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The obligate mutualism and exquisite specificity of many plant-pollinator interactions lead to the expectation that flower phenotypes (e.g., corolla tube length) and corresponding pollinator traits (e.g., hawkmoth proboscis length) are congruent as a result of coevolution by natural selection. However, the effect of variation in flower morphology on the fitness of plants and their pollinators has not been quantified systematically. In this study, we employed the theoretical morphospace paradigm using a combination of 3D printing, electronic sensing, and machine vision technologies to determine the influence of two flower morphological features (corolla curvature and nectary diameter) on the fitness of both parties: the artificial flower and its hawkmoth pollinator. Contrary to the expectation that the same flower morphology maximizes the fitness of both plant and pollinator, we found that the two parties have divergent optima for corolla curvature, with non-overlapping fitness peaks in flower morphospace. The divergent fitness optima between plants and pollinators could lead to evolutionary diversification in both groups. [ABSTRACT FROM AUTHOR]

PREMISE OF THE STUDY:: Pollinator‐mediated selection on flower phenotypes (e.g., shape, color, scent) is key to understanding the adaptive radiation of angiosperms, many of which have evolved specialized relationships with a particular guild of animal pollinators (e.g., birds, bats, moths, bees). E‐β‐Ocimene, a monoterpene produced by OCIMENE SYNTHASE (OS) in Mimulus lewisii, is a floral scent important in attracting the species’ bumblebee pollinators. The taxa closely related to M. lewisii have evolved several different pollination syndromes, including hummingbird pollination and self pollination (autogamy). We are interested in how floral scent variation contributed to species diversification in this clade. METHODS:: We analyzed variation in E‐β‐ocimene emission within this Mimulus clade and explored its molecular basis through a combination of DNA sequencing, reverse transcriptase PCR, and enzyme functional analysis in vitro. KEY RESULTS:: We found that none of the taxa, other than M. lewisii, emitted E‐β‐ocimene from flowers. But the molecular basis underlying loss of E‐β‐ocimene emission is unique in each taxon, including deletion, missense, or frameshift mutations in the OS gene, and potential posttranscriptional downregulation. CONCLUSIONS:: The molecular evidence suggests that parallel loss‐of‐function in OS is the best explanation for the observed pattern of E‐β‐ocimene emission, likely as the result of natural selection. [ABSTRACT FROM AUTHOR]

Flowering plants employ a wide variety of signals, including scent, to attract the attention of pollinators. In this study we investigated the role of floral scent in mediating differential attraction between two species of monkeyflowers (Mimulus) reproductively isolated by pollinator preference. The emission rate and chemical identity of floral volatiles differ between the bumblebee-pollinated Mimulus lewisii and the hummingbird-pollinated M. cardinalis. Mimulus lewisii flowers produce an array of volatiles dominated by D-limonene, β-myrcene and E-β-ocimene. Of these three monoterpenes, M. cardinalis flowers produce only D-limonene, released at just 0.9% the rate of M. lewisii flowers. Using the Bombus vosnesenskii bumblebee, an important pollinator of M. lewisii, we conducted simultaneous gas chromatography with extracellular recordings in the bumblebee antennal lobe. Results from these experiments revealed that these three monoterpenes evoke significant neural responses, and that a synthetic mixture of the three volatiles evokes the same responses as the natural scent. Furthermore, the neural population shows enhanced responses to the M. lewisii scent over the scent of M. cardinalis. This neural response is reflected in behavior; in two-choice assays, bumblebees investigate artificial flowers scented with M. lewisii more frequently than ones scented with M. cardinalis, and in synthetic mixtures the three monoterpenes are necessary and sufficient to recapitulate responses to the natural scent of M. lewisii. In this system, floral scent alone is sufficient to elicit differential visitation by bumblebees, implying a strong role of scent in the maintenance of reproductive isolation between M. lewisii and M. cardinalis. [ABSTRACT FROM AUTHOR]

The role of major mutations in adaptive evolution has been debated for more than a century. The classical view is that adaptive mutations are nearly infinite in number with infinitesimally small phenotypic effect, but recent theory suggests otherwise. To provide empirical estimates of the magnitude of adaptive mutations in wild plants, we conducted field studies to determine the adaptive value of alternative alleles at a single locus, YELLOW UPPER (YUP). YUP controls the presence or absence of yellow carotenoid pigments in the petals of pink-flowered Mimulus lewisii, which is pollinated by bumblebees, and its red-flowered sister species M. cardinalis, which is pollinated by hummingbirds. We bred near-isogenic lines (NILs) in which the YUP allele from each species was substituted into the other. M. cardinalis NILs with the M. lewisii YUP allele had dark pink flowers and received 74-fold more bee visits than the wild type, whereas M. lewisii NILs with the M. cardinalis yup allele had yellow-orange flowers and received 68-fold more hummingbird visits than the wild type. These results indicate that an adaptive shift in pollinator preference may be initiated by a single major mutation. [ABSTRACT FROM AUTHOR]