The evolution of olfactory gene families in Drosophila and the genomic basis of chemical-ecological adaptation in Drosophila suzukii

How the evolution of olfactory genes correlates with adaption to new ecological niches is still a debated topic. We explored this issue in Drosophila suzukii, an emergin gmodel that reproduces on fresh fruit rather than in fermenting substrates likemost other Drosophila. We first annotated the repertoire of odorant receptors (ORs), odorant binding proteins (OBPs), and antennal ionotropic receptors (aIRs) in the genomes of two strains of D. suzukii and of its close relative Drosophila biarmipes. We then analyzed these genes on the phylogeny of 14 Drosophila species: whereas ORs and OBPs are characterized by higher turnover rates in some lineages including D. suzukii, aIRs are conserved throughout the genus. Drosophila suzukii is further characterized by a non-random distribution of OR turnover on the gene phylogeny, consistent with a change in selective pressures. In D. suzukii, we found duplications and signs of positive selection in ORs with affinity for short-chain esters, and loss of function of ORs with affinity for volatiles produced during fermentation. These receptors-Or85a and Or22a-are characterized by divergent alleles in the European and American genomes, and we hypothesize that they may have been replaced by some of the duplicated ORs in corresponding neurons, a hypothesis reciprocally confirmed by electrophysiological recordings. Our study quantifies the evolution of olfactory genes in Drosophila and reveals an array of genomic events that can be associated with the ecological adaptations of D. suzukii

Detection of the spotted wing drosophila, Drosophila suzukii, in continental sub-Saharan Africa

The spotted wing drosophila, Drosophila suzukii Matsumura, is an insect pest of soft-skinned fruit, native to Eastern Asia. Since 2008, a world-wide dispersal of D. suzukii is seen, characterized by the establishment of the pest in many Asian, American and European countries. While the potential for invasion of continental Africa by D. suzukii has been predicted, its presence has only been shown for Morocco in Northern Africa. Knowledge about a possible establishment in other parts of the continent is needed as a basis for pest management. In 2019, we carried out a first survey in three counties in Kenya to monitor for the presence of D. suzukii using traps baited with a blend of apple cider vinegar and red wine. A total of 389 D. suzukii flies were captured in a fruit farm at Nakuru county, with more female flies being trapped than males. We confirmed the morphological identification of D. suzukii using DNA barcoding. In 2020, we performed a follow-up survey at 14 locations in six counties to delimit the distribution of D. suzukii in the main berry growing zones in Kenya. The survey indicated that so far D. suzukii is restricted to Nakuru county where it was initially detected. This is the first study to provide empirical evidence of D. suzukii in continental sub-Saharan Africa, confirming that the pest is expanding its geographic range intercontinentally. Given the high dispersal potential of D. suzukii, a concerted effort to develop management strategies is a necessity for containment of the pest

Linking genomics and ecology to investigate the complex evolution of an invasive Drosophila pest

Drosophilid fruit flies have provided science with striking cases of behavioural adaptation and genetic innovation. A recent example is the invasive pest Drosophila suzukii, which, unlike most other Drosophila, lays eggs and feeds on undamaged, ripening fruits. This poses a serious threat for fruit cultivation, but also offers an interesting model to study evolution of behavioural innovation. We developed genome and transcriptome resources for D. suzukii. Coupling analyses of these data with field observations, we propose a hypothesis of the origin of its peculiar ecology. Using nuclear and mitochondrial phylogenetic analyses, we confirm its Asian origin, and reveal a surprising sister relationship between the eugracilis and the melanogaster subgroups. While the D. suzukii genome is comparable in size and repeat content to other Drosophila species, it has the lowest nucleotide substitution rate among the species analysed in this study. This finding is compatible with the overwintering diapause of D. suzukii, which results in a reduced number of generations per year compared to its sister species. Genome-scale relaxed clock analyses support a late Miocene origin of D. suzukii, concomitant with paleogeological and climatic conditions that suggest an adaptation to temperate montane forests, a hypothesis confirmed by field trapping. We propose a causal link between the ecological adaptations of D. suzukii in its native habitat and its invasive success in Europe and North America

Larval response to frass and guaiacol: detection of an attractant produced by bacteria from Spodoptera littoralis frass

Larval frass in herbivorous lepidopterans is mainly composed of plant-derived material and microbes from the gut. Despite the fact that frass from conspecific larvae repels female moths in Spodoptera littoralis from oviposition, the role of frass volatiles on larval foraging behavior is largely unknown. Here, we show that larvae of S. littoralis walk upwind to larval frass volatiles in a wind tunnel assay. We identified the frass volatile guaiacol (2-methoxyphenol) as key ligand for the S. littoralis odorant receptor (OR) SlitOr59 which we expressed heterologously. We isolated guaiacol-producing bacteria identified as Serratia marcescens from frass of larvae that were fed on cotton, and Enterobacter cloacae, E. ludwigii and Klebsiella sp. from frass derived from cabbage-fed larvae. In addition to guaiacol, we also identified volatiles acetoin, 3-methyl-1-butanol and dimethyl disulfide, in large proportions in headspace collections from the bacteria. A Y-tube olfactometer assay showed that fourth instar S. littoralis larvae are attracted to guaiacol. Moreover, cotton leaves treated with the insecticide Spinosad and guaiacol were highly attractive to the larvae. Our results provide a basis for management of the pest by directly targeting larvae, based on an attract-and-kill strategy. Further studies are needed to test the application of guaiacol for semiochemical-based pest management of Spodoptera pest species

Sexual Behavior of Drosophila suzukii

A high reproductive potential is one reason for the rapid spread of Drosophila suzukii in Europe and in the United States. In order to identify mechanisms that mediate mating and reproduction in D. suzukii we studied the fly’s reproductive behavior, diurnal mating activity and sexual maturation. Furthermore, we studied the change of female cuticular hydrocarbons (CHCs) with age and conducted a preliminary investigation on the role of female-derived chemical signals in male mating behavior. Sexual behavior in D. suzukii is characterized by distinct elements of male courtship leading to female acceptance for mating. Time of day and age modulate D. suzukii mating activity. As with other drosophilids, female sexual maturity is paralleled by a quantitative increase in CHCs. Neither female CHCs nor other olfactory signals were required to induce male courtship, however, presence of those signals significantly increased male sexual behavior. With this pilot study we hope to stimulate research on the reproductive biology of D. suzukii, which is relevant for the development of pest management tools

Plant reproductive traits mediate tritrophic feedback effects within an obligate brood-site pollination mutualism

Plants, herbivores and parasitoids affect each other directly and indirectly; however, feedback effects mediated by host plant traits have rarely been demonstrated in these tritrophic interactions. Brood-site pollination mutualisms (e.g. those involving figs and fig wasps) represent specialised tritrophic communities where the progeny of mutualistic pollinators and of non-mutualistic gallers (both herbivores) together with that of their parasitoids develop within enclosed inflorescences called syconia (hence termed brood-sites or microcosms). Plant reproductive phenology (which affects temporal brood-site availability) and inflorescence size (representing brood-site size) are plant traits that could affect reproductive resources, and hence relationships between trees, pollinators and non-pollinating wasps. Analysing wasp and seed contents of syconia, we examined direct, indirect, trophic and non-trophic relationships within the interaction web of the fig-fig wasp community of Ficus racemosa in the context of brood site size and availability. We demonstrate that in addition to direct resource competition and predator-prey (host-parasitoid) interactions, these communities display exploitative or apparent competition and trait-mediated indirect interactions. Inflorescence size and plant reproductive phenology impacted plant-herbivore and plant-parasitoid associations. These plant traits also influenced herbivore-herbivore and herbivore-parasitoid relationships via indirect effects. Most importantly, we found a reciprocal effect between within-tree reproductive asynchrony and fig wasp progeny abundances per syconium that drives a positive feedback cycle within the system. The impact of a multitrophic feedback cycle within a community built around a mutualistic core highlights the need for a holistic view of plant-herbivore-parasitoid interactions in the community ecology of mutualisms

Data from: Enhanced yeast feeding following mating facilitates control of the invasive fruit pest Drosophila suzukii

The highly invasive spotted wing Drosophila Drosophila suzukii is a key pest of soft fruit and berries in Europe and North America, and development of control techniques is an urgent research challenge. Drosophila suzukii is widely associated with the yeast Hanseniaspora uvarum. Yeasts are symbionts of drosophilid flies and communicate with insects through volatile metabolites for spore dispersal. Accordingly, yeasts and behaviour-modifying chemicals produced by yeasts are prospective tools for environmentally sound insect management. We first bioassayed flight attraction, feeding and oviposition of D. suzukii females in response to H. uvarum yeast and blueberries, which are a preferred host fruit. We then investigated the combined effect of yeast and insecticide on adult female oviposition behaviour and mortality towards the development of a yeast-based control method. Following mating, attraction of female flies to blueberry and yeast odour cues was strongly enhanced. Yeast feeding significantly increased in mated females, while yeast did not increase oviposition on blueberries. This observation suggests that mated flies become attracted to yeast for feeding and to fruit for egg laying. A combined feeding–oviposition assay demonstrated different roles and interference between yeast and fruit stimuli: during the day after mating, females laid fewer eggs when yeast was available. The post-mating yeast-feeding response is an opportunity for the development of an attract-and-kill technique for population control of D. suzukii. Exposing flies to a blend of yeast and insecticide reduced oviposition and greatly enhanced adult fly mortality compared with an insecticide treatment alone. Synthesis and applications. Mated females are the key life stage for Drosophila suzukii population control. Egg-laying females perforate fruit skin and fungal infestations ensue, even when eggs and larvae are killed off by insecticide sprays. Behaviour-modifying chemicals, including yeast metabolites, enable environmentally safe insect management via manipulation of olfactory-mediated reproductive behaviour. Our results highlight that yeast and yeast semiochemicals hold potential for D. suzukii management and that response modulation to olfactory stimuli following mating is a vital element for the development of D. suzukii control methods. Yeast feeding is enhanced in mated D. suzukii females, and this change in post-mating behaviour can be exploited by an attract-and-kill strategy, combining a fly-associated yeast with an insecticide. Furthermore, using the D. suzukii yeast mutualist, H. uvarum, may reduce non-target effects and increase species specificity, which further contributes to the development of an efficient and safe control method

Path diagrams representative of the best-fit models for each season.

<p>These path models describe the relationships between various factors for: (a) Season 1 (winter), (b) Season 2 (hot days and cold nights), (c) Season 3 (summer) and (d) Season 4 (wet). Measures of syconium volume, number of pollinators, parasites and seeds per syconium for the path analyses were obtained from 237 syconia in season 1, 340 in season 2, 427 in season 3 and 405 in season 4. The intra-crop variations in syconium inhabitants and syconium volume were described using the standard deviation (SD) in numbers of that inhabitant and syconium volumes amongst syconia within a tree during a particular reproductive episode. All arrows indicate direct relationships between factors. Solid arrows indicate positive relationships and dotted arrows indicate negative relationships. Numbers next to these arrows indicate standardised path coefficients for direct effects. *** p<0.001, ** p<0.01 and >0.001, *p<0.05 and >0.01, n.s.</p

High Temperatures Result in Smaller Nurseries which Lower Reproduction of Pollinators and Parasites in a Brood Site Pollination Mutualism

<div><p>In a nursery pollination mutualism, we asked whether environmental factors affected reproduction of mutualistic pollinators, non-mutualistic parasites and seed production via seasonal changes in plant traits such as inflorescence size and within-tree reproductive phenology. We examined seasonal variation in reproduction in <i>Ficus racemosa</i> community members that utilise enclosed inflorescences called syconia as nurseries. Temperature, relative humidity and rainfall defined four seasons: winter; hot days, cold nights; summer and wet seasons. Syconium volumes were highest in winter and lowest in summer, and affected syconium contents positively across all seasons. Greater transpiration from the nurseries was possibly responsible for smaller syconia in summer. The 3–5°C increase in mean temperatures between the cooler seasons and summer reduced fig wasp reproduction and increased seed production nearly two-fold. Yet, seed and pollinator progeny production were never negatively related in any season confirming the mutualistic fig–pollinator association across seasons. Non-pollinator parasites affected seed production negatively in some seasons, but had a surprisingly positive relationship with pollinators in most seasons. While within-tree reproductive phenology did not vary across seasons, its effect on syconium inhabitants varied with season. In all seasons, within-tree reproductive asynchrony affected parasite reproduction negatively, whereas it had a positive effect on pollinator reproduction in winter and a negative effect in summer. Seasonally variable syconium volumes probably caused the differential effect of within-tree reproductive phenology on pollinator reproduction. Within-tree reproductive asynchrony itself was positively affected by intra-tree variation in syconium contents and volume, creating a unique feedback loop which varied across seasons. Therefore, nursery size affected fig wasp reproduction, seed production and within-tree reproductive phenology via the feedback cycle in this system. Climatic factors affecting plant reproductive traits cause biotic relationships between plants, mutualists and parasites to vary seasonally and must be accorded greater attention, especially in the context of climate change.</p></div

Average daily maximum and minimum values of: (a) Temperature; (b) Relative humidity for each month.

<p>Based on these values, months were grouped into 4 seasons, with 1 denoting the cold season, 2 denoting the season with hot days and cold nights, 3 denoting the hot season and 4 denoting the wet season.</p