Mating systems, insect pollination and chemical ecology of grassland Protea species (Proteaceae)
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Date
2012
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Abstract
Major transitions between vertebrate and insect pollination systems have occurred many times
during the angiosperm radiation and are associated with evolutionary modifications in floral
traits. In the large ancestrally bird-pollinated African genus Protea (Proteaceae), an
evolutionary shift from bird to insect pollination in the genus is suggested by the fruity
diurnal scent of flowers in a recently evolved clade of grassland species. In this study, I
confirm that four of these grassland Protea species have mixed mating systems and are indeed
insect pollinated, and furthermore demonstrate the functional significance of their floral
presentation and scent chemistry for attraction of pollinators, specifically cetoniine beetles.
The study species, Protea caffra, Protea dracomontana, Protea simplex and Protea
welwitschii, have colourful bowl-shaped inflorescences that produce copious amounts of
pollen and dilute, xylose-rich nectar. Cetoniine beetles were found to be the most suitable
pollinators due to their abundance, size, relatively pure Protea pollen loads, and their
preference for the fruity scent and low growth form of these scented Protea species, as
demonstrated by choice experiments in which inflorescences were offered at either end of a y-maze
or at various heights above the ground, respectively.
Bagging and hand pollinations revealed that these Protea species are self-compatible
and capable of autonomous selfing. Self progeny of P. caffra were as vigorous as cross
progeny in terms of germinability and survivorship to two months. Vertebrate-excluded and
open-pollinated inflorescences yielded similar seed numbers for all species. Supplemental
hand-pollinations, however, failed to increase seed set substantially, an indication of resource
limitation. Outcrossing rates estimated using polymorphisms at eight allozyme loci in progeny
from vertebrate-excluded and open-pollinated treatments of P. caffra were no different
(t=0.59), indicating outcrossing by insects and an equal or insubstantial contribution from bird
pollinators.
The fruity-sweet scents of these species were more complex, with higher whole flower
and mass-specific emission rates, than those in eight bird-pollinated congenerics. The overall
floral scent is shown to be a blend of emissions from various plant parts, especially nectar.
Electroantennography (EAG) revealed that the generalist pollinator Atrichelaphinis tigrina
responds to a variety of volatile compounds found in fruity Protea scents. Field trapping
confirmed that this cetoniine beetle is strongly attracted to ß-linalool (up to 60% of scent
profile) and methyl benzoate.
In conclusion, this study demonstrates the evolution of beetle pollination and mixed
mating systems in a grassland clade of Protea. Volatile compounds that make up the unique
(within Protea) fruity scent of the study species are shown to attract beetles, and the emission
of large amounts of these compounds was probably a key step in the transition from bird to
insect pollination in Protea.
Description
Thesis (Ph.D.)-University of KwaZulu-Natal, Pietermaritzburg, 2012.
Keywords
Protea., Grassland ecology--South Africa., Fertilization of plants., Plant chemical ecology., Pollinators., Beetles., Theses--Botany.