Julianne Naomi Pelaez

Plant secondary metabolites pose a challenge for generalist herbivorous insects because they are not only potentially toxic, they also may trigger aversion. On the contrary, some highly specialized herbivorous insects evolved to use these same compounds as ‘token stimuli’ for unambiguous determination of their host plants. Two questions that emerge from these observations are how recently derived herbivores evolve to overcome this aversion to plant secondary metabolites and the extent to which they evolve increased attraction to these same compounds. In this study, we addressed these questions by focusing on the evolution of bitter taste preferences in the herbivorous drosophilid Scaptomyza flava , which is phylogenetically nested deep in the paraphyletic Drosophila . We measured behavioral and neural responses of S. flava and a set of non-herbivorous species representing a phylogenetic gradient ( S. pallida, S. hsui , and D. melanogaster ) towards host- and non-host derived bitter plant compounds. We observed that S. flava evolved a shift in bitter detection, rather than a narrow shift towards glucosinolates, the precursors of mustard-specific defense compounds. In a dye-based consumption assay, S. flava exhibited shifts in aversion toward the non-mustard bitter, plant-produced alkaloids caffeine and lobeline, and reduced aversion towards glucosinolates, whereas the non-herbivorous species each showed strong aversion to all bitter compounds tested. We then examined whether these changes in bitter preferences of S. flava could be explained by changes in sensitivity in the peripheral nervous system and compared electrophysiological responses from the labellar sensilla of S. flava , S. pallida , and D. melanogaster . Using scanning electron microscopy, we also created a map of labellar sensilla in S. flava and S. pallida . We assigned each sensillum to a functional sensilla class based on their morphology and initial response profiles to bitter and sweet compounds. Despite a high degree of conservation in the morphology and spatial placement of sensilla between S. flava and S. pallida , electrophysiological studies revealed that S. flava had reduced sensitivity to glucosinolates to varying degrees. We found this reduction only in I type sensilla. Finally, we speculate on the potential role that evolutionary genetic changes in gustatory receptors between S. pallida and S. flava may play in driving these patterns. Specifically, we hypothesize that the evolution of bitter receptors expressed in I type sensilla may have driven the reduced sensitivity observed in S. flava , and ultimately, its reduced bitter aversion. The S. flava system showcases the importance of reduced aversion to bitter defense compounds in relatively young herbivorous lineages, and how this may be achieved at the molecular and physiological level.

Herbivorous insects are exceptionally diverse, accounting for a quarter of all known eukaryotic species, but the genomic basis of adaptations that enabled this dietary transition remains poorly understood. Many studies have suggested that expansions and contractions of chemosensory and detoxification gene families—genes directly mediating interactions with plant chemical defenses—underlie successful plant colonization. However, this hypothesis has been challenging to test because the origins of herbivory in many insect lineages are ancient (>150 million years ago (mya)), obscuring genomic evolutionary patterns. Here, we characterized chemosensory and detoxification gene family evolution across Scaptomyza, a genus nested within Drosophila that includes a recently derived (Scaptomyza has among the smallest chemosensory and detoxification gene repertoires across 12 drosophilid species surveyed. Rates of gene turnover averaged across the herbivore clade were significantly higher than background rates in over half of the surveyed gene families. However, gene turnover was more limited along the ancestral herbivore branch, with only gustatory receptors and odorant-binding proteins experiencing strong losses. The genes most significantly impacted by gene loss, duplication, or changes in selective constraint were those involved in detecting compounds associated with feeding on living plants (bitter or electrophilic phytotoxins) or their ancestral diet (fermenting plant volatiles). These results provide insight into the molecular and evolutionary mechanisms of plant-feeding adaptations and highlight gene candidates that have also been linked to other dietary transitions in Drosophila.

Herbivorous insects are exceptionally diverse, accounting for a quarter of all known eukaryotic species, but the genetic basis of adaptations that enabled this dietary transition remains poorly understood. Many studies have suggested that expansions and contractions of chemosensory and detoxification gene families – genes directly mediating interactions with plant chemical defenses – underlie successful plant colonization. However, this hypothesis has been challenging to test because the origins of herbivory in many lineages are ancient (>150 million years ago [mya]), obscuring genomic evolutionary patterns. Here, we characterized chemosensory and detoxification gene family evolution across Scaptomyza, a genus nested within Drosophila that includes a recently derived (Scaptomyza have among the smallest chemosensory and detoxification gene repertoires across 12 drosophilid species surveyed. Rates of gene turnover averaged across the herbivore clade were significantly higher than background rates in over half of the surveyed gene families. However, gene turnover was more limited along the ancestral herbivore branch, with only gustatory receptors and odorant binding proteins experiencing strong losses. The genes most significantly impacted by gene loss, duplication, or changes in selective constraint were those involved in detecting compounds associated with feeding on plants (bitter or electrophilic phytotoxins) or their ancestral diet (yeast and fruit volatiles). These results provide insight into the molecular and evolutionary mechanisms of plant-feeding adaptations and highlight strong gene candidates that have also been linked to other dietary transitions in Drosophila .

Herbivorous insects are extraordinarily diverse, yet are found in only one-third of insect orders. This skew may result from barriers to plant colonization, coupled with phylogenetic constraint on plant-colonizing adaptations. The plant-penetrating ovipositor, however, is one trait that surmounts host plant physical defences and may be evolutionarily labile. Ovipositors densely lined with hard bristles have evolved repeatedly in herbivorous lineages, including within the Drosophilidae. However, the evolution and genetic basis of this innovation has not been well studied. Here, we focused on the evolution of this trait inScaptomyza, a genus sister to HawaiianDrosophila, that contains a herbivorous clade. Our phylogenetic approach revealed that ovipositor bristle number increased as herbivory evolved in theScaptomyzalineage. Through a genome-wide association study, we then dissected the genomic architecture of variation in ovipositor bristle number withinS. flava. Top-associated variants were enriched for transcriptional repressors, and the strongest associations included genes contributing to peripheral nervous system development. Individual genotyping supported the association at a variant upstream ofGαi, a neural development gene, contributing to a gain of 0.58 bristles/major allele. These results suggest that regulatory variation involving conserved developmental genes contributes to this key morphological trait involved in plant colonization.

The model organism Drosophila melanogaster has become a focal system for investigations of rapidly evolving genital morphology as well as the development and functions of insect reproductive structures. To follow up on a previous paper outlining unifying terminology for the structures of the male terminalia in this species, we offer here a detailed description of the female terminalia of D. melanogaster. Informative diagrams and micrographs are presented to provide a comprehensive overview of the external and internal reproductive structures of females. We propose a collection of terms and definitions to standardize the terminology associated with the female terminalia in D. melanogaster and we provide a correspondence table with the terms previously used. Unifying terminology for both males and females in this species will help to facilitate communication between various disciplines, as well as aid in synthesizing research across publications within a discipline that has historically focused principally on male features. Our efforts to refine and standardize the terminology should expand the utility of this important model system for addressing questions related to the development and evolution of animal genitalia, and morphology in general.

It has become increasingly clear that the microbiome plays a critical role in shaping the host organism’s response to disease. There also exists mounting evidence that an organism’s ploidy level is important in their response to pathogens and parasites. However, no study has determined if or how these two factors influence one another. We investigate the effect of whole-genome duplication in Arabidopsis thaliana on their above-ground (phyllosphere) microbiome, and determine the interacting impacts of ploidy and the microbiome on disease outcome. Using seven independently derived synthetic auto-tetraploid Arabidopsis accessions, a synthetic leaf-associated bacterial community, and the model pathogen Pseudomonas syringae pv. Tomato DC3000, we confirm that polyploids are generally more resistant to pathogens, but illustrate that this resistance may be in part due to a decrease in the reliance on beneficial bacteria. Polyploids fare better against the pathogen than diploids regardless of microbial inoculation, while we observed that diploids harboring an intact microbiome have lower pathogen densities than those without. We then use RNA sequencing to show that diploids have many more differentially expressed defense-related genes in the presence of their phyllosphere microbiota, while polyploids exhibit constitutively activated defenses regardless of exposure to the synthetic community. These results imply that whole-genome duplication can disrupt historical host-microbiome associations, and suggest that a potential cause or consequence of disruption is a heightened capacity for pathogen defense that is less impacted by the microbiome.

Herbivorous insects are extraordinarily diverse, yet are found in only one-third of insect orders. This skew may result from barriers to plant colonization, coupled with phylogenetic constraint on plant-colonizing adaptations. Physical barriers have been surmounted through the evolution of key morphological innovations, such as the plant-penetrating ovipositor. Despite their significance, the evolution and genetic basis of such innovations have not been well studied. Ovipositors densely lined with hard bristles have evolved repeatedly in herbivorous lineages within the Drosophilidae. Here, we focus on the evolution of this trait in Scaptomyza, an herbivorous radiation nested in a microbe-feeding clade, sister to Hawaiian Drosophila. Our phylogenetic approach revealed that ovipositor bristle number increased as herbivory evolved. We then dissected the genomic architecture of variation in ovipositor bristle number within S. flava through a genome wide association study. Top associated variants were enriched for transcriptional repressors, and the strongest associations included genes contributing to peripheral nervous system development. Genotyping individual flies replicated the association at a variant upstream of Gαi, a neural development gene, contributing to a gain of 0.58 bristles/major allele. These results suggest that regulatory variation involving conserved developmental genes contributes to a key morphological adaptation required for plant colonization.

The wings of Lepidoptera contain a matrix of living cells whose function requires appropriate temperatures. However, given their small thermal capacity, wings can overheat rapidly in the sun. Here we analyze butterfly wings across a wide range of simulated environmental conditions, and find that regions containing living cells are maintained at cooler temperatures. Diverse scale nanostructures and non-uniform cuticle thicknesses create a heterogeneous distribution of radiative cooling that selectively reduces the temperature of structures such as wing veins and androconial organs. These tissues are supplied by circulatory, neural and tracheal systems throughout the adult lifetime, indicating that the insect wing is a dynamic, living structure. Behavioral assays show that butterflies use wings to sense visible and infrared radiation, responding with specialized behaviors to prevent overheating of their wings. Our work highlights the physiological importance of wing temperature and how it is exquisitely regulated by structural and behavioral adaptations.

Three ant species nest obligately in the swollen-thorn domatia of the African ant-plantVachellia(Acacia)drepanolobium, a model system for the study of ant-defence mutualisms and species coexistence. Here we report on the characteristic fungal communities generated by these ant species in their domatia. First, we describe behavioural differences between the ant species when presented with a cultured fungal isolate in the laboratory. Second, we use DNA metabarcoding to show that each ant species has a distinctive fungal community in its domatia, and that these communities remain characteristic of the ant species over two Kenyan sampling locations separated by 190 km. Third, we find that DNA extracted from female alates ofTetraponera penzigiandCrematogaster nigricepscontained matches for most of the fungal metabarcodes from those ant species' domatia, respectively. Fungal hyphae and other debris are also visible in sections of these alates' infrabuccal pockets. Collectively, our results indicate that domatium fungal communities are associated with the ant species occupying the tree. To the best of our knowledge, this is the first record of such ant-specific fungal community-level differences on the same myrmecophytic host species. These differences may be shaped by ant behaviour in the domatia, and by ants vectoring fungi when they disperse to establish new colonies. The roles of the fungi with respect to the ants and their host plant remain to be determined.