One of the greatest challenges in the study of coevolution, indeed, for biology in general,
is to understand how evolutionary and ecological processes shape patterns in nature.
Ecologists routinely observe patterns of association among organisms, such as parasites
infecting hosts or insects pollinating flowers and systematists routinely infer patterns of
phylogenetic relationship. Such patterns invite explanation and suggest hypotheses about
the evolutionary process, but it is difficult to investigate contemporary processes,
including natural selection and, of course, impossible to directly observe historical
processes. Observation of patterns in various ecological contexts, inference of
phylogenetic patterns, model and simulation of processes, and direct experimentation aim
to test specific predictions about the role of ecology in shaping evolutionary trajectories,
and evolutionary processes in shaping ecological associations.
The fig-wasp pollinator mutualism provides a unique opportunity to examine
fundamental processes of coevolution, namely, reciprocal adaptation where interacting
partners are the agents of selection. Because pollinating wasp reproduction is directly
linked to host plant reproduction, it is possible to estimate the fitness consequences of
interaction for both partners simultaneously. By manipulation of interacting individuals and species, or by examination of natural variation within and among populations, it may
be possible to estimate the strength and direction of selection on each mutualistic partner.
This work employs molecular genetic patterns, ecological observations, and direct
experimentation to investigate host specificity in Ceratosolen (Agaonidae, Hymenoptera) pollinators of Ficus subgenus Sycomorus (Moraceae) and potential processes affecting
the origin and evolution of species diversity in this system.
The first chapter examines genetic variation in Ceratosolen pollinators of
widespread Ficus across the geographic range of several host species. Deep
mitochondrial DNA sequence divergence between host-specific populations distributed
across Wallacea suggests host conservatism during ancient range expansion and
subsequent isolation by distance. Geographic patterns of sequence divergence and host
association are more consistent with a model of allopatric speciation than speciation by
host switching. The second chapter investigates pollinator host choice by
morphotyping and DNA barcoding of floral visitors in a community of closely related
and sympatric fig species. Host specificity was very high, but rare pollinator sharing among sympatric fig species was observed at a rate of 1-2%. Even such rare events could
be evolutionarily significant and pose challenges for species delimitation.
The third chapter examines fitness consequences of pollinator sharing by
experiment. A new method of manipulating fig pollinators investigated the reproductive
consequences of intra- and interspecific pollinator visitation for both mutualistic partners.
When pollinators were introduced to a novel host species, hybrid seed set was
comparable to results of conspecific crosses. Hybrids germinated, established, and grew
at rates comparable to non-hybrids. Pollinator fitness, however, was compromised after
oviposition in the novel host. Although heterospecific pollinators induced gall formation,
offspring did not develop to maturity in the new host. Microsatellite genotypes of a New
Guinea fig community indicated a substantial but not absolute barrier to gene flow among sympatric species. That hybrids constituted fewer than 2% of individuals in populations
may be explained by selection against pollinator host switching in this system.
Collectively, these studies suggest that the extreme species-specificity of
associations between Ceratosolen pollinators and Sycomorus figs is maintained by the
fitness cost of colonizing new hosts. At the same time, hybridization resulting from rare
instances of pollinator sharing in even the most extremely specialized of pollination
mutualisms has the potential to influence diversification and coevolution.
University of Minnesota Ph.D. dissertation. September 2011. Major: Ecology, Evolution and Behavior. Advisor: George D. Weiblen. 1 computer file (PDF); x, 115 pages, appendices I-III.
Moe, Annika M..
From pattern to process: ecology and evolution of host specificity in the fig-pollinator mutualism..
Retrieved from the University of Minnesota Digital Conservancy,
Content distributed via the University of Minnesota's Digital Conservancy may be subject to additional license and use restrictions applied by the depositor.