Claßen-Bockhoff, 2006). Nectar sugar composition has been analysed in two
of them (
S. lanceolata,
S. africana-lutea
) and is predominantly hexose (B-E
van Wyk, unpublished data). Pollination shifts in
Salvia
from the original
condition of bee pollination seem to involve adding water to the nectar for
hummingbirds, but hydrolysing sucrose to hexoses for the passerine-pollinated
species.
A volume increase without dilution of the nectar has occurred in ginger
species (Zingiberaceae and Costaceae) flowering on the forest floor in Bor-
neo and grouped by Sakai et al. (1999) into three pollination guilds. The
nectar sugar concentration averaged 26-29%, but the daily sugar production
per inflorescence was 24 and 60 times higher for species pollinated by spi-
derhunters (Nectariniidae) than for those pollinated by anthophorid or halictid
bees, respectively. The sugar composition of nectar of these ginger flowers is
unknown; it would be interesting to know whether the elevated nectar vol-
umes required for spiderhunters are hexose-based.
In order to understand the genetic basis of pollinator shifts, it is necessary
to study single floral traits in isolation, preferably using plant species for
which the appropriate molecular tools are available (Galliot et al., 2006).
The few studies of heritability in nectar traits concern mainly nectar produc-
tion rate, which may show more plasticity than nectar chemistry (Mitchell,
2004). Changes in nectar may not always be involved in transitions between
pollination systems. Schemske and Bradshaw (1999) crossed two sister spe-
cies of
Mimulus
(Scrophulariaceae), hummingbird-pollinated
M. cardinalis
and bee-pollinated
M. lewisii
, and used quantitative trait locus (QTL) mapping
