| How and why angiosperms self-pollinate is one of the oldest questions in evolutionary biology. Angiosperm mating systems range from obligate xenogamy to obligate autogamy, spanned by a continuum of mixed mating. This dissertation focuses on how mixed mating angiosperms use floral characteristics to balance autogamy and xenogamy through communication with pollinators. To do this, I studied a mixed mating species with extensive floral variation.;Oenothera flava (Onagraceae) is a hawkmoth-pollinated plant found in western North America. It produces yellow flowers that open in the evening and wilt by the following morning. Oenothera flava subsp. taraxacoides flowers are the deepest in North America (≈ 25 cm floral tubes), and petals emit large quantities of volatiles; predominantly aldoximes. Positive herkogamy at dusk disappears by dawn, when the flowers self-pollinate. This subspecies grows in disjunct "sky-island" populations and exhibits variation suggesting repeated evolution toward xenogamy. Oenothera flava subsp. flava is broadly distributed in xeric habitats and has weakly scented, short-tubed (≈ 6 cm) flowers that self-pollinate relatively early.;I tracked floral development to determine whether isometric or allometric scaling best explains differences in floral morphology and found allometry in hypanthium elongation and anther/stigma placement. Allometric growth between neighboring large and small-flowered populations indicates multiple developmental paths to variable floral morphology, supporting the hypothesis of repeated derivations of one or both subspecies.;The floral scent is dominated by two aldoximes, a class of compounds that are common to hawkmoth-pollinated flowers, but are better known as intermediates in the glucosinolate and cyanogenic glucoside defense pathways. In these pathways, they result from the oxidation of an amino acid by a cytochrome P450. Total RNA sequencing from the petals of each subspecies yielded 13,000 contiguous sequences, including a candidate gene significantly similar to an aldoxime-producing Populus trichocarpa Cyp79. Transient expression of this gene in the leaves of Nicotiana benthamiana demonstrated that this gene is sufficient for production of the floral aldoximes. Lastly, a known pollinator, Hyles lineata, demonstrated increased visitation to artificial flowers supplemented with 3-methylbutylaldoxime. These experiments investigate the developmental, molecular and behavioral strategies used by a versatile plant species to generate adaptive variation in floral phenotype; maintaining populations of showy flowers on sky islands and smaller, weakly scented, self-pollinating flowers in xeric habitats. |
