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The evolution of animal behaviour is poorly understood(1,2). Despite numerous correlations between interspecific divergence in behaviour and nervous system structure and function, demonstrations of the genetic basis of these behavioural differences remain rare(3-5). Here we develop a neurogenetic model, Drosophila sechellia, a species that displays marked differences in behaviour compared to its close cousin Drosophila melanogaster(6,7), which are linked to its extreme specialization on noni fruit (Morinda citrifolia)(8-16). Using calcium imaging, we identify olfactory pathways in D. sechellia that detect volatiles emitted by the noni host. Our mutational analysis indicates roles for different olfactory receptors in long- and short-range attraction to noni, and our cross-species allele-transfer experiments demonstrate that the tuning of one of these receptors is important for species-specific host-seeking. We identify the molecular determinants of this functional change, and characterize their evolutionary origin and behavioural importance. We perform circuit tracing in the D. sechellia brain, and find that receptor adaptations are accompanied by increased sensory pooling onto interneurons as well as species-specific central projection patterns. This work reveals an accumulation of molecular, physiological and anatomical traits that are linked to behavioural divergence between species, and defines a model for investigating speciation and the evolution of the nervous system.

A neurogenetic model, Drosophila sechellia-a relative of Drosophila melanogaster that has developed an extreme specialization for a single host plant-sheds light on the evolution of interspecific differences in behaviour.

Elucidating elementary mechanisms that underlie bacterial diversity is central to ecology(1,2) and microbiome research(3). Bacteria are known to coexist by metabolic specialization(4), cooperation(5) and cyclic warfare(6-8). Many species are also motile(9), which is studied in terms of mechanism(10,11), benefit(12,13), strategy(14,15), evolution(16,17) and ecology(18,19). Indeed, bacteria often compete for nutrient patches that become available periodically or by random disturbances(2,20,21). However, the role of bacterial motility in coexistence remains unexplored experimentally. Here we show that-for mixed bacterial populations that colonize nutrient patches-either population outcompetes the other when low in relative abundance. This inversion of the competitive hierarchy is caused by active segregation and spatial exclusion within the patch: a small fast-moving population can outcompete a large fast-growing population by impeding its migration into the patch, while a small fast-growing population can outcompete a large fast-moving population by expelling it from the initial contact area. The resulting spatial segregation is lost for weak growth-migration trade-offs and a lack of virgin space, but is robust to population ratio, density and chemotactic ability, and is observed in both laboratory and wild strains. These findings show that motility differences and their trade-offs with growth are sufficient to promote diversity, and suggest previously undescribed roles for motility in niche formation and collective expulsion-containment strategies beyond individual search and survival.

In mixed bacterial populations that colonize nutrient patches, a growth-migration trade-off can lead to spatial exclusion that provides an advantage to populations that become rare, thereby stabilizing the community.