Evolution of Insect Olfaction

Neuroethology utilizes a wide range of multidisciplinary approaches to decipher neural correlates of natural behaviors associated with an animal's ecological niche. By placing emphasis on comparative analyses of adaptive and evolutionary trends across species, a neuroethological perspective is uniquely suited to uncovering general organizational and biological principles that shape the function and anatomy of the nervous system. In this review, we focus on the application of neuroethological principles in the study of insect olfaction and discuss how ecological environment and other selective pressures influence the development of insect olfactory neurobiology, not only informing our understanding of olfactory evolution but also providing broader insights into sensory processing

Olfactory Proxy Detection of Dietary Antioxidants in Drosophila

SummaryBackgroundDietary antioxidants play an important role in preventing oxidative stress. Whether animals in search of food or brood sites are able to judge the antioxidant content, and if so actively seek out resources with enriched antioxidant content, remains unclear.ResultsWe show here that the vinegar fly Drosophila melanogaster detects the presence of hydroxycinnamic acids (HCAs)—potent dietary antioxidants abundant in fruit—via olfactory cues. Flies are unable to smell HCAs directly but are equipped with dedicated olfactory sensory neurons detecting yeast-produced ethylphenols that are exclusively derived from HCAs. These neurons are housed on the maxillary palps, express the odorant receptor Or71a, and are necessary and sufficient for proxy detection of HCAs. Activation of these neurons in adult flies induces positive chemotaxis, oviposition, and increased feeding. We further demonstrate that fly larvae also seek out yeast enriched with HCAs and that larvae use the same ethylphenol cues as the adults but rely for detection upon a larval unique odorant receptor (Or94b), which is co-expressed with a receptor (Or94a) detecting a general yeast volatile. We also show that the ethylphenols act as reliable cues for the presence of dietary antioxidants, as these volatiles are produced—upon supplementation of HCAs—by a wide range of yeasts known to be consumed by flies.ConclusionsFor flies, dietary antioxidants are presumably important to counteract acute oxidative stress induced by consumption or by infection by entomopathogenic microorganisms. The ethylphenol pathway described here adds another layer to the fly’s defensive arsenal against toxic microbes

The Cayman Crab Fly Revisited — Phylogeny and Biology of Drosophila endobranchia

BACKGROUND: The majority of all known drosophilid flies feed on microbes. The wide spread of microorganisms consequently mean that drosophilids also can be found on a broad range of substrates. One of the more peculiar types of habitat is shown by three species of flies that have colonized land crabs. In spite of their intriguing lifestyle, the crab flies have remained poorly studied. Perhaps the least investigated of the three crab flies is the Cayman Island endemic Drosophila endobranchia. Apart from its life cycle very little is known about this species, including its phylogenetic position, which has remained unresolved due to a cryptic set of characteristics. PRINCIPAL FINDINGS: Based on molecular data, corroborated by a re-analysis of the morphological make up, we have resolved the phylogenetic position of D. endobranchia and show that it somewhat surprisingly belongs to the large Neotropical repleta radiation, and should be considered as an aberrant member of the canalinea species group. Furthermore we also provide additional data on the behavior of these remarkable flies. CONCLUSION: Our findings reveal that the two Caribbean crab flies are not as distantly related as first thought, as both species are members of the derived repleta radiation. That this lineage has given rise to two species with the same odd type of breeding substrate is curious and prompts the question of what aspects of their shared ancestry has made these flies suitable for a life on (and inside) land crabs. Knowledge of the phylogenetic position of D. endobranchia will allow for comparative explorations and will aid in efforts aimed at understanding processes involved in drastic host shifts and extreme specialization

Towards plant-odor-related olfactory neuroethology in Drosophila

Drosophila melanogaster is today one of the three foremost models in olfactory research, paralleled only by the mouse and the nematode. In the last years, immense progress has been achieved by combining neurogenetic tools with neurophysiology, anatomy, chemistry, and behavioral assays. One of the most important tasks for a fruit fly is to find a substrate for eating and laying eggs. To perform this task the fly is dependent on olfactory cues emitted by suitable substrates as e.g. decaying fruit. In addition, in this area, considerable progress has been made during the last years, and more and more natural and behaviorally active ligands have been identified. The future challenge is to tie the progress in different fields together to give us a better understanding of how a fly really behaves. Not in a test tube, but in nature. Here, we review our present state of knowledge regarding Drosophila plant-odor-related olfactory neuroethology to provide a basis for new progress

The fly nose

This thesis summarizes and discusses the results of four separate studies on fly olfaction. The aim of the thesis has been to investigate how odor information is decoded by the fly peripheral olfactory system, how this code has evolved and how it is used by the insects in their daily life. Here I show that insect odorant receptors (ORs), as indicated by the response profile of olfactory receptor neurons (ORNs), are narrowly tuned to specific compounds. The ligands of the system are volatile chemicals characteristic for favored or rejected traits of preferred or avoided resources. These compounds each carry important information. By relying on key-ligands even a narrowly tuned system comprising a fairly small number of ORs can be used to locate and evaluate a large number of resources in complex odor environments. This conclusion is supported by experimental data contained in the four publications comprising this thesis. In the first study we show that fruitfly ORNs respond most selectively to generic fruit and fungal volatiles, typical for favored characteristics of the flies’ preferred resources. In the second study we show how the olfactory code has evolved among the eight close relatives of D. melanogaster. The evolutionary pattern we observe illustrates how drastic alterations in odor space and food choice can have direct effects on specific ORNs tuned to important key-ligands. The concept of key-ligand tuning is further nicely illustrated in the third and fourth study by the deceptive pollination system of the dead horse arum. This plant copies in remarkable detail a cadaver in order to attract carrion blowflies. Of particular importance is the chemical mimicry, through which the plant copies three specific cadaver volatiles. Our study shows that these three compounds are the sole mean through which blowfly identify carrion, even though carrion produces a large range of volatile chemicals. The odor mimicry is accompanied by further adaptations reinforcing the carrion mimicry, among which heat is most important. Our study provides rare evidence for a direct functional role of plant thermogeny as we show that the generated heat (up to 20°C above ambient temperature) is important for fine tuning the behavior of the flies

Insect Olfaction : Once Swatted, Twice Shy

While some mosquitoes are known to have an innate penchant for human hosts, new research details that they can learn, what they can learn and how they can learn. While some mosquitoes are known to have an innate penchant for human hosts, new research details that they can learn, what they can learn and how they can learn

Evolutionary Genetics : Smells like a Pseudo-pseudogene

A new study reports the presence of a chemosensory pseudogene in Drosophila sechellia, which in spite of carrying a premature stop-codon nevertheless encodes a fully functional and full-length protein. Such ‘pseudo-pseudogenes’ might well be a widespread phenomenon

Mosquito Biology : How a Quest for Water Spawned a Thirst for Blood

The invasive yellow-fever mosquito Aedes aegypti preferentially feeds on human blood. A new study finds that human-biting in this important disease vector might just be an unfortunate side effect of breeding in human-stored water