Ecology and evolution of the specialized hemipepsis-wasp (Hymenoptera : Pompilidae) pollination guild in South Africa.
Loading...
Date
2011
Authors
Journal Title
Journal ISSN
Volume Title
Publisher
Abstract
Pollinators are believed to have played a key role in the radiation of flowering plants. The Grant-Stebbins model of pollinator-mediated speciation, in which evolutionary shifts between pollinators
result in phenotypic diversification and enforce reproductive isolation, is one of the most compelling
hypotheses for the rapid diversification of angiosperms. A key principle in this model is that plant
pollination systems tend towards specialization, resulting in convergent suites of floral traits
(syndromes) associated with particular types of pollinators. However, the expectation of pollination
system specialization is not always supported by ecological data and has also been questioned on
theoretical grounds. In this thesis, I examine pollination by Hemipepsis spider-hunting wasps
(Hymenoptera, Pompilidae, Pepsinae) and use this system to address questions about levels and
proximal mechanisms of floral specialization, floral shifts and convergent evolution of floral traits.
Specialized pollination by Hemipepsis wasps is a newly described pollination system within the
angiosperms. I document pollination by these wasps for the first time in 15 South African grassland
plant species, including two species of Eucomis (Hyacinthaceae) and 13 asclepiads (Apocynaceae:
Asclepiadoideae). In one of the asclepiads, Xysmalobium undulatum, I describe a bimodal pollination
system involving both Hemipepsis wasps and a cetoniine beetle. I also describe an unusual and
potentially antagonistic pollination mechanism whereby wasps are systematically dismembered
during the insertion of pollinia in the two asclepiads Pachycarpus asperifolius and P. appendiculatus.
I have used these and previous case studies to establish the existence of a new pollination guild,
consisting of at least 21 plant species (across 10 genera and three families), that are reliant on four
functionally similar species of Hemipepsis wasp for pollination. Plants in the guild are distributed
throughout the moist grasslands of eastern South Africa and flower from September through until early May, peaking in December/January.
The Hemipepsis-wasp pollination guild is characterized by high levels of functional
specialization (17 of the 21 known guild members are pollinated exclusively by Hemipepsis wasps),
despite the absence of morphological adaptations to prevent non-pollinating insects from accessing
nectar. I used field and laboratory based experiments to explore the function of floral traits in
enforcing specialization. These showed that Hemipepsis wasps primarily use scent, rather than visual
cues, to locate flowers, but I was unable to firmly identify specific compounds responsible for the
attraction of these wasps (compounds that elicited antennal responses in preliminary GC-EAD experiments did not attract wasps in bioassays). The chemical composition of the floral scents of guild
members was examined for 71 individuals representing 14 species in addition to previous studies, and
found to comprise complex blends of volatiles (usually containing between 30 and 50 compounds),
typically dominated by aliphatics and monoterpenes with small amounts of aromatics. I also showed
that the floral colours of guild members are similar to background vegetation, suggesting that floral
colours are adapted for crypsis to avoid detection by non-pollinating insects. Palatability choice experiments with honeybees showed that non-pollinating insects find the nectars of at least three of
the asclepiad guild members distasteful. Plants in this guild thus appear to achieve specialization
through biochemical filters (scent as an attractant and differentially palatable nectar) and cryptic coloration.
Pollinator-mediated convergence in floral traits is the fundamental basis for pollination
syndromes, but has seldom been rigorously analyzed. Flowers in the Hemipepsis-wasp pollination
guild share several qualitative traits, including dull greenish- or brownish-white colour, often with
purple blotches, exposed sucrose dominant nectar with a relatively high sugar concentration (typically
over 50% sugar by weight) and a sweet/spicy fragrance to the human nose. To test for convergent
evolution in guild members, I compared scent, nectar and colour traits of guild members to those of
congeners with different pollinators. Although traits often differed between guild members and their
congeners, I found little evidence for overall convergence in floral scent profiles and nectar
properties, but floral colours in the guild were significantly closer to the colour of background
vegetation than those of congeners. At this stage, the lack of knowledge about specific floral volatiles
that influence Hemipepsis-wasp behaviour and secondary nectar constituents that limit non-pollinator
visits makes it difficult to identify the extent of biochemical convergent evolution within the guild.
The directions and functional traits involved in evolutionary transitions between pollination by
Hemipepsis wasps and other vectors are currently difficult to ascertain as there is limited phylogenetic
data for the plant families concerned. In the genus Eucomis, fly and Hemipepsis-wasp pollinated
species are very similar in floral morphology and colour, but differ strongly in floral scent. Using
manipulative field experiments in conjunction with detailed analyses of colour, scent and
morphology, I was able to show that a shift between wasp and fly pollination could be induced simply
by manipulating oligosulphides in the scent emission from inflorescences. When considered in
combination with other experiments highlighting the importance of scent as a pollinator attractant for
all guild members, this suggests that scent properties may have played a key role in the evolutionary
transitions between pollination by Hemipepsis wasps and other vectors.
This research has established that pollination by Hemipepsis spider-hunting wasps is more
geographically and phylogenetically widespread than was previously known, and has confirmed that
these wasps are important and consistent pollinators in southern African grassland ecosystems. I have
shown that a distinct guild of plants is specialized for pollination by these wasps. The high levels of
specialization within this guild highlight the effectiveness of biochemical filters and cryptic coloration
in limiting the spectrum of flower visitors. The major challenge ahead will be to identify the floral
volatiles that attract Hemipepsis wasps and the non-sugar constituents that make the nectars of some
guild members differentially palatable. These would both contribute greatly to our understanding of
floral specialization and the mechanisms involved in the radiation of the angiosperms.
Description
Thesis (Ph.D.)-University of KwaZulu-Natal, Pietermaritzburg, 2011.
Keywords
Spider wasps--South Africa., Asclepiads--Pollination--South Africa., Flowers--Colour--South Africa., Odours--South Africa., Grassland plants--Evolution--South Africa., Theses--Ecology.