Anton Pauw

FLIES AND FLOWERS IN DARWIN'S RACE

The idea of coevolution originated with Darwins proposal that long-proboscid pollinators and long-tubed flowers might be engaged in reciprocal selection, but this has not been demonstrated. Here we test key aspects of Darwins hypothesis of reciprocal selection in an experiment with naturally interacting populations of extremely long-proboscid flies (Moegistorhynchus longirostris: Nemestinidae) and long-tubed irises (Lapeirousia anceps: Iridaceae). We show that the benefit derived by both the fly (volume of nectar consumed) and the plant (number pollen grains received) depends on the relative length of their interacting organs. Each trait is shown to act both as agent and target in directional reciprocal selection, potentially leading to a race. This understanding of how fitness in both species varies in relation to the balance of their armament allows us to make tentative predictions about the nature of selection across multiple communities. We find that in each community a core group of long-tubed plant species might together be involved in diffuse coevolution with the fly. In poorly matched populations, the imbalance in armament is too great to allow reciprocal selection to act, and these species might instead experience one-sided selection that leads to convergence with the core species. Reciprocal selection drives the evolution of the community, then, additional species become attached to the network of interacting mutualists by convergence.

The Effects of Above- and Belowground Mutualisms on Orchid Speciation and Coexistence

Both pollination by animals and mycorrhizal symbioses with fungi are believed to have been important for the diversification of flowering plants. However, the mechanisms by which these above- and belowground mutualisms affect plant speciation and coexistence remain obscure. We provide evidence that shifts in pollination traits are important for both speciation and coexistence in a diverse group of orchids, whereas shifts in fungal partner are important for coexistence but not for speciation. Phylogenetic analyses show that recently diverged orchid species tend either to use different pollinator species or to place pollen on different body parts of the same species, consistent with the role of pollination-mode shifts in speciation. Field experiments provide support for the hypothesis that colonization of new geographical areas requires adaptation to new pollinator species, whereas co-occurring orchid species share pollinator species by placing pollen on different body parts. In contrast to pollinators, fungal partners are conserved between closely related orchid species, and orchids recruit the same fungal species even when transplanted to different areas. However, co-occurring orchid species tend to use different fungal partners, consistent with their expected role in reducing competition for nutrients. Our results demonstrate that the two dominant mutualisms in terrestrial ecosystems can play major but contrasting roles in plant community assembly and speciation.

The scope of Baker's law

Bakers law refers to the tendency for species that establish on islands by long-distance dispersal to show an increased capacity for self-fertilization because of the advantage of self-compatibility when colonizing new habitat. Despite its intuitive appeal and broad empirical support, it has received substantial criticism over the years since it was proclaimed in the 1950s, not least because it seemed to be contradicted by the high frequency of dioecy on islands. Recent theoretical work has again questioned the generality and scope of Bakers law. Here, we attempt to discern where the idea is useful to apply and where it is not. We conclude that several of the perceived problems with Bakers law fall away when a narrower perspective is adopted on how it should be circumscribed. We emphasize that Bakers law should be read in terms of an enrichment of a capacity for uniparental reproduction in colonizing situations, rather than of high selfing rates. We suggest that Bakers law might be tested in four different contexts, which set the breadth of its scope: the colonization of oceanic islands, metapopulation dynamics with recurrent colonization, range expansions with recurrent colonization, and colonization through species invasions.

Inversostyly: a new stylar polymorphism in an oil-secreting plant, Hemimeris racemosa (Scrophulariaceae)

A new kind of stylar polymorphism, provisionally called inversostyly, is described. The polymorphism occurs in Hemimeris racemosa (Scrophulariaceae), a common annual herb of the Cape region of South Africa. Most populations are dimorphic for style orientation: the style alternates with the two stamens and is deflected either upwards or downwards. Thus, there is reciprocal placement of the style and stamens in a vertical plane in zygomorphic flowers. The flowers are symmetrical, and the floral parts do not vary in length. All flowers on a given plant are of the same stylar orientation. Pollination is by specialized oil-collecting bees (Rediviva spp.), which carry the pollen of the two morphs separately in discrete anterior or posterior locations on the underside of the body. Most inversostylous populations have a slightly higher proportion of the style-down morph, and this bias increases with decreasing pollinator abundance. In contrast with inversostylous populations, all individuals in homostylous populations of H. racemosa have the style and the stamens clustered together in the down position and high levels of autogamous seed set. Homostylous populations of H. racemosa, as well as the homostylous species Hemimeris sabulosa, occur where oil-collecting bees are less abundant.

Adaptive divergence in Darwin's race: How coevolution can generate trait diversity in a pollination system

Understanding how reciprocal selection shapes interacting species in Darwins coevolutionary race is a captivating pursuit in evolutionary ecology. Coevolving traits can potentially display following three patterns: (1) geographical variation in matched traits, (2) bias in trait matching, and (3) bimodal distribution of a trait in certain populations. Based on the framework of adaptive dynamics, we present an evolutionary model for a coevolving pollination system involving the long‐proboscid fly (Moegistorhynchus longirostris) and the long‐tubed iris (Lapeirousia anceps). The model successfully demonstrates that Darwins hypothesis can lead to all three patterns if costs are involved. Geographical variation in matched traits could be driven by geographical variation in environmental factors that affect the cost rate of trait escalation. Unequal benefits derived from the interaction by the fly and the flower could potentially cause the bias in trait matching of the system. Different cost rates to trait elongation incurred by the two species and weak assortative interactions in the coevolutionary race can drive divergent selection (i.e., an evolutionary branching) that leads to the bimodal distribution of traits. Overall, the model highlights the importance of assortative interactions and the balance of costs incurred by coevolving species as factors determining the eventual phenotypic outcome of coevolutionary interactions.

Pollinators underestimated: A molecular phylogeny reveals widespread floral convergence in oil-secreting orchids (sub-tribe Coryciinae) of the Cape of South Africa

The oil-secreting orchids of southern Africa belong to the sub-tribe Coryciinae within Diseae. A phylogeny of Diseae is inferred using sequence data from all genera in the tribe, with an emphasis on resolving generic classifications within Coryciinae. Nuclear (ITS) and plastid (trnLF and matK) gene region sequences were analysed for 79 ingroup taxa and three outgroup taxa. Coryciinae is confirmed to be diphyletic, with Disperis and Coryciinae sensu stricto (s.s.) forming separate monophyletic clades. The current genera Corycium and Pterygodium are not monophyletic according to our analysis and we propose a subdivision of Coryciinae s.s. into 10 monophyletic clades including three monotypic groups. Previous generic classifications of Coryciinae s.s. have been hampered by convergent evolution of floral parts, a consequence of few pollinator species and limited pollinia attachment sites in the oil-bee pollination system common to this group.

Adaptation for rodent pollination in Leucospermum arenarium (Proteaceae) despite rapid pollen loss during grooming.

BACKGROUND AND AIMS Plants are adapted for rodent pollination in diverse and intricate ways. This study explores an extraordinary example of these adaptations in the pincushion Leucospermum arenarium (Proteaceae) from South Africa. METHODS Live trapping and differential exclusion experiments were used to test the role of rodents versus birds and insects as pollinators. To explore the adaptive significance of geoflory, inflorescences were raised above ground level and seed production was compared. Captive rodents and flowers with artificial stigmas were used to test the effect of grooming on the rate of pollen loss. Microscopy, nectar composition analysis and manipulative experiments were used to investigate the bizarre nectar production and transport system. KEY RESULTS Differential exclusion of rodents, birds and insects demonstrated the importance of rodents in promoting seed production. Live trapping revealed that hairy-footed gerbils, Gerbillurus paeba, and striped field mice, Rhabdomys pumilio, both carried L. arenarium pollen on their forehead and rostrum, but much larger quantities ended up in faeces as a result of grooming. Terrarium experiments showed that grooming exponentially diminished the pollen loads that they carried. The nectar of L. arenarium was found to be unusually viscous and to be presented in a novel location on the petal tips, where rodents could access it without destroying the flowers. Nectar was produced inside the perianth, but was translocated to the petal tips via capillary ducts. In common with many other rodent-pollinated plants, the flowers are presented at ground level, but when raised to higher positions seed production was not reduced, indicating that selection through female function does not drive the evolution of geoflory. CONCLUSIONS Despite the apparent cost of pollen lost to grooming, L. arenarium has evolved remarkable adaptations for rodent pollination and provides the first case of this pollination system in the genus.

Reduced flower visitation by nectar-feeding birds in response to fire in Cape fynbos vegetation, South Africa

AbstractNectar-feeding birds are important pollinators in fire-prone regions of the world, but the impact of fires on these bird communities has seldom been studied. Nectar-feeding bird communities were censused during peak flowering, before and after fire in the Cape fynbos of South Africa. The abundance and species richness of nectar-feeding birds decreased at all sites. In a controlled experiment, two common bird-pollinated plants, which flower profusely after a fire, were presented in floral arrays in burnt and unburnt vegetation. Birds visited flowers only in the unburnt areas. The results are surprising given the large number of bird-pollinated plants that have fire-stimulated flowering.ZusammenfassungWeniger Blütenbesuche Nektar fressender Vögel nach Buschfeuern in der Kap-Fynbos Vegetation Südafrikas Nektarfressende Vögel sind wichtige Bestäuber in Regionen, die häufigen Feuern ausgesetzt sind. Dennoch wurde der Einfluss von Feuer auf die Gemeinschaften dieser Vögel nur selten untersucht. Erhebungen von Gemeinschaften Nektar fressender Vögel wurden vor und nach Feuerereignissen im südafrikanischen Kap-Fynbos durchgeführt. In allen untersuchten Gebieten waren Anzahl und Artenreichtum der Vögel nach dem Feuer geringer. In einem kontrollierten Experiment wurden zwei von Vögeln bestäubte Pflanzenarten, die nach Feuerereignissen stark blühen, in abgebrannter und in nicht von Feuern betroffener Vegetation angeboten. Die Vögel besuchten nur jene Blüten, die in der nicht abgebrannten Vegetation angeboten wurden. Dies ist ein überraschendes Ergebnis, da das Blühen in vielen von Vögeln bestäubten Pflanzen durch Feuer stimuliert wird.

Farming with native bees (Apis mellifera subsp. capensis Esch.) has varied effects on nectar-feeding bird communities in South African fynbos vegetation

Outside their natural range, honeybees (Apis mellifera) are known to have detrimental effects on indigenous pollinators through exploitative or interference competition, but little is known about the effect of honeybee farming in areas where honeybees occur naturally. In the Cape Floristic Region of South Africa, where honeybees are indigenous, managed hives potentially elevate the abundance of honeybees far above natural levels, but impacts on other floral resource-dependent species have not been studied. Here we use experimental manipulation of honeybee density to test whether honeybee farming affects nectar-feeding birds. We selected the common sugarbush (Protea repens), utilized by both birds and bees, and analysed the time (before/after) by treatment (control/experiment) interaction to explore changes in bee abundance, nectar availability and bird abundance at three sites. Hive addition increased honeybee abundance in inflorescences of P. repens above expected levels. Despite experimental increase in honeybee numbers, there is no reduction in nectar sugar availability relative to the control areas. Where honeybee density was highest, sugarbird (Promerops cafer) numbers declined relative to expected, but sunbirds (Nectarinidae) were not affected at any of the sites. We conclude that stocking rates of more than one honey bee per P. repens inflorescence have detrimental effects on bird abundance due to interference, rather than resource competition.

Floral divergence in closely related Leucospermum tottum (Proteaceae) varieties pollinated by birds and long-proboscid flies

Abstract The Proteaceae are renowned for their floral diversity but surprisingly the role of pollinators in driving evolutionary divergence in this family has been underexplored. Here we focus on recently diverged taxa to gain insight into the processes that generate diversity by testing whether two varieties of Leucospermum tottum might have originated by pollinator mediated adaptive divergence. L. tottum var. tottum has pale salmon-coloured horizontally-oriented flowers, long nectar tubes, and small volumes of concentrated nectar. L. tottum var. glabrum has red and yellow vertically oriented flowers, short nectar tubes, and large volumes of dilute nectar. Despite the morphological divergence, the varieties are indistinguishable using eight molecular markers, indicating a very early stage of differentiation. Consistent with their morphologies, L. tottum var. tottum is pollinated by long-proboscid flies (Philoliche rostrata and Philoliche gulosa), Cape sugarbirds (Promerops cafer), and, to a lesser extent, by Orange-breasted sunbirds (Anthobaphes violacea), whereas, L. tottum var. glabrum is pollinated only by Orange-breasted sunbirds. A. violacea visits both varieties, but makes more frequent contact with pollen presenters when foraging on L. tottum var. glabrum. The exclusion of birds caused a steeper reduction in seed production in L. tottum var. glabrum than in L. tottum var. tottum, consistent with specialization for bird-pollination in this variety. Additionally, L. tottum var. glabrum exhibits autogamy, whereas L. tottum var. tottum does not. Floral divergence between the two L. tottum varieties corresponds with divergence in pollinator use.