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Evolution of Spur Length in a Moth-pollinated Orchid
Uppsala University, Disciplinary Domain of Science and Technology, Biology, Department of Ecology and Evolution, Ecological Botany.
2010 (English)Doctoral thesis, comprehensive summary (Other academic)
Abstract [en]

There is considerable evidence that pollinator shifts can explain many differences in flower morphology between closely related plant species, but the extent to which pollinator shifts can explain the maintenance of among-population variation in floral traits within species is poorly known. In this thesis, I combined comparative and experimental approaches to examine the evolution of floral traits in the moth-pollinated orchid Platanthera bifolia. More specifically, I investigated (1) the relationship between flower and pollinator morphology, (2) phenotypic selection on morphology and phenology in populations in contrasting environments, (3) components of prezygotic reproductive isolation among divergent populations, and (4) the adaptive and functional significance of two correlated floral traits.

A study of Scandinavian of P. bifolia populations revealed that spur length was positively correlated with proboscis length of local pollinators, which suggests that variation in spur length reflects adaptive evolution in response to geographically variable pollinator-mediated selection.

A phenotypic selection study on Öland, SE Sweden, suggested that disruptive selection on spur length contributes to the maintenance of a bimodal distribution of spur length in mixed habitats, but provided very limited evidence of divergent selection on plant morphology and flowering phenology in grassland and woodland habitats.

Field experiments revealed strong reproductive isolation between divergent populations on Öland, due to differences in spatial distribution, flowering phenology, and pollinators, and among-population incompatibility. The results suggest that prezygotic reproductive isolation contributes to the maintenance of population differentiation in floral traits in P. bifolia.

A field manipulation experiment demonstrated that spur length but not perianth size affects pollination success and seed production. This suggests that among-population differentiation in perianth size may be the result of a genetic correlation with spur length.

Taken together, the results of this thesis suggest that pollinator-mediated selection can shape the evolution of intraspecific floral variation.

Place, publisher, year, edition, pages
Uppsala: Acta Universitatis Upsaliensis , 2010. , p. 33
Series
Digital Comprehensive Summaries of Uppsala Dissertations from the Faculty of Science and Technology, ISSN 1651-6214 ; 712
Keywords [en]
Platanthera bifolia, floral evolution, natural selection, pollination, spur length, population differentiation, divergent selection, disruptive selection, reproductive isolation
National Category
Biological Sciences
Research subject
Ecological Botany
Identifiers
URN: urn:nbn:se:uu:diva-113086ISBN: 978-91-554-7714-1 (print)OAI: oai:DiVA.org:uu-113086DiVA, id: diva2:289669
Public defence
2010-03-05, Zootissalen, Villavägen 9, Uppsala, 10:00 (English)
Opponent
Supervisors
Available from: 2010-02-12 Created: 2010-01-25 Last updated: 2011-02-09Bibliographically approved
List of papers
1. Pollinator shifts and the evolution of spur length in the moth-pollinated orchid Platanthera bifolia
Open this publication in new window or tab >>Pollinator shifts and the evolution of spur length in the moth-pollinated orchid Platanthera bifolia
Show others...
2014 (English)In: Annals of Botany, ISSN 0305-7364, E-ISSN 1095-8290, Vol. 113, no 2, p. 267-275Article in journal (Refereed) Published
Abstract [en]

Plantpollinator interactions are thought to have shaped much of floral evolution. Yet the relative importance of pollinator shifts and coevolutionary interactions for among-population variation in floral traits in animal-pollinated species is poorly known. This study examined the adaptive significance of spur length in the moth-pollinated orchid Platanthera bifolia. Geographical variation in the length of the floral spur of P. bifolia was documented in relation to variation in the pollinator fauna across Scandinavia, and a reciprocal translocation experiment was conducted in south-east Sweden between a long-spurred woodland population and a short-spurred grassland population. Spur length and pollinator fauna varied among regions and habitats, and spur length was positively correlated with the proboscis length of local pollinators. In the reciprocal translocation experiment, long-spurred woodland plants had higher pollination success than short-spurred grassland plants at the woodland site, while no significant difference was observed at the grassland site. The results are consistent with the hypothesis that optimal floral phenotype varies with the morphology of the local pollinators, and that the evolution of spur length in P. bifolia has been largely driven by pollinator shifts.

Keywords
Pollinator shifts, floral traits, spur length, proboscis length, pollination success
National Category
Biological Sciences
Identifiers
urn:nbn:se:uu:diva-113080 (URN)10.1093/aob/mct217 (DOI)000330193100006 ()
Available from: 2010-01-25 Created: 2010-01-25 Last updated: 2018-06-05Bibliographically approved
2. Phenotypic selection on floral traits in divergent populations of the moth-pollinated orchid Platanthera bifolia
Open this publication in new window or tab >>Phenotypic selection on floral traits in divergent populations of the moth-pollinated orchid Platanthera bifolia
(English)Manuscript (preprint) (Other academic)
Abstract [en]

Among-population differences in flower morphology is commonly attributed to divergent natural selection mediated by pollinators, but comparisons of current selection in populations growing in contrasting environments are scarce. In the moth-pollinated orchid Platanthera bifolia, floral morphology and flowering phenology differ between grassland and woodland populations on the island Öland, SE Sweden. To examine whether among-population variation in floral morphology is consistent with differences in current selection, we documented phenotypic selection through female and male function on flowering phenology, plant height, flower production and spur length in three populations. One grassland and one woodland population were studied in three years, and one population bimodal for spur length and occurring in a mixed habitat was studied in one year. There was evidence of disruptive selection on spur length in the population in the mixed habitat. In one of three years, there was evidence of divergent selection on plant height between the grassland and woodland population.  In all three years selection favoured longer nectar spurs; selection gradients were statistically significant in the short-spurred grassland population, but not in the long-spurred woodland population. In both populations, there was directional selection for earlier flowering and more flowers through female function. Seed production was pollen limited in the grassland population. The results support the hypothesis that disruptive selection through female reproductive success contributes to the maintenance of a bimodal distribution of spur length in P. bifolia in mixed habitats, but provides very limited evidence of divergent selection on plant morphology and flowering phenology in grassland and woodland populations of P. bifolia. The results are discussed in relation to the evolution of floral traits and the challenges associated with detecting stabilizing selection in natural populations.

Keywords
Disruptive selection, divergent selection, flower morphology, flowering phenology, selection analysis, seed output, pollen removal
National Category
Biological Sciences
Identifiers
urn:nbn:se:uu:diva-113083 (URN)
Available from: 2010-01-25 Created: 2010-01-25 Last updated: 2011-02-09Bibliographically approved
3. Reproductive isolation among divergent populations of the moth-pollinated orchid Platanthera bifolia
Open this publication in new window or tab >>Reproductive isolation among divergent populations of the moth-pollinated orchid Platanthera bifolia
(English)Manuscript (preprint) (Other academic)
Abstract [en]

Differentiation in floral traits has been documented between populations in contrasting habitats in several species of flowering plants, but few studies have investigated whether barriers to reproduction contribute to the maintenance of this variation. We investigated components of prezygotic reproductive isolation between short-spurred and long-spurred populations of the moth-pollinated orchid Platanthera bifolia on the island of Öland, SE Sweden. Short-spurred and long-spurred populations were reproductively isolated due to spatial isolation, and differences in flowering time, pollinator morphology and behaviour. The index of reproductive isolation due to pollinators and flowering time was higher for the early-flowering long-spurred plants than it was for the late-flowering short-spurred plants. In addition, we documented a postpollination isolation mechanism manifested as reduced fruit formation in short-spurred plants when pollinated with pollen from long-spurred plants. The results indicate that strong reproductive isolation contribute to the maintenance of population differentiation in morphology and phenology between long-spurred and short-spurred populations of P. bifolia.

Keywords
Reproductive isolation, spur length, population differentiation, speciation
National Category
Biological Sciences
Identifiers
urn:nbn:se:uu:diva-113085 (URN)
Available from: 2010-01-25 Created: 2010-01-25 Last updated: 2011-02-09Bibliographically approved
4. Despite their apparent integration, spur length but not perianth size affects reproductive success in the moth-pollinated orchid Platanthera bifolia
Open this publication in new window or tab >>Despite their apparent integration, spur length but not perianth size affects reproductive success in the moth-pollinated orchid Platanthera bifolia
2009 (English)In: Functional Ecology, ISSN 0269-8463, E-ISSN 1365-2435, Vol. 23, no 5, p. 1022-1028Article in journal (Refereed) Published
Abstract [en]

The evolution of many floral traits is thought to have been shaped  by pollinator-mediated selection for increased attractiveness or an   improved mechanical fit of flowers to pollinators. Yet, few studies have examined experimentally the independent and interactive effects of   different aspects of flower morphology on plant reproductive success.   In the orchid Platanthera bifolia, perianth size and spur length are   positively correlated within and among populations. To test the hypothesis that pollination success and seed output increases with   increasing perianth size and spur length, we manipulated the two traits   in a factorial design in a long-spurred population of P. bifolia   pollinated by long-tongued hawkmoths. Additionally, to determine   whether differences in selfing rate can explain variation in fruit set   and fruit size, we performed controlled self- and cross-pollination.   Plants with long spurs had more flowers pollinated, more pollen removed   and produced more and larger fruits compared to plants with short   spurs. In contrast, perianth size did not affect the pollination   success or fruit production of P. bifolia.   Fruit production and fruit size did not differ among flowers pollinated   with self- and cross pollen, respectively. This indicates that reduced   pollen deposition rather than pollinator-mediated self-pollination   caused the reduction in fruit set and fruit volume observed after   shortening of the spur.   The results demonstrate that spur length, but not perianth size, is   critical for reproductive success in P. bifolia, and suggest that   among-population differentiation in perianth size may reflect a  correlated response to selection on spur length. The results are  consistent with the hypothesis that visual display is less important   than other cues for the reproductive success of P. bifolia, and   underscore the necessity to experimentally examine the functional   significance of putatively adaptive traits.

Keywords
pollination success, seed output, floral evolution, floral display, flower morphology, pollen removal
National Category
Biological Sciences
Identifiers
urn:nbn:se:uu:diva-108552 (URN)10.1111/j.1365-2435.2009.01595.x (DOI)000269976800020 ()
Available from: 2009-09-22 Created: 2009-09-22 Last updated: 2017-12-13Bibliographically approved

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