Heiko Vogel

Butterfly genome reveals promiscuous exchange of mimicry adaptations among species

The evolutionary importance of hybridization and introgression has long been debated. Hybrids are usually rare and unfit, but even infrequent hybridization can aid adaptation by transferring beneficial traits between species. Here we use genomic tools to investigate introgression in Heliconius, a rapidly radiating genus of neotropical butterflies widely used in studies of ecology, behaviour, mimicry and speciation. We sequenced the genome of Heliconius melpomene and compared it with other taxa to investigate chromosomal evolution in Lepidoptera and gene flow among multiple Heliconius species and races. Among 12,669 predicted genes, biologically important expansions of families of chemosensory and Hox genes are particularly noteworthy. Chromosomal organization has remained broadly conserved since the Cretaceous period, when butterflies split from the Bombyx (silkmoth) lineage. Using genomic resequencing, we show hybrid exchange of genes between three co-mimics, Heliconius melpomene, Heliconius timareta and Heliconius elevatus, especially at two genomic regions that control mimicry pattern. We infer that closely related Heliconius species exchange protective colour-pattern genes promiscuously, implying that hybridization has an important role in adaptive radiation.

Disarming the mustard oil bomb

Plants are attacked by a broad array of herbivores and pathogens. In response, plants deploy an arsenal of defensive traits. In Brassicaceae, the glucosinolate–myrosinase complex is a sophisticated two-component system to ward off opponents. However, this so-called “mustard oil bomb” is disarmed by a glucosinolate sulfatase of a crucifer specialist insect, diamondback moth, Plutella xylostella (Lepidoptera: Plutellidae). Sulfatase activity of this enzyme largely prevents the formation of toxic hydrolysis products arising from this plant defense system. Importantly, the enzyme acts on all major classes of glucosinolates, thus enabling diamondback moths to use a broad range of cruciferous host plants.

The genetic basis of a plant-insect coevolutionary key innovation.

Ehrlich and Raven formally introduced the concept of stepwise coevolution using butterfly and angiosperm interactions in an attempt to account for the impressive biological diversity of these groups. However, many biologists currently envision butterflies evolving 50 to 30 million years (Myr) after the major angiosperm radiation and thus reject coevolutionary origins of butterfly biodiversity. The unresolved central tenet of Ehrlich and Ravens theory is that evolution of plant chemical defenses is followed closely by biochemical adaptation in insect herbivores, and that newly evolved detoxification mechanisms result in adaptive radiation of herbivore lineages. Using one of their original butterfly-host plant systems, the Pieridae, we identify a pierid glucosinolate detoxification mechanism, nitrile-specifier protein (NSP), as a key innovation. Larval NSP activity matches the distribution of glucosinolate in their host plants. Moreover, by using five different temporal estimates, NSP seems to have evolved shortly after the evolution of the host plant group (Brassicales) (≈10 Myr). An adaptive radiation of these glucosinolate-feeding Pierinae followed, resulting in significantly elevated species numbers compared with related clades. Mechanistic understanding in its proper historical context documents more ancient and dynamic plant–insect interactions than previously envisioned. Moreover, these mechanistic insights provide the tools for detailed molecular studies of coevolution from both the plant and insect perspectives.

An ABC Transporter Mutation Is Correlated with Insect Resistance to Bacillus thuringiensis Cry1Ac Toxin

Transgenic crops producing insecticidal toxins from Bacillus thuringiensis (Bt) are commercially successful in reducing pest damage, yet knowledge of resistance mechanisms that threaten their sustainability is incomplete. Insect resistance to the pore-forming Cry1Ac toxin is correlated with the loss of high-affinity, irreversible binding to the mid-gut membrane, but the genetic factors responsible for this change have been elusive. Mutations in a 12-cadherin-domain protein confer some Cry1Ac resistance but do not block this toxin binding in in vitro assays. We sought to identify mutations in other genes that might be responsible for the loss of binding. We employed a map-based cloning approach using a series of backcrosses with 1,060 progeny to identify a resistance gene in the cotton pest Heliothis virescens that segregated independently from the cadherin mutation. We found an inactivating mutation of the ABC transporter ABCC2 that is genetically linked to Cry1Ac resistance and is correlated with loss of Cry1Ac binding to membrane vesicles. ABC proteins are integral membrane proteins with many functions, including export of toxic molecules from the cell, but have not been implicated in the mode of action of Bt toxins before. The reduction in toxin binding due to the inactivating mutation suggests that ABCC2 is involved in membrane integration of the toxin pore. Our findings suggest that ABC proteins may play a key role in the mode of action of Bt toxins and that ABC protein mutations can confer high levels of resistance that could threaten the continued utilization of Bt–expressing crops. However, such mutations may impose a physiological cost on resistant insects, by reducing export of other toxins such as plant secondary compounds from the cell. This weakness could be exploited to manage this mechanism of Bt resistance in the field.

Antennal transcriptome of Manduca sexta

In recent years, considerable progress has been made in understanding the molecular mechanisms underlying olfaction in insects. Because of the diverse nature of the gene families involved, this process has largely relied on genomic data. As a consequence, studies have focused on a small subset of species with extensive genomic information. For Lepidoptera, a large order historically crucial to olfactory research, this circumstance has mostly limited advances to the domesticated species Bombyx mori, with some progress in the noctuid Heliothis virescens based on a nonpublic partial genome database. Because of the limited behavioral repertoire and nonexistent ecological importance of Bombyx, molecular data on the tobacco hornworm Manduca sexta are of utmost importance, especially with regards to its position as a classical olfactory model and its complex natural behavior. Here we present the use of transcriptomic and microarray data to identify members of the main olfactory gene families of Manduca. To assess the quality of our data, we correlate information on expressed receptor genes with detailed morphological data on the antennal lobe. Finally, we compare the expression of the near-complete transcript sets in male and female antennae.

Molecular traces of alternative social organization in a termite genome

Although eusociality evolved independently within several orders of insects, research into the molecular underpinnings of the transition towards social complexity has been confined primarily to Hymenoptera (for example, ants and bees). Here we sequence the genome and stage-specific transcriptomes of the dampwood termite Zootermopsis nevadensis (Blattodea) and compare them with similar data for eusocial Hymenoptera, to better identify commonalities and differences in achieving this significant transition. We show an expansion of genes related to male fertility, with upregulated gene expression in male reproductive individuals reflecting the profound differences in mating biology relative to the Hymenoptera. For several chemoreceptor families, we show divergent numbers of genes, which may correspond to the more claustral lifestyle of these termites. We also show similarities in the number and expression of genes related to caste determination mechanisms. Finally, patterns of DNA methylation and alternative splicing support a hypothesized epigenetic regulation of caste differentiation.

Parallel Evolution of Bacillus thuringiensis Toxin Resistance in Lepidoptera

Despite the prominent and worldwide use of Bacillus thuringiensis (Bt) insecticidal toxins in agriculture, knowledge of the mechanism by which they kill pests remains incomplete. Here we report genetic mapping of a membrane transporter (ABCC2) to a locus controlling Bt Cry1Ac toxin resistance in two lepidopterans, implying that this protein plays a critical role in Bt function.

A comprehensive transcriptome and immune-gene repertoire of the lepidopteran model host Galleria mellonella

BackgroundThe larvae of the greater wax moth Galleria mellonella are increasingly used (i) as mini-hosts to study pathogenesis and virulence factors of prominent bacterial and fungal human pathogens, (ii) as a whole-animal high throughput infection system for testing pathogen mutant libraries, and (iii) as a reliable host model to evaluate the efficacy of antibiotics against human pathogens. In order to compensate for the lack of genomic information in Galleria, we subjected the transcriptome of different developmental stages and immune-challenged larvae to next generation sequencing.ResultsWe performed a Galleria transcriptome characterization on the Roche 454-FLX platform combined with traditional Sanger sequencing to obtain a comprehensive transcriptome. To maximize sequence diversity, we pooled RNA extracted from different developmental stages, larval tissues including hemocytes, and from immune-challenged larvae and normalized the cDNA pool. We generated a total of 789,105 pyrosequencing and 12,032 high-quality Sanger EST sequences which clustered into 18,690 contigs with an average length of 1,132 bases. Approximately 40% of the ESTs were significantly similar (E ≤ e-03) to proteins of other insects, of which 45% have a reported function. We identified a large number of genes encoding proteins with established functions in immunity related sensing of microbial signatures and signaling, as well as effector molecules such as antimicrobial peptides and inhibitors of microbial proteinases. In addition, we found genes known as mediators of melanization or contributing to stress responses. Using the transcriptomic data, we identified hemolymph peptides and proteins induced upon immune challenge by 2D-gelelectrophoresis combined with mass spectrometric analysis.ConclusionHere, we have developed extensive transcriptomic resources for Galleria. The data obtained is rich in gene transcripts related to immunity, expanding remarkably our knowledge about immune and stress-inducible genes in Galleria and providing the complete sequences of genes whose primary structure have only partially been characterized using proteomic methods. The generated data provide for the first time access to the genetic architecture of immunity in this model host, allowing us to elucidate the molecular mechanisms underlying pathogen and parasite response and detailed analyses of both its immune responses against human pathogens, and its coevolution with entomopathogens.

The butterfly plant arms-race escalated by gene and genome duplications

Significance This research uncovers the mechanisms of an ancient arms race between butterflies and plants, seen today in countless gardens as caterpillars of cabbage butterflies that devour cabbage crop varieties. Nearly 90 million years ago, the ancestors of Brassica (mustards, cabbage) and related plants developed a chemical defense called glucosinolates. While very toxic to most insects, humans experience glucosinolates as the sharp taste in wasabi, horseradish and mustard. Here we report that this triggered a chemical arms race that escalated in complexity over time. By investigating the evolutionary histories of these plants and insects, we found that major increases in chemical defense complexity were followed by butterflies evolving countertactics to allow them to continue to attack and feed on the plants. Coevolutionary interactions are thought to have spurred the evolution of key innovations and driven the diversification of much of life on Earth. However, the genetic and evolutionary basis of the innovations that facilitate such interactions remains poorly understood. We examined the coevolutionary interactions between plants (Brassicales) and butterflies (Pieridae), and uncovered evidence for an escalating evolutionary arms-race. Although gradual changes in trait complexity appear to have been facilitated by allelic turnover, key innovations are associated with gene and genome duplications. Furthermore, we show that the origins of both chemical defenses and of molecular counter adaptations were associated with shifts in diversification rates during the arms-race. These findings provide an important connection between the origins of biodiversity, coevolution, and the role of gene and genome duplications as a substrate for novel traits.

Paleopolyploidy in the Brassicales: Analyses of the Cleome Transcriptome Elucidate the History of Genome Duplications in Arabidopsis and Other Brassicales

The analysis of the Arabidopsis genome revealed evidence of three ancient polyploidy events in the evolution of the Brassicaceae, but the exact phylogenetic placement of these events is still not resolved. The most recent event is called the At-α (alpha) or 3R, the intermediate event is referred to as the At-β (beta) or 2R, and the oldest is the At-γ (gamma) or 1R. It has recently been established that At-γ is shared with other Rosids, including papaya (Carica), poplar (Populus), and grape (Vitis), whereas data to date suggest that At-α is Brassicaceae specific. To address more precisely when the At-α and At-β events occurred and which plant lineages share these paleopolyploidizations, we sequenced and analyzed over 4,700 normalized expressed sequence tag sequences from the Cleomaceae, the sister family to the Brassicaceae. Analysis of these Cleome data with homologous sequences from other Rosid genomes (Arabidopsis, Carica, Gossypium, Populus, and Vitis) yielded three major findings: 1) confirmation of a Cleome-specific paleopolyploidization (Cs-α) that is independent of the Brassicaceae At-α paleopolyploidization; 2) Cleome and Arabidopsis share the At-β duplication, which is lacking from papaya within the Brassicales; and 3) rates of molecular evolution are faster for the herbaceous annual taxa Arabidopsis and Cleome than the other predominantly woody perennial Rosid lineages. These findings contribute to our understanding of the dynamics of genome duplication and evolution within one of the most comprehensively surveyed clades of plants, the Rosids, and clarify the complex history of the At-α, At-β, and At-γ duplications of Arabidopsis.