Geographic Variation in Signals
Do fragrances come in dialects? Considering that floral scents contribute to plant siring success, seed fitness, reproductive isolation and gene flow, this question is not as glib as it sounds. The foundational studies on white crowned sparrows by Peter Marler and his students connected regional dialects in birdsong with biogeography and gene flow. Given widespread evidence for geographic mosaics of coevolution, pioneered by John Thompson, Craig Benkman and the Brodie family, it is reasonable to expect geographic variation in floral scent as a sexual signal under complex selection pressures.
Do fragrances come in dialects? Considering that floral scents contribute to plant siring success, seed fitness, reproductive isolation and gene flow, this question is not as glib as it sounds. The foundational studies on white crowned sparrows by Peter Marler and his students connected regional dialects in birdsong with biogeography and gene flow. Given widespread evidence for geographic mosaics of coevolution, pioneered by John Thompson, Craig Benkman and the Brodie family, it is reasonable to expect geographic variation in floral scent as a sexual signal under complex selection pressures.
Our first study of this nature was a collaboration with John Thompson and his postdoc, Magne Friberg, working on Lithophragma herbs in a nursery pollination system with seed-eating Greya moths as pollinators. As in yucca-yucca moth interactions, female Greya moths pollinate Lithophragma flowers during oviposition bouts. Unlike the yucca system, some populations of Lithophragma also have more conventional pollinators (andrenid bees and bombyliid flies), altering the degree to which plants tolerate ovule loss to Greya larvae. Soon, we discovered more profound differences between the Lithophragma-Greya and Yucca-yucca moth systems.
[Left, John Thomson (L) and Magne Friberg (R) flanking a pair of blooming Hesperoyucca whipplei scapes on the General's Highway, Sequoia National Park, CA, 2012]
[Left, John Thomson (L) and Magne Friberg (R) flanking a pair of blooming Hesperoyucca whipplei scapes on the General's Highway, Sequoia National Park, CA, 2012]
Working in a common greenhouse garden including most species of Lithophragma and supplementing with exhaustive field surveys across western North America, Magne found a) extreme biosynthetic richness of scent composition in Lithophragma, b) hyper-variable scent composition at the scale of population as well as species, and c) strong philopatry (preference for the scent of local hosts) by female Greya moths in behavioral assays. These findings suggest that poor dispersal by both plants and moths, combined with local differences in co-pollinators and selective pressures, results in a highly structured, diverse landscape of scent-mediated interactions in this system.
Evidence of biosynthetic, phylogenetic and geographic variability of floral scent in the genus Lithophragma. Color coding is shared between the left (NMDS plot of scent composition) and center (phylogenetic mapping) panels. The right panel depicts only the 4 taxa in the "parviflorum" species group; each pie is a population.
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These figures summarize philopatric responses of female Greya moths to the floral volatiles of Lithophragma plants from different populations in western North America, showing (L) binary choice Y-maze assays and (R) no-choice oviposition trials. Note that moths significantly preferred floral scent from their home population in each assay.
Friberg, M., C. Schwind, P.R. Guimarães, R.A. Raguso, J.N. Thompson. 2019. Extreme diversification of floral volatiles within and among species of Lithophragma
(Saxifragaceae). Proceedings of the National Academy of Sciences, USA 116: 4406-4415. https://doi.org/10.1073/pnas.1809007116
Friberg, M., C. Schwind, R.A. Raguso, J.N. Thompson. 2013. Extreme divergence in floral scent among woodland star species (Lithophragma) pollinated by floral
parasites. Annals of Botany 111: 539-550. https://doi.org/10.1093/aob/mct007
Friberg, M., C. Schwind, L.C. Roark, R.A. Raguso, J.N. Thompson. 2014. Floral scent contributes to interaction specificity in coevolving plants and their insect
pollinators. Journal of Chemical Ecology 40: 955-965. https://doi.org/10.1007/s10886-014-0497-y
Friberg, M., C. Schwind, P.R. Guimarães, R.A. Raguso, J.N. Thompson. 2019. Extreme diversification of floral volatiles within and among species of Lithophragma
(Saxifragaceae). Proceedings of the National Academy of Sciences, USA 116: 4406-4415. https://doi.org/10.1073/pnas.1809007116
Friberg, M., C. Schwind, R.A. Raguso, J.N. Thompson. 2013. Extreme divergence in floral scent among woodland star species (Lithophragma) pollinated by floral
parasites. Annals of Botany 111: 539-550. https://doi.org/10.1093/aob/mct007
Friberg, M., C. Schwind, L.C. Roark, R.A. Raguso, J.N. Thompson. 2014. Floral scent contributes to interaction specificity in coevolving plants and their insect
pollinators. Journal of Chemical Ecology 40: 955-965. https://doi.org/10.1007/s10886-014-0497-y