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The evolution of flowers that offer oils as rewards and are pollinated by specialized bees represents a distinctive theme in plant-pollinator co-diversification. Some plants that offer acetylated glycerols as floral oils emit diacetin, a volatile by-product of oil metabolism, which is utilized by oil-collecting bees as an index signal for the presence of floral oil. However, floral oils in the genus Krameria (Krameriaceae) contain ß-acetoxy-substituted fatty acids instead of acetylated glycerols, making them unlikely to emit diacetin as an oil-bee attractant. We analyzed floral headspace composition from K. bicolor and K. erecta, native to the Sonoran Desert of southwestern North America, in search of alternative candidates for volatile index signals. Using solid-phase microextraction, combined with gas chromatography-mass spectrometry, we identified 26 and 45 floral volatiles, respectively, from whole flowers and dissected flower parts of these two Krameria species. As expected, diacetin was not detected. Instead, ß-ionone emerged as a strong candidate for an index signal, as it was uniquely present in dissected oil-producing floral tissues (elaiophores) of K. bicolor, as well as the larval cells and provisions from its oil-bee pollinator, Centris cockerelli. This finding suggests that the floral oil of K. bicolor is perfused with ß-ionone in its tissue of origin and retains the distinctive raspberry-like scent of this volatile after being harvested by C. cockerelli bees. In contrast, the elaiophores of K. erecta, which are not thought to be pollinated by C. cockerelli, produced a blend of anise-related oxygenated aromatics not found in the elaiophores of K. bicolor. Our findings suggest that ß-ionone has the potential to impact oil-foraging by C. cockerelli bees through several potential mechanisms, including larval imprinting on scented provisions or innate or learned preferences by foraging adults.

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The concept of mimicry has been developed for animals, but it also applies to plants. Plant species may be Müllerian mimics if they have similar reproductive traits and offer similar rewards to the pollinators. Several Oncidiinae orchids offer floral oils to their pollinators and have been suggested to form a Müllerian complex with species of Malpighiaceae. We provide a test of this hypothesis using Gomesa flexuosa (Orchidaceae) and Janusia guaranitica (Malpighiaceae), which are sympatric and phenologically synchronous, secrete the same floral resource (oils), and show similar flower morphology. We exposed individuals of Gomesa near and far from individuals of Janusia and monitored floral visitation. Both species were exclusively pollinated by oil-collecting bees, sharing Centris trigonoides as a pollinator. Nevertheless, the probability of bee visitation, number of contact approximations, and number of visits to flowers of Gomesa were similar when individuals were near and far from Janusia. These findings do not support the Müllerian mimicry hypothesis in these two species. Their resemblances can be better explained by the "exploitation of perceptual biases" hypothesis. According to this hypothesis, pre-existing traits in Oncidiinae orchids (e.g. colour, shape, rewards) may coopt oil-collecting bees that usually search for rewards in Malpighiaceae species with similar flower traits.

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It is plausible that specialized ecological interactions constrain geographic ranges. We address this question in neotropical bees, Centris and Epicharis, that collect oils from flowers of Calceolariaceae, Iridaceae, Krameriaceae, Malpighiaceae, Plantaginaceae, or Solanaceae, with different species exploiting between one and five of these families, which either have epithelial oil glands or hair fields. We plotted the level of oil-host specialization on a clock-dated phylogeny for 22 of the 35 species of Epicharis and 72 of the 230 species of Centris (genera that are not sister genera) and calculated geographic ranges (km(2) ) for 23 bee species based on collection data from museum specimens. Of the oil-offering plants, the Malpighiaceae date to the Upper Cretaceous, whereas the other five families are progressively younger. The stem and crown groups of the two bee genera date to the Cretaceous, Eocene, and Oligocene. Shifts between oil hosts from different families are common in Centris, but absent in Epicharis, and the direction is from flowers with epithelial oil glands to flowers with oil hairs, canalized by bees' oil-collecting apparatuses, suitable for piercing epithelia or mopping oil from hair fields. With the current data, a link between host specialization and geographic range size could not be detected.

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Floral oils as reward to pollinators occur in eleven plant families and appeared at least 28 times in the evolutionary history of flowering plants. They are produced in epithelial or tricomatic glands and collected by oil bee visitors. The present paper focuses on floral-oil-producing species of Plantaginaceae, a Neotropical group namely Angelonia clade. This group comprises around 40 described species in the genera Angelonia, Basistemon, Monttea, Monopera and the oil-less Melosperma. We present a revision of all species of the Angelonia clade, their geographical distribution, resources offered to pollinators and records of flower visitors, especially oil-collecting bees. These plants rely only on oil-collecting species in the tribe Centridini and Tapinotaspidini for a successful pollination, being the interaction between both partners an especial case of bee/flower adaptation in Neotropical region. Some bee species depend only on the oil of Plantaginaceae flowers to survive, while others can collect on several floral oil sources. The pollinating bees explore the oil glands located in sacs using specialized hairs in the forelegs. With this study, we hope to inspire further research relating to this fascinating group of plants, in which most species are rare and occur in highly endangered habitats in South American open vegetation biomes.

Os óleos florais como recompensa a polinizadores estão presentes em onze famílias e surgiram ao menos 28 vezes na história evolutiva das plantas floríferas. Estes são produzidos em glândulas tricomáticas ou epiteliais e coletados por abelhas coletoras de óleo. O presente trabalho foi focado nas espécies produtoras de óleo floral de Plantaginaceae, um grupo neotropical conhecido como clado Angelonia. Este grupo compreende cerca de 40 espécies nos gêneros Angelonia, Basistemon, Monttea, Monopera e Melosperma, que não produz óleo floral. Nós apresentamos uma revisão de todas as espécies no clado Angelonia, sua distribuição geográfica, recursos oferecidos e registros de visitantes florais, especialmente abelhas coletoras de óleo. Estas plantas dependem das abelhas coletoras de óleo das tribos Centridini e Tapinotaspidini para uma polinização bem-sucedida, sendo a interação entre ambos os parceiros um caso especial de adaptação abelha/planta na Região Neotropical. Algumas espécies dependem somente do óleo coletado em espécies do clado Angelonia, enquanto outras também podem coletar em várias fontes de óleos florais. Essas abelhas exploram as glândulas de óleo localizadas em bolsas utilizando pelos especializados nas pernas anteriores. Com o presente trabalho esperamos inspirar estudos futuros com este fascinante grupo de plantas, que são em sua maioria espécies raras e que ocorrem nos altamente ameaçados biomas de vegetação aberta da América do Sul.

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It is reported for the first time oil collecting by bees of the genus Caenonomada on flowers of Plantaginaceae. Females of Caenonomada unicalcarata were observed collecting oil on flowers of Angelonia cornigera, and males and females of Caenonomada bruneri and C. aff. unicalcarata were observed on flowers of Angelonia and Monopera (Plantaginaceae). The record of Caenonomada on Plantaginaceae suggests the use of trichomatic oil glands as a primitive condition in the tribe Tapinotaspidini.

Pela primeira vez é reportado a coleta de óleo por Caenonomada em Plantaginaceae. Fêmeas de Caenonomada unicalcarata foram observadas coletando óleo em flores de Angelonia cornigera, e fêmeas e machos de Caenonomada bruneri e Caenonomada aff. unicalcarata em flores de Angelonia e Monopera (Plantaginaceae). O registro de Caenonomada em Plantaginaceae sugere uma condição mais primitiva para o uso de glândulas tricomáticas de óleo na tribo Tapinotaspidini.

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