Elisa Frasnelli

Left–right asymmetries of behaviour and nervous system in invertebrates

Evidence of left-right asymmetries in invertebrates has begun to emerge, suggesting that lateralization of the nervous system may be a feature of simpler brains as well as more complex ones. A variety of studies have revealed sensory and motor asymmetries in behaviour, as well as asymmetries in the nervous system, in invertebrates. Asymmetries in behaviour are apparent in olfaction (antennal asymmetries) and in vision (preferential use of the left or right visual hemifield during activities such as foraging or escape from predators) in animals as different as bees, fruitflies, cockroaches, octopuses, locusts, ants, spiders, crabs, snails, water bugs and cuttlefish. Asymmetries of the nervous system include lateralized position of specific brain structures (e.g., in fruitflies and snails) and of specific neurons (e.g., in nematodes). As in vertebrates, lateralization can occur both at the individual and at the population-level in invertebrates. Theoretical models have been developed supporting the hypothesis that the alignment of the direction of behavioural and brain asymmetries at the population-level could have arisen as a result of social selective pressures, when individually asymmetrical organisms had to coordinate with each other. The evidence reviewed suggests that lateralization at the population-level may be more likely to occur in social species among invertebrates, as well as vertebrates.

Intraspecific competition and coordination in the evolution of lateralization.

Recent studies have revealed a variety of left–right asymmetries among vertebrates and invertebrates. In many species, left- and right-lateralized individuals coexist, but in unequal numbers (‘population-level’ lateralization). It has been argued that brain lateralization increases individual efficiency (e.g. avoiding unnecessary duplication of neural circuitry and reducing interference between functions), thus counteracting the ecological disadvantages of lateral biases in behaviour (making individual behaviour more predictable to other organisms). However, individual efficiency does not require a definite proportion of left- and right-lateralized individuals. Thus, such arguments do not explain population-level lateralization. We have previously shown that, in the context of prey–predator interactions, population-level lateralization can arise as an evolutionarily stable strategy when individually asymmetrical organisms must coordinate their behaviour with that of other asymmetrical organisms. Here, we extend our model showing that populations consisting of left- and right-lateralized individuals in unequal numbers can be evolutionarily stable, based solely on strategic factors arising from the balance between antagonistic (competitive) and synergistic (cooperative) interactions.

Behavioural and electrophysiological lateralization in a social (Apis mellifera) but not in a non-social (Osmia cornuta) species of bee.

Recent evidence suggests that asymmetry between the left and right sides of the brain is not limited to vertebrates but extends to invertebrates as well. We compared olfactory lateralization in two species of Hymenoptera Apoidea, the honeybee (Apis mellifera), a social species, and the mason bee (Osmia cornuta), a solitary species. Recall of the olfactory memory 1 h after training to associate an odour with a sugar reward, as revealed by the bee extending its proboscis when presented with the trained odour, was better in honeybees trained with their right than with their left antenna. No such asymmetry was observed in mason bees. Similarly, electroantennographic responses to a floral volatile compound and to an alarm pheromone component were higher in the right than in the left antenna in honeybees but not in mason bees. These findings seem to support recent game-theoretical models suggesting that population-level lateralization is more likely to have evolved in social than in non-social species.

Brain and behavioral lateralization in invertebrates

Traditionally, only humans were thought to exhibit brain and behavioral asymmetries, but several studies have revealed that most vertebrates are also lateralized. Recently, evidence of left–right asymmetries in invertebrates has begun to emerge, suggesting that lateralization of the nervous system may be a feature of simpler brains as well as more complex ones. Here I present some examples in invertebrates of sensory and motor asymmetries, as well as asymmetries in the nervous system. I illustrate two cases where an asymmetric brain is crucial for the development of some cognitive abilities. The first case is the nematode Caenorhabditis elegans, which has asymmetric odor sensory neurons and taste perception neurons. In this worm left/right asymmetries are responsible for the sensing of a substantial number of salt ions, and lateralized responses to salt allow the worm to discriminate between distinct salt ions. The second case is the fruit fly Drosophila melanogaster, where the presence of asymmetry in a particular structure of the brain is important in the formation or retrieval of long-term memory. Moreover, I distinguish two distinct patterns of lateralization that occur in both vertebrates and invertebrates: individual-level and population-level lateralization. Theoretical models on the evolution of lateralization suggest that the alignment of lateralization at the population level may have evolved as an evolutionary stable strategy in which individually asymmetrical organisms must coordinate their behavior with that of other asymmetrical organisms. This implies that lateralization at the population-level is more likely to have evolved in social rather than in solitary species. I evaluate this new hypothesis with a specific focus on insects showing different level of sociality. In particular, I present a series of studies on antennal asymmetries in honeybees and other related species of bees, showing how insects may be extremely useful to test the evolutionary hypothesis.

A right antenna for social behaviour in honeybees

Sophisticated cognitive abilities have been documented in honeybees, possibly an aspect of their complex sociality. In vertebrates brain asymmetry enhances cognition and directional biases of brain function are a putative adaptation to social behaviour. Here we show that honeybees display a strong lateral preference to use their right antenna in social interactions. Dyads of bees tested using only their right antennae (RA) contacted after shorter latency and were significantly more likely to interact positively (proboscis extension) than were dyads of bees using only their left antennae (LA). The latter were more likely to interact negatively (C-responses) even though they were from the same hive. In dyads from different hives C-responses were higher in RA than LA dyads. Hence, RA controls social behaviour appropriate to context. Therefore, in invertebrates, as well as vertebrates, lateral biases in behaviour appear to be associated with requirements of social life.

Morpho-functional asymmetry of the olfactory receptors of the honeybee (Apis mellifera).

Lateralization, i.e., the different functional specialisation of the left and right side of the brain, has been documented in many vertebrate species and, recently, in invertebrate species as well. In the Honeybee, Apis mellifera L. (Hymenoptera Apidae), it has been shown that short-term (<1h) recall of olfactory memories would be possible mainly from the right rather than from the left antenna. Here we confirmed this finding showing that recall of the olfactory memory 1h after training to associate (-)-linalool, a floral volatile compound, with a sugar reward, as revealed by the bee extending its proboscis when presented with the trained odour, was better when the odour was presented to the right rather than to the left antenna. We then measured the number of sensilla present on the left and right antenna by scanning electron microscopy. Results showed that putative olfactory sensilla (placodea, trichodea, basiconica) were significantly more abundant on the right antenna surface than on the left antenna surface, whereas sensilla not involved in olfaction (campaniformia, coeloconica and chaetica) tended to be more abundant on the left than on the right antenna surface.

Lateralization in the invertebrate brain: left-right asymmetry of olfaction in bumble bee, Bombus terrestris.

Brain and behavioural lateralization at the population level has been recently hypothesized to have evolved under social selective pressures as a strategy to optimize coordination among asymmetrical individuals. Evidence for this hypothesis have been collected in Hymenoptera: eusocial honey bees showed olfactory lateralization at the population level, whereas solitary mason bees only showed individual-level olfactory lateralization. Here we investigated lateralization of odour detection and learning in the bumble bee, Bombus terrestris L., an annual eusocial species of Hymenoptera. By training bumble bees on the proboscis extension reflex paradigm with only one antenna in use, we provided the very first evidence of asymmetrical performance favouring the right antenna in responding to learned odours in this species. Electroantennographic responses did not reveal significant antennal asymmetries in odour detection, whereas morphological counting of olfactory sensilla showed a predominance in the number of olfactory sensilla trichodea type A in the right antenna. The occurrence of a population level asymmetry in olfactory learning of bumble bee provides new information on the relationship between social behaviour and the evolution of population-level asymmetries in animals.

Response competition associated with right-left antennal asymmetries of new and old olfactory memory traces in honeybees

Lateralized recall of olfactory memory in honeybees was tested, following conditioning of the proboscis extension reflex (PER), at 1 or 6h after training. After training with lemon (+)/vanilla (-) or cineol (+)/eugenol (-) recall at 1h was better when the odour was presented to the right side of the bee than when it was presented to the left side. In contrast, recall at 6h was better when the odour was presented to the left than to the right side. This confirmed previous evidence of shorter-term recall via the right antenna and long-term memory recall via the left antenna. However, when trained with either a familiar appetitive odour (rose) as a negative stimulus, or with a naturally aversive odour (isoamyl acetate, IAA) as a positive stimulus, bees showed suppression of the response from both the right and the left side at 1h after training (likely due to retroactive inhibition) and at 6h responded to both odours on both sides. We argued that at 6h, when access to memory has completed the shift from the right to the left side, memory of these familiar odours in the left side of the brain would be present as both positive (rose)/negative (IAA) (as a result of long-term memory either biologically encoded or acquired well before testing) and negative (rose)/positive (IAA) (as a result of the long-term memory of training) stimuli, thus producing response competition. As a direct test of this hypothesis, bees were first trained with unfamiliar lemon (+)/vanilla (-) and then (16h later) re-trained with vanilla (+)/lemon (-); as predicted, 6h after re-training bees responded to both odours on both the left and right side.

Searching for anatomical correlates of olfactory lateralization in the honeybee antennal lobes: a morphological and behavioural study.

The honeybee, Apis mellifera L. (Hymenoptera: Apidae), has recently become a model for studying brain asymmetry among invertebrates. A strong lateralization favouring the right antenna was discovered in odour learning and short-term memory recall experiments, and a lateral shift favouring the left antenna for long-term memory recall. Corresponding morphological asymmetries have been found in the distribution of olfactory sensilla between the antennae and confirmed by electrophysiological odour response measurements in isolated right and left antennae. The aim of this study was to investigate whether a morphological asymmetry can be observed in the volume of the primary olfactory centres of the central nervous system, the antennal lobes (ALs). Precise volume measurements of a subset of their functional units, the glomeruli, were performed in both sides of the brain, exploiting the advantages of two-photon microscopy. This novel method allowed minimal invasive acquisition of volume images of the ALs, avoiding artefacts from brain extraction and dehydration. The study was completed by a series of behavioural experiments in which response asymmetry in odour recall following proboscis extension reflex conditioning was assessed for odours, chosen to stimulate strong activity in the same glomeruli as in the morphological study. The volumetric measurements found no evidence of lateralization in the investigated glomeruli within the experimental limits. Instead, in the behavioural experiments, a striking odour dependence of the lateralization was observed. The results are discussed on the basis of recent neurophysiological and ethological experiments in A. mellifera.

Difference in Visual Social Predispositions Between Newborns at Low- and High-risk for Autism

Some key behavioural traits of Autism Spectrum Disorders (ASD) have been hypothesized to be due to impairments in the early activation of subcortical orienting mechanisms, which in typical development bias newborns to orient to relevant social visual stimuli. A challenge to testing this hypothesis is that autism is usually not diagnosed until a child is at least 3 years old. Here, we circumvented this difficulty by studying for the very first time, the predispositions to pay attention to social stimuli in newborns with a high familial risk of autism. Results showed that visual preferences to social stimuli strikingly differed between high-risk and low-risk newborns. Significant predictors for high-risk newborns were obtained and an accurate biomarker was identified. The results revealed early behavioural characteristics of newborns with familial risk for ASD, allowing for a prospective approach to the emergence of autism in early infancy.