Making scents of olfactory neurogenesis.

Olfaction was long considered to belong more to the realm of art than to that of science. As a result, how the brain perceives, discriminates, and recognizes odorant molecules is still a mystery. Recent progress has nonetheless been made at early stages of the olfactory pathway when olfactory studies entered into the molecular era to elucidate the first contact of an odor molecule with a receptor. Our group focuses on the analysis of odor information in the olfactory bulb, the first processing relay in the mammalian brain. Using this model, we are attempting to decipher the code for odorant information. Furthermore, the olfactory bulb also provides an attractive model to investigate neuronal proliferation, differentiation, migration, and neuronal death, processes involving an interplay between genetic and epigenetic influences. Finally, our goal is to explore the possible consequences of the olfactory bulb plasticity, in olfactory performance. For these purposes, we aim to combine morphological, electrophysiological and behavioral approaches to investigate: (1) how the olfactory bulb processes odor molecule information, (2) how neural precursors differentiate into olfactory bulb interneurons, (3) how these newly-generated neurons integrate into an operational neural network, (4) what role they play in the adult olfactory bulb, and (5) how are basic olfactory functions maintained in such a sensory system subjected to continuous renewal of a large percentage of its neuronal population. These questions should provide new fuel for the molecular and cellular bases of sensory perception and shed light onto cellular bases of learning and memory.     

Sparse encoding of natural scents

Natural scents are mixtures of tens to hundreds of individual chemical compounds. While previous studies have investigated how the olfactory bulb responds to simple odorants, how the nose and the brain respond to and interpret complex mixtures is less clear. In a paper in this issue of Neuron, Lin et al. combined gas chromatography and intrinsic signal imaging to examine the responses of individual olfactory bulb glomeruli in the mouse to natural odors and their component parts.     

Making sense of nectar scents: the effects of nectar secondary metabolites on floral visitors of Nicotiana attenuata

Flowers produce a plethora of secondary metabolites but only nectar sugars, floral pigments and headspace volatiles have been examined in the context of pollinator behavior. We identify secondary metabolites in the headspace and nectar of glasshouse- and field-grown Nicotiana attenuata plants, infer within-flower origins by analyzing six flower parts, and compare the attractiveness of 16 constituents in standardized choice tests with two guilds of natural pollinators (Manducasexta moths and Archilochus alexandri and Selasphorus rufus hummingbirds) and one nectar thief (Solenopsis xyloni ants) to determine whether nectar metabolites can 'filter' flower visitors: only two could. Moths responded more strongly than did hummingbirds to headspace presentation of nicotine and benzylacetone, the most abundant repellent and attractant compounds, respectively. For both pollinators, nectar repellents decreased nectaring time and nectar volume removed, but increased visitation number, particularly for hummingbirds. Fewer ants visited if the nectar contained repellents. To determine whether nicotine reduced nectar removal rates in nature, we planted transformed, nicotine-silenced plants into native populations in Utah over 2 years. Plants completely lacking nicotine in their nectar had 68-70% more nectar removed per night by the native community of floral visitors than did wild-type plants. We hypothesize that nectar repellents optimize the number of flower visitors per volume of nectar produced, allowing plants to keep their nectar volumes small.     

Flower scents from the Pacific

For a long time, exotic scents from the islands of the South Pacific have universally been appreciated. Most frequently, fragrant flowers (e.g., frangipani, jasmine sambac, tiaré, pua kenikeni) are used locally for ornamental purposes such as flower garlands (leis). Despite their powerful and delightful fragrance, very few of these flowers have been commercially employed in this part of the world for perfume manufacturing. Creative perfumers are nevertheless strongly interested to better understand these fragrances and to use them, either genuine or artificially reconstituted. Analytical results on the fragrance of these flowers are reported, together with some economical considerations.     

Floral scents of hawkmoth-pollinated flowers in Japan

Similarity among the floral scents of hawkmoth-pollinated plants was investigated with headspace samplings. Six of seven plant species belonging to different families were found to be rich in isoprenoids, among which linalool was the most common compound. Linalool showed rhythmicity with a nocturnal increase inLonicera japonica. These findings suggest that linalool is a common attractant for nocturnal hawkmoths. However, the composition of other isoprenoids, benzenoids and fatty acid derivatives varied markedly among the plant species examined. There was a significant correlation between species composition of flower-visiting hawkmoths and specific floral scents, suggesting that attractiveness to each hawkmoth species is dependent upon floral scent.     

Olfactory coding: random scents make sense

Stimulating arbitrary collections of as few as 300 neurons in the primary olfactory cortex of mice suffices for associative learning independent of any odor stimulation. Thus, programmed spatial relationships may not exist in piriform cortex, making flexible random associations the rule.     

Floral scents of chafer-pollinated asclepiads and a potential hybrid

Floral scent is a key functional trait for pollinator attraction to flowers, but is poorly documented in many plant lineages and pollination systems. In South African grasslands, chafer beetles (Scarabaeidae: Cetoniinae), particularly Atrichelaphinis tigrina, Cyrtothyrea marginalis and Leucoscelis spp., are common floral visitors and specialized pollination by these beetles has recently been established in several asclepiad, orchid and protea species. Chafer beetles are known to be attracted by a variety of floral volatile compounds and scent has been suggested to be an important signal in these chafer-operated pollination systems. In this study, we used dynamic headspace extraction methods and coupled gas chromatography–mass spectrometry (GC–MS) to examine the chemical composition of the floral scents of seven putatively chafer-pollinated asclepiad species in the genera Asclepias, Pachycarpus and Xysmalobium. We identified 15–57 compounds in the scents of these species, of which seven were common to all species examined. The scent profiles of each species separate into discrete clusters in two dimensional space based on non-metric multidimensional scaling (NMDS), indicating clear distinctions between species and suggesting that plants may use different combinations of volatiles to attract beetles. Two plants suspected to be intergeneric hybrids were also examined. Data on pollination systems, morphology and scent chemistry are consistent with the hypothesis that these plants are hybrids between the chafer-pollinated species Asclepias woodii and Pachycarpus concolor. The results of this study are discussed in relation to the role of chafer beetles as generalist pollinators of specialized asclepiads.     

Horse signals: The sounds and scents of fury

Summary During contests animals typically exchange information about fighting ability. Among feral horses these signals involve olfactory or acoustical elements and each type can effectively terminate contests before physical contact becomes necessary. Dung transplant experiments show that for stallions, irrespective of rank, olfactory signals such as dung sniffing encode information about familiarity suggesting that such signals can be used as signatures. As such they can provide indirect information about fighting ability as long as opponents associate identity with past performance. Play-back experiments, however, show that vocalizations, such as squeals, directly provide information about status regardless of stallion familiarity. Sonographs reveal that squeals of dominants are longer than those of subordinates and that only those of dominants have at their onset high-frequency components.     

Crop scents affect the occurrence of trophallaxis among forager honeybees

Previous evidence indicates that the recognition of the nectar delivered by forager honeybees within the colony may have been a primitive method of communication on food resources. Thus, the association between scent and reward that nectar foragers establish while they collect on a given flower species should be retrieved during trophallaxis, i.e., the transfer of liquid food by mouth, and, accordingly, foraging experience could affect the occurrence of these interactions inside the nest. We used experimental arenas to analyze how crop scents carried by donor bees affect trophallaxis among foragers, i.e., donors and receivers, which differ in their foraging experience. Results showed that whenever the foragers had collected unscented sugar solution from a feeder the presence of scents in the solution carried by donors did not affect the occurrence of trophallaxis nor its dynamics. In contrast, whenever the foragers had previous olfactory information, new scents present in the crop of the donors negatively affected the occurrence, but not the dynamics of trophallaxis. Thus, the association learned at the food source seems to be retrieved during trophallaxis, and it is possible that known scents present in the mouthparts of nest-mates may operate as a triggering stimulus to elicit trophallactic behavior within the hive.     

Plant scents modify innate colour preference in foraging swallowtail butterflies

Flower-visiting insects exhibit innate preferences for particular colours. A previous study demonstrated that naive Papilio xuthus females prefer yellow and red, whereas males are more attracted to blue. Here, we demonstrate that the innate colour preference can be modified by olfactory stimuli in a sexually dimorphic manner. Naive P. xuthus were presented with four coloured discs: blue, green, yellow and red. The innate colour preference (i.e. the colour first landed on) of the majority of individuals was blue. When scent from essential oils of either orange flower or lily was introduced to the room, females’ tendency to select the red disc increased. Scents of lavender and flowering potted Hibiscus rosa-sinensis, however, were less effective. Interestingly, the odour of the non-flowering larval host plant, Citrus unshiu, shifted the preference to green in females. In males, however, all plant scents were less effective than in females, such that blue was always the most favoured colour. These observations indicate that interactions between visual and olfactory cues play a more prominent role in females.     

Start making scents: the challenge of integrating chemistry into pollination ecology

Although research on plant volatiles and pollination ecology has grown explosively over the past 15 years, there remains little dialogue between these fields. Here I examine the historical and cultural reasons for this impasse, focusing on the ways that questions in each field are addressed, and the potential for productive cross-talk. The specialization–generalization debate in pollination has cast doubt on the importance of sensory biology in mediating plant–pollinator interactions on the community scale. However, chemical 'filters' of volatile or nectar-borne repellents are likely to explain the absence of specific interactions in plant–pollinator webs. In addition, the omission of plant volatiles from path analyses measuring the relative impacts of herbivores and pollinators on plant fitness may be one reason for large unexplained variance terms in such models. Floral scent functions in concert with visual and gustatory cues by attracting pollinators from a distance, increasing approaches and landings, and mediating outcrossing rates through changes in visitation frequency and duration. All dimensions of floral chemistry, including ontogenetic and diel variation in scent emissions, have the potential to respond to balancing selection between herbivores and pollinators. The available data reveal that chemical aspects of floral phenotypes are important across the specialization–generalization spectrum, and thus are widely applicable to mainstream pollination ecology.     

Vanishing flora--lost chemistry: the scents of endangered plants around the world

As part of our broad and ongoing evaluation of the olfactory components of fragrant plants and flowers during the past 25 years, we have encountered an astounding number of interestingly scented, but endangered plant species. In appreciation of nature's marvels in these species, we are compiling a report on their scent compositions and complementary information in an upcoming book 'Vanishing Flora--Lost Chemistry'. In this paper, a few examples of endangered plant species and their scent components are presented as a brief introduction to the concept of the book project.     

Multisensory integration of colors and scents: insights from bees and flowers

Karl von Frisch’s studies of bees’ color vision and chemical senses opened a window into the perceptual world of a species other than our own. A century of subsequent research on bees’ visual and olfactory systems has developed along two productive but independent trajectories, leaving the questions of how and why bees use these two senses in concert largely unexplored. Given current interest in multimodal communication and recently discovered interplay between olfaction and vision in humans and Drosophila, understanding multisensory integration in bees is an opportunity to advance knowledge across fields. Using a classic ethological framework, we formulate proximate and ultimate perspectives on bees’ use of multisensory stimuli. We discuss interactions between scent and color in the context of bee cognition and perception, focusing on mechanistic and functional approaches, and we highlight opportunities to further explore the development and evolution of multisensory integration. We argue that although the visual and olfactory worlds of bees are perhaps the best-studied of any non-human species, research focusing on the interactions between these two sensory modalities is vitally needed.     

Floral scents produced by Lilium and Cardiocrinum species native to China

Floral scents of thirteen Lilium and one Cardiocrinum species native to China were collected by headspace adsorption during day and night periods and analyzed by gas chromatography and mass spectrometry. There was a positive correlation between volatile emissions and scent intensities. The scented and light-scented Lilium species emitted greater amounts of volatiles at night, and their predominant volatiles were monoterpenoids and benzenoids or monoterpenoids alone. The scent profiles provided useful taxonomic information in section Leucolirion. The major scent compounds of L. regale, L. sulphureum and L. sargentiae were 1,8-cineole (rarely linalool) and methyl benzoate. The closely related Cardiocrinum cathayanum was differentiated from scented Lilium species based on its high emission of (E,E)-α-farnesene and methyl salicylate. L. bakerianum var. delavayi and L. primulinum var. ochraceum emitted the key compound linalool only at night as well as trace quantities of methyl benzoate. Two chemotypes of floral scent were found in L. sargentiae and L. bakerianum var. delavayi, and their differences in volatile compositions were related to the monoterpenoids and benzenoids emissions, respectively. This work provides a basis for advanced studies on the reproductive biology of Lilium species.