Dramatic vestigialization of floral fragrance across a transition from outcrossing to selfing in Abronia umbellata (Nyctaginaceae).

PREMISE OF THE STUDY

Vestigialization of traits that no longer enhance fitness is a common theme in evolution. Plants often use colorful, scented flowers to attract pollinators that mediate outcross pollination. After an evolutionary shift from outcrossing to self-fertilization, where cross-pollination is no longer necessary, attractive traits may be reduced, especially because these traits may also attract herbivores. Selection may be particularly strong in moth-pollinated lineages where pollinators are also herbivores.

METHODS

We used field surveys and common garden glasshouse experiments to test this hypothesis by quantifying components of flower size and display and floral volatiles in outcrossing vs. selfing populations of the moth-pollinated Pacific coastal dune endemic Abronia umbellata.

KEY RESULTS

Floral face diameter and flower tube length were 43% and 54% smaller in selfing than outcrossing populations, and selfers displayed 15% fewer flowers per umbel. Selfers emitted 99% less total floral volatiles per flower per hour than outcrossers; a much stronger reduction. The chemical composition of volatiles also differed between outcrossers and selfers. Similar differences were observed in a common glasshouse environment, suggesting genetic differentiation in these floral attractive traits among populations. Contrary to expectations, there were no differences in leaf or flower herbivory between selfing and outcrossing populations.

CONCLUSIONS

Floral fragrance is much more dramatically reduced in selfing compared to outcrossing populations than other floral attractive traits, but probably not due to selection exerted by moth herbivory. Reduction in aspects of flower size may be constrained by developmental linkages with fruit and seed size.