A candidate locus for variation in dispersal rate in a butterfly metapopulation

Frequent extinctions of local populations in metapopulations create opportunities for migrant females to establish new populations. In a metapopulation of the Glanville fritillary butterfly (Melitaea cinxia), more mobile individuals are more likely to establish new populations, especially in habitat patches that are poorly connected to existing populations. Here we show that flight metabolic rate and the frequency of a specific allele of the metabolic enzyme phosphoglucose isomerase (pgi) were both highest in newly established, isolated populations. Furthermore, genotypes with this pgi allele had elevated flight metabolic rates. These results suggest that genetic variation in pgi or a closely linked locus has a direct effect on flight metabolism, dispersal rate, and thereby on metapopulation dynamics in this species. These results also contribute to an emerging understanding of the mechanisms by which population turnover in heterogeneous landscapes may maintain genetic and phenotypic variation across populations.

Molecular-level variation affects population growth in a butterfly metapopulation

The dynamics of natural populations are thought to be dominated by demographic and environmental processes with little influence of intraspecific genetic variation and natural selection, apart from inbreeding depression possibly reducing population growth in small populations. Here we analyse hundreds of well-characterised local populations in a large metapopulation of the Glanville fritillary butterfly (Melitaea cinxia), which persists in a balance between stochastic local extinctions and recolonisations in a network of 4,000 discrete habitat patches. We show that the allelic composition of the glycolytic enzyme phosphoglucose isomerase (Pgi) has a significant effect on the growth of local populations, consistent with previously reported effects of allelic variation on flight metabolic performance and fecundity in the Glanville fritillary and Colias butterflies. The strength and the sign of the molecular effect on population growth are sensitive to the ecological context (the area and spatial connectivity of the habitat patches), which affects genotype-specific gene flow and the influence of migration on the dynamics of local populations. The biological significance of the results for Pgi is underscored by lack of any association between population growth and allelic variation at six other loci typed in the same material. In demonstrating, to our knowledge for the first time, that molecular variation in a candidate gene affects population growth, this study challenges the perception that differential performance of individual genotypes, leading to differential fitness, is irrelevant to population dynamics. These results also demonstrate that the spatial configuration of habitat and spatial dynamics of populations contribute to maintenance of Pgi polymorphism in this species.

Researchers observe a link between the genetic composition of the Pgi locus (an enzyme linked with flight metabolic performance) and local population growth within a metapopulation of butterflies.

Inhibition of CYP6B1-mediated detoxification of xanthotoxin by plant allelochemicals in the black swallowtail (Papilio polyxenes)

The structural and biosynthetic diversity of allelochemicals in plants is thought to arise from selection for additive toxicity as a consequence of toxin mixture or for enhanced toxicity as a result of synergism. In order to understand how insects cope with this type of plant defense, we tested the effects of some allelochemicals in host plants of the black swallowtail Papilio polyxenes on the xanthotoxin-metabolic activity of CYP6B1, the principal enzyme responsible for the detoxification of furanocoumarins in this caterpillar. Additionally, the effects of some synthetic compounds not normally encountered by P. polyxenes on CYP6B1 were tested. These studies demonstrate that the integrity of furanocoumarin structure is important for competitive binding to the active site of CYP6B1, even though the carbonyl group on the pyranone ring apparently does not affect its inhibitory capacity, as in the case of furanochromones. Angular furanocoumarins are generally less phototoxic to many organisms than linear furanocoumarins due to their reduced capacity for cross-linking DNA strands, yet they are more toxic than linear furanocoumarins to black swallowtail larvae. This enhanced toxicity in vivo may be due to the ability of angular furanocoumarins to bind to the active site of CYP6B1 without being rapidly metabolized. This binding reduces the availability of CYP6B1 to metabolize other linear furanocoumarins. The structure-activity relationships for methylenedioxyphenyl compounds, flavonoids, imidazole, and imidazole derivatives are also discussed in light of their capacity to inhibit the xanthotoxin-metabolic activity of CYP6B1.

From DNA to fitness differences: sequences and structures of adaptive variants of Colias phosphoglucose isomerase (PGI)

Colias eurytheme butterflies display extensive allozyme polymorphism in the enzyme phosphoglucose isomerase (PGI). Earlier studies on biochemical and fitness effects of these genotypes found evidence of strong natural selection maintaining this polymorphism in the wild. Here we analyze the molecular features of this polymorphism by sequencing multiple alleles and modeling their structures. PGI is a dimer with rotational symmetry. Each monomer provides a critical residue to the other monomer's catalytic center. Sequenced alleles differ at multiple amino acid positions, including cryptic charge-neutral variation, but most consistent differences among the electromorph alleles are at the charge-changing amino acid sites. Principal candidate sites of selection, identified by structural and functional analyses and by their variants' population frequencies, occur in interpenetrating loops across the interface between monomers, where they may alter subunit interactions and catalytic center geometry. Comparison to a second (and basal) species, Colias meadii, also polymorphic for PGI under natural selection, reveals one fixed amino acid difference between their PGIs, which is located in the interpenetrating loop and accompanies functional differences among their variants. We also study nucleotide variability among the PGI alleles, comparing these data to similar data from another glycolytic enzyme gene, glyceraldehyde-3-phosphate dehydrogenase. Despite extensive nonsynonymous and synonymous polymorphism at PGI in each species, the only base changes fixed between species are the two causing the amino acid replacement; this absence of synonymous fixation yields a significant McDonald-Kreitman test. Analyses of these data suggest historical population expansion. Positive peaks of Tajima's D statistic, representing regions of neutral "hitchhiking," are found around the principal candidate sites of selection. This study provides novel views of molecular-structural mechanisms, and beginnings of historical evidence, for a long-persistent balanced enzyme polymorphism at PGI in these and perhaps other species.

Disjunct distributions during glacial and interglacial periods in mountain butterflies: Erebia epiphron as an example

Over several decades, the distribution patterns and evolution of alpine disjunct species has become an increasingly discussed subject. Large scale genetic analysis has allowed the resolution of the past range changes and intraspecific evolution of many species, in Europe especially of Mediterranean origin. However, the phylogeographic structures of species with arctic-alpine disjunct distribution patterns are relatively poorly studied. The existing phylogeographic analysis (mostly of alpine plant species) supports disjunct distributions during glacial as well as post-glacial periods for a number of species. However, several questions still remain unresolved and we therefore analysed the Mountain Ringlet Erebia epiphron as a model for such alpine disjunct species. We found strong differentiation into five different lineages supporting five differentiation centres: (i) the eastern Pyrenees, (ii) the mountain ranges between the central Pyrenees and south-western Alps, (iii, iv) two areas along the southern Alps margin and (v) the northern Alps margin. We propose that these patterns evolved due to the humidity requirements of this species, which did not allow survival in the dry glacial steppes, but along the margins of the wetter glaciated high mountain ranges.

Expression of one isoform of GTP cyclohydrolase I coincides with the larval black markings of the swallowtail butterfly, Papilio xuthus

The larva of the swallowtail butterfly Papilio xuthus changes its body markings during the fourth ecdysis. We found that stage-specific cuticular black markings are mainly regulated by co-localization of two melanin synthesis enzymes; tyrosine hydroxylase (TH) and dopa decarboxylase (DDC). TH converts tyrosine to dihydroxyphenylalanine (dopa), and tyrosine itself is converted from phenylalanine by phenylalanine hydroxylase (PAH). Guanosine triphosphate cyclohydrolase I (GTPCHI) is essential for the synthesis of tetrahydrobiopterin (BH4) that is a cofactor of TH and PAH. In this report, we found that a GTPCHI inhibitor prevents pigmentation in cultured integuments, suggesting that the GTPCHI activity is also involved in cuticle pigmentation. We have cloned GTPCHI and PAH cDNAs from P. xuthus and investigated their spatial expression patterns in epidermis by whole-mount in situ hybridization. There are two isoforms of GTPCHI in larval epidermis (GTPCHIa and GTPCHIb). GTPCHIa is expressed at the black markings of the subsequent instar, similar to TH, whereas GTPCHIb is expressed uniformly, similar to PAH. This suggests that the region-specific expression of GTPCHIa supplies sufficient BH(4) reinforcing the TH activity in black marking area. Our results imply that larval markings are regulated by not only melanin synthesis enzymes but also the cofactor supplying enzyme.

Melanin-synthesis enzymes coregulate stage-specific larval cuticular markings in the swallowtail butterfly, Papilio xuthus

Like the adult wing, butterfly larvae are unique in their coloring. However, the molecular mechanisms underlying the formation of insect larval color patterns are largely unknown. The larva of the swallowtail butterfly Papilio xuthus changes its color pattern markedly during the 4th ecdysis. We investigated its cuticular color pattern, which is thought to be composed of melanin and related pigments derived from tyrosine. We cloned three enzymes involved in the melanin-synthesis pathway in P. xuthus: tyrosine hydroxylase (TH), dopa decarboxylase (DDC), and ebony. Whole-mount in situ hybridization showed that the expression of both TH and DDC is strongly correlated with the black markings. ebony is strongly expressed only in the reddish-brown area. The expression pattern of each enzyme coincides with the cuticular color pattern of the subsequent instar. We also investigated the uptake of melanin precursors into cultured integument. Inhibition of either TH or DDC activity prevents in vitro pigmentation completely. Addition of dopamine to integuments in the presence of TH inhibitor causes overall darkening without specific markings. From these results, specific larval cuticular color patterns are regulated by stage-specific colocalization of enzymes in epidermal cells rather than by the differential uptake of melanin precursors into individual epidermal cells. Epidermal cells expressing TH and DDC, but not ebony, produce the black cuticle, and epidermal cells expressing TH, DDC, and ebony produce the reddish-brown cuticle.

Phylogenetic relationships of the New World Troidini swallowtails (Lepidoptera: Papilionidae) based on COI, COII, and EF-1α genes

A phylogeny of the Neotropical members of the Tribe Troidini (Lepidoptera: Papilionidae) was obtained with sequences of three protein-coding genes: two mitochondrial (COI and COII), and one nuclear (EF-1alpha). Parsimony and Bayesian analyses of 33 taxa resulted in very similar trees regardless of method used with the 27 troidines always forming a monophyletic clade. Within Troidini, the genus Battus is sister group to the remaining troidines, followed by a clade formed by the Paleotropical taxa (here represented by three exemplars). The genus Euryades is the next branch, and sister group of Parides. The genus Parides is monophyletic, and is divided into four main groups by Maximum Parsimony analysis, with the most basal group composed of tailed species restricted to SE Brazil. Character optimization of ecological and morphological traits over the phylogeny proposed for troidines indicated that the use of several species of Aristolochia is ancestral over the use of few or a single host-plant. For the other three characters, the ancestral states were the absence of long tails, forest as the primary habitat and oviposition solitary or in loose group of several eggs.

Regulation of an insect cytochrome P450 monooxygenase gene (CYP6B1) by aryl hydrocarbon and xanthotoxin response cascades

Many organisms respond to toxic compounds in their environment by inducing regulatory networks controlling the expression and activity of cytochrome P450 monooxygenase (P450s) detoxificative enzymes. In particular, black swallowtail (Papilio polyxenes) caterpillars respond to xanthotoxin, a toxic phytochemical in their hostplants, by activating transcription of the CYP6B1 promoter via several regions located within 150 nt of the transcription initiation site. One such element is the xenobiotic response element to xanthotoxin (XRE-Xan) that lies upstream of consensus XRE-AhR (xenobiotic response element to the aryl hydrocarbon receptor) and OCT-1 (octamer-1 binding site) element known to be utilized in mammalian aryl hydrocarbon response cascades. Two-plasmid transfections conducted in Sf9 cells have indicated that XRE-Xan, XRE-AhR and a number of other proximal elements, but not OCT-1, are critical for basal as well as xanthotoxin- and benzo[alpha]pyrene-induced transcription of the CYP6B1 promoter. Four-plasmid transfections with vectors co-expressing the spineless (Ss) and tango (Tgo) proteins, the Drosophila melanogaster homologues of mammalian AhR and ARNT, have indicated that these proteins enhance basal expression of the CYP6B1 promoter but not the magnitude of its xanthotoxin and benzo[alpha]pyrene induction. Based on these results, we propose that these Drosophila transcription factors modulate basal expression of this promoter in a ligand-independent manner and attenuate its subsequent responses to planar aryl hydrocarbons (benzo[alpha]pyrene) and allelochemicals (xanthotoxin).

Identification of cytochrome P450 and glutathione-S-transferase genes preferentially expressed in chemosensory organs of the swallowtail butterfly, Papilio xuthus L

The swallowtail butterfly, Papilio xuthus L., feeds exclusively on members of the plant family, Rutaceae. Female butterflies lay eggs in response to specific chemicals contained in their host plants. They perceive a variety of polar compounds as oviposition stimulants through the tarsal chemosensilla of the foreleg by drumming upon the leaf surface. We undertook an expressed sequence tag (EST) analysis to identify the chemosensory-related genes that are expressed in chemosensilla on the tarsus of P. xuthus. Several genes that showed similarity with biotransformation enzymes were identified from the ESTs. Among them, a cytochrome P450 and a glutathione-S-transferase (GST) were preferentially expressed in the chemosensory organs. We have determined the structure of both cDNA and genomic sequences encoding these enzymes and designated the P450 as CYP341A2, a novel member of CYP341A subfamily, and the GST as GST-pxcs1, respectively. We observed a localized expression of CYP341A2 at the base of tarsal chemosensilla by in situ hybridization. These results suggest that these degrading enzymes play a role in the chemosensory reception for host plant recognition.

Ile115Leu mutation in the SRS1 region of an insect cytochrome P450 (CYP6B1) compromises substrate turnover via changes in a predicted product release channel

CYP6B1 represents the principal cytochrome P450 monooxygenase responsible for metabolizing furanocoumarins in Papilio polyxenes, an insect that specializes on host plants containing these toxins. Investigations of the amino acids responsible for the efficient metabolism of these plant toxins has identified Ile115 as one that modulates the rate of furanocoumarin metabolism even though it is predicted to be positioned at the edge of the heme plane and outside substrate contact regions. In contrast to previous expression studies conducted under conditions of limiting P450 reductase showing that the Ile115-to-Leu replacement enhances turnover of xanthotoxin and other furanocoumarins, studies conducted at high P450 reductase indicate that the Ile115-to-Leu replacement reduces turnover of these substrates. Further analysis of substrate binding affinities, heme spin state and NADPH consumption rates indicate that, whereas the I115L replacement mutant displays higher substrate affinity and heme spin state than the wild-type CYP6B1 protein, it utilizes NADPH more slowly than the wild-type CYP6B1 protein at high P450 reductase levels. Molecular models developed for the wild-type CYP6B1 and mutant protein suggest that more constricted channels extending from the catalytic site in the I115L mutant to the P450 surface limit the rate of product release from this mutant catalytic site under conditions not limited by the rate of electron transfer from NADPH.

Enzymatic properties of pierisin-1 and its N-terminal domain, a guanine-specific ADP-ribosyltransferase from the cabbage butterfly

The cabbage butterfly, Pieris rapae, produces an ADP-ribosylating cytotoxic protein, pierisin-1. Unlike other ADP-ribosylating toxins, the acceptor site for ADP-ribosylation by pierisin-1 is the N-2 position of guanine bases in DNA. The present study was designed to characterize this novel guanine-specific ADP-ribosyltransferase, pierisin-1. The N-terminal polypeptide from Met-1 to Arg-233, but not the C-terminal Ser-234-Met-850 polypeptide, was found to exhibit guanine ADP-ribosyltransferase activity. Trypsin-treated pierisin-1, which is considered to be a "nicked" full-length form composed of associated N- and C-terminal fragments, also demonstrated such activity. Optimum conditions for the N-terminal polypeptide of pierisin-1 were pH 8-10, 37-40 degrees C, in the presence of 100-200 mM NaCl or KCl. Other metal ions such as Ca(2+) or Mg(2+) were not required. Kinetic studies demonstrated potent ADP-ribosyltransferase activity with a K(M) value for NAD of 0.17 mM and k(cat) of 55 per second. Under these optimum conditions, the specific activity of trypsin-treated pierisin-1 was about half (k(cat) = 25 per second). When the conditions were changed to pH 5-7 or 10-20 degrees C, some activity (6-55% or 5-20%, respectively, of that under optimal conditions) of the N-terminal polypeptide was still evident; however, almost all of the trypsin-treated enzyme activity disappeared. This implies the inhibition of the N-terminal enzyme domain by the associated C-terminal fragment. Long-term reactions indicated that a single molecule of pierisin-1 has the capacity to generate more than 10(6) ADP-ribosylated DNA adducts, which could cause the death of a mammalian cell.

Conserved regulatory elements in the promoters of two allelochemical-inducible cytochrome P450 genes differentially regulate transcription

CYP6B4, a cytochrome P450 gene from the tiger swallowtail Papilio glaucus, is transcriptionally induced in the midgut by dietary furanocoumarins, plant allelochemicals that can crosslink DNA in their UV-activated form. The CYP6B4 promoter contains an overlapping EcRE/ARE/XRE-xan element similar to that used for basal and xanthotoxin-inducible expression of the CYP6B1 promoter from the black swallowtail Papilio polyxenes. Transfection of the CYP6B4 promoter:CAT reporter construct into Sf9 cells demonstrates that the basal and xanthotoxin-inducible expression levels observed reflect the relative expression levels of this gene in the midguts of tiger swallowtail larvae. Transfections of mutant CYP6B4 promoter constructs into Sf9 cells indicate that the EcRE/ARE/XRE-xan element is necessary for CYP6B4 induction by xanthotoxin but not for its minimal basal expression. In addition to these elements, the CYP6B4 and CYP6B1 promoters also contain putative XRE-AhR elements identical to the aryl hydrocarbon response elements present in mammalian phase I detoxification genes. Transfections of CYP6B4 and CYP6B1 promoters containing EcRE/ARE/XRE-xan and XRE-AhR elements indicate that both are induced significantly by benzo(alpha)pyrene, an aryl hydrocarbon widespread in the environment, as well as by xanthotoxin, an allelochemical encountered in their hostplants.

Purification and properties of isocitrate lyase from pupas of the butterfly Papilio machaon L

Key enzymes of the glyoxylate cycle, isocitrate lyase and malate synthase, were identified in pupas of the butterfly Papilio machaon L. The activities of these enzymes in pupas were 0.056 and 0.108 unit per mg protein, respectively. Isocitrate lyase was purified by a combination of various chromatographic steps including ammonium sulfate fractionation, ion-exchange chromatography on DEAE-Toyopearl, and gel filtration. The specific activity of the purified enzyme was 5.5 units per mg protein, which corresponded to 98-fold purification and 6% yield. The enzyme followed Michaelis-Menten kinetics (Km for isocitrate, 1.4 mM) and was competitively inhibited by succinate (Ki = 1.8 mM) and malate (Ki = 1 mM). The study of physicochemical properties of the enzyme showed that it is a homodimer with a subunit molecular weight of 68 +/- 2 kD and a pH optimum of 7.5 (in Tris-HCl buffer).

Diversification of furanocoumarin-metabolizing cytochrome P450 monooxygenases in two papilionids: Specificity and substrate encounter rate

Diversification of cytochrome P450 monooxygenases (P450s) is thought to result from antagonistic interactions between plants and their herbivorous enemies. However, little direct evidence demonstrates the relationship between selection by plant toxins and adaptive changes in herbivore P450s. Here we show that the furanocoumarin-metabolic activity of CYP6B proteins in two species of swallowtail caterpillars is associated with the probability of encountering host plant furanocoumarins. Catalytic activity was compared in two closely related CYP6B4 and CYP6B17 groups in the polyphagous congeners Papilio glaucus and Papilio canadensis. Generally, P450s from P. glaucus, which feeds occasionally on furanocoumarin-containing host plants, display higher activities against furanocoumarins than those from P. canadensis, which normally does not encounter furanocoumarins. These P450s in turn catalyze a larger range of furanocoumarins at lower efficiency than CYP6B1, a P450 from Papilio polyxenes, which feeds exclusively on furanocoumarin-containing host plants. Reconstruction of the ancestral CYP6B sequences using maximum likelihood predictions and comparisons of the sequence and geometry of their active sites to those of contemporary CYP6B proteins indicate that host plant diversity is directly related to P450 activity and inversely related to substrate specificity. These predictions suggest that, along the lineage leading to Papilio P450s, the ancestral, highly versatile CYP6B protein presumed to exist in a polyphagous species evolved through time into a more efficient and specialized CYP6B1-like protein in Papilio species with continual exposure to furanocoumarins. Further diversification of Papilio CYP6Bs has likely involved interspersed events of positive selection in oligophagous species and relaxation of functional constraints in polyphagous species.

Metabolism of linear and angular furanocoumarins by Papilio polyxenes CYP6B1 co-expressed with NADPH cytochrome P450 reductase

One challenge in the heterologous expression of microsomal cytochrome P450 monooxygenases (P450s) is fulfilling their obligatory requirement for electrons transferred from NADPH P450 reductase. We have established co-expression parameters for Papilio polyxenes CYP6B1 and house fly P450 reductase in baculovirus-infected Sf9 cells that allow for efficient expression of both components and significantly enhance metabolic turnover of this insect P450's substrates. These expression conditions have allowed us to reexamine the turnover capacities of CYP6B1 toward linear and angular furanocoumarins present in the host plants for the specialist caterpillar P. polyxenes. Coexpression of CYP6B1 and P450 reductase at equivalent viral concentrations [MOI (multiplicity of infection) ratio of 1] results in turnover rates for the linear furanocoumarins xanthotoxin and psoralen, which are increased 32-33 fold over the turnover rates obtained with CYP6B1 expressed alone. The turnover rate for the angular furanocoumarin angelicin is also significantly increased to 4.76 nmol/min/nmol P450 compared to its barely detectable level obtained with CYP6B1 expressed alone. Substrate binding analyses indicate that all three of these compounds elicit typical type I binding spectra but with varying magnitudes and affinities that are indicative of each substrate's effectiveness at coordinating with the heme iron. The relative proportions of high spin state generated with these substrates are consistent with CYP6B1 metabolizing these furanocoumarins in the rank order xanthotoxin>psoralen>angelicin. These differential activities for CYP6B1 suggest that it may have been an ancient participant in the coevolutionary arms race between papilionid butterflies and their apiaceous host plants. Due to its ability to handle a range of furanocoumarin structures, CYP6B1 may have contributed to P. polyxenes' early colonization of linear furanocoumarin-containing plants and to its subsequent colonization of angular furanocoumarin-containing plants.

CYP6B cytochrome p450 monooxygenases from Papilio canadensis and Papilio glaucus: potential contributions of sequence divergence to host plant associations

Two groups of furanocoumarin-inducible cytochrome p450 genes, the CYP6B4 group and the CYP6B17 group, characterized in two closely related tiger swallowtails, Papilio glaucus and Papilio canadensis, are induced to different extents, with generally higher levels of CYP6B transcripts in P. glaucus. To investigate the evolutionary history of these CYP6B genes in the context of their association with furanocoumarin detoxification, we isolated thirteen CYP6B genes from these species. Each of these genes contains an intron at a conserved position (1334 nucleotides from the translation start site), which varies in length due to three insertion/deletions. The proximal 5' end flanking sequence from the transcription initiation site is highly conserved (91-98% nt identity). The sequence 5' to -640 is significantly variable due largely to the presence of three insertion/deletions. The sequence at the 3' end of this region contains a putative xenobiotic response element to xanthotoxin (XRE-xan), important for basal and xanthotoxin-inducible transcription of the P. polyxenes CYP6B1v3 gene, and multiple elements known to regulate vertebrate phase I and II promoters, including an XRE-AhR (Xenobiotic Response Element to Aryl hydrocarbon Receptor), an OCT-1 element (octamer protein binding site), an ARE (Antioxidant Response Element), an EcRE (Ecdysone Response Element), and an imperfect PXR (Pregnane X Receptor) responsive element (PRE). Our analyses of CYP6B genes in these two species indicate that these genes are in an early stage of divergence and that differential exposure of these two species to chemically distinct host plants resulting from geographical isolation has had functional impacts not only on the coding regions of these genes but also on their promoter regions. Thus, changes in p450 regulation as well as catalytic activity may play a role in the evolution of host plant associations in herbivorous insects.

Hormonal control of GTP cyclohydrolase I gene expression and enzyme activity during color pattern development in wings of Precis coenia

Color patterns of butterfly wings are composed of single color points represented by each scale. In the case of Precis coenia, at the end of pupal development, different types of pigments are synthesized sequentially in the differently colored scales beginning with white (pterins) followed by red (ommatins) and then black (melanin). In order to explain how formation of these different colors is regulated, we examined the expression of an mRNA-encoding guanosine triphosphate-cyclohydrolase I (GTP-CH I; EC 3.5.4.16), the first key enzyme in the biosynthesis of pteridines, during pigment formation in the wings of P. coenia. The strongest positive signal was recognized around pigment formation one day before butterfly emergence. This GTP-CH I gene expression is paralleled by GTP-CH I enzyme activity measured in wing extracts. We also investigated the effect of 20-hydroxyecdysone on the expression of GTP-CH I mRNA and the enzyme activity during color formation. The results strongly suggest that the onset and duration of the expression of a GTP-CH I mRNA is triggered by a declining ecdysteroid hormone titer during late pupal development.

Herbivory: caterpillar saliva beats plant defences

Blood-feeding arthropods secrete special salivary proteins that suppress the defensive reaction they induce in their hosts. This is in contrast to herbivores, which are thought to be helpless victims of plant defences elicited by their oral secretions. On the basis of the finding that caterpillar regurgitant can reduce the amount of toxic nicotine released by the tobacco plant Nicotiana tabacum, we investigate here whether specific salivary components from the caterpillar Helicoverpa zea might be responsible for this suppression. We find that the enzyme glucose oxidase counteracts the production of nicotine induced by the caterpillar feeding on the plant.

Amino acids in SRS1 and SRS6 are critical for furanocoumarin metabolism by CYP6B1v1, a cytochrome P450 monooxygenase

CYP6B1v1 is the principal cytochrome P450 monooxygenase (P450) that detoxifies dietary furanocoumarins in the guts of Papilio polyxenes, the black swallowtail caterpillar. Sequence alignments and structure comparisons of CYP6B1v1 with the mouse CYP2A5 and bacterial CYP102 proteins, which are also capable of metabolizing the linear furanocoumarin xanthotoxin (8-methoxypsoralen), suggested that Phe116, His117, Val368 and Phe484 might be active site residues. In a homology model developed for CYP6B1v1, the side chains of Phe116 and His117 located in the B'-C loop of SRS1 are predicted to be positioned above the haem plane, while the side chain of Phe484 located in SRS6 is predicted near the entrance of the catalytic pocket. Site-directed mutagenesis of residues Phe116, His117 and Phe484 indicated that these residues represent several of those that determine this protein's stability and substrate specificity. Whereas all aromatic mutants of Phe116 and Phe484 generated CO-difference spectra with maxima at 450 nm indicative of correctly configured monooxygenases, aromatic mutants of Phe116 exhibited reduced reactivities toward some furanocoumarins and aromatic mutants of Phe484 eliminated all reactivities toward furanocoumarins. All single and double aliphatic mutants of Phe116, His117 and Phe484 and aromatic mutants of His117 generated carbon monoxide (CO) difference spectra with maxima at 420 nm (P420) indicative of incorrectly configured monooxygenases. These studies define residues Phe116, His117 and Phe484 as determinants of this insect P450's catalytic site integrity and residues Phe116 and Phe484 as determinants of its substrate specificity. Conservation of Phe116 and His117 in an array of lepidopteran CYP6B proteins implies that these amino acids serve a similar function in other monooxygenases of the insect CYP6B subfamily.

Developmental variation in cytochrome P450 expression in Papilio polyxenes in response to xanthotoxin, a hostplant allelochemical

Although developmental variation in activity and inducibility is typical of cytochrome P450 monooxygenases (P450s) in insects, the adaptive significance of such variation is often unclear, in part because the natural function of insect P450s is rarely known. In this study, we examined developmental variation in expression of CYP6B1 and CYP6B3 in Papilio polyxenes, the black swallowtail. Enzymes encoded by these genes have been implicated in the metabolism of xanthotoxin, a furanocoumarin characteristic of the apiaceous hostplants of P. polyxenes. In each life stage-egg, five larval instars, pupa, and adult-we examined individuals exposed to foliage with and without supplemental xanthotoxin. For each stage, we conducted enzyme assays to estimate xanthotoxin metabolism, Northern analysis to detect constitutive and induced mRNA levels, and RT-PCR amplification and Southern analysis to differentiate among P450 genes expressed. Inducible xanthotoxin metabolism, previously reported in fifth instars, was observed in four of five larval stages but was absent or undetectable in all stages that do not feed on foliage; the highest levels of activity were in early larval instars. The same pattern was observed in both Northern and RT-PCR gel blot analyses. In inducible larval stages, inducibility of CYP6B1 transcripts by xanthotoxin was greater than the inducibility of CYP6B3 transcripts. These findings support earlier suggestions that these two P450s contribute to xanthotoxin metabolism in this species and that expression of these P450 genes is regulated in an adaptive fashion with respect to probability of exposure to hostplant toxins over the course of development.

Molecular analysis of multiple CYP6B genes from polyphagous Papilio species

Papilio glaucus (eastern tiger swallowtail) and Papilio. canadensis (Canadian tiger swallowtail) are two closely related species with broad but overlapping hostplant ranges. P. glaucus encounters toxic furanocoumarins occasionally in its diet in its rutaceous hostplants, whereas P. canadensis rarely if ever encounters these compounds. Analysis of their furanocoumarin-metabolic profiles indicates that these species induce cytochrome P450 monooxygenases (P450s) capable of metabolizing linear and angular furanocoumarins to varying degrees in response to dietary supplementation with xanthotoxin (a linear furanocoumarin). In P. glaucus, metabolism is induced to a significantly higher level than in P. canadensis. Cloning of multiple P450 genes from each species has revealed that both species contain and express two groups of P450s, designated CYP6B4 and CYP6B17, that are related to the P. glaucus CYP6B4v1 enzyme known to metabolize an array of furanocoumarins. Expression patterns of the CYP6B4 and CYP6B17 group transcripts differ in these species in both their basal and furanocoumarin-inducible levels. In P. glaucus, CYP6B4 transcripts, which are not detectable constitutively, are 311-fold induced by xanthotoxin and CYP6B17 transcripts, which are detectable constitutively, are 3-fold induced by xanthotoxin. In P. canadensis, CYP6B4 transcripts are only 8-fold induced and CYP6B17 transcripts are 13-fold induced. These findings are consistent with the postulated evolutionary history of these two species, according to which P. glaucus maintains its association with rutaceous hostplants and P. canadensis has differentiated to utilize hostplants in other families more extensively.

Expression of CYP6B1 and CYP6B3 cytochrome P450 monooxygenases and furanocoumarin metabolism in different tissues of Papilio polyxenes (Lepidoptera: Papilionidae)

The CYP6B1 and CYP6B3 cytochrome P450 monooxygenases in the midgut of the black swallowtail participate in the metabolism of toxic furanocoumarins present in its host plants. In this study, biochemical analyses indicate that the fat body metabolizes significant amounts of the linear furanocoumarins bergapten and xanthotoxin after larvae feed on xanthotoxin. Northern analyses of the combined CYP6B1/3 transcript expression patterns indicate that transcripts in this P450 subfamily are induced in the midgut and fat body by xanthotoxin. Semi-quantitative RT-PCR analyses of individual CYP6B1/CYP6B3 mRNAs indicate that CYP6B1 transcripts are induced by xanthotoxin in all tissues examined and that CYP6B3 transcripts are induced in the fat body only. These results indicate that the fat body participates in the P450-mediated metabolism of excess furanocoumarins unmetabolized by the midgut. Although transcripts of both genes were detected and CYP6B1 transcripts were induced by xanthotoxin in the integument, furanocoumarin metabolism was not detected. Comparison of these P450 promoters with the promoters of alcohol dehydrogenase genes expressed in the fat bodies of several Drosophila species suggest that the xanthotoxin inducibilities of these P450 genes in fat bodies are regulated by elements other than those modulating expression of Adh genes.

Role of proteolysis in determining potency of Bacillus thuringiensis Cry1Ac delta-endotoxin

Bacillus thuringiensis protein delta-endotoxins are toxic to a variety of different insect species. Larvicidal potency depends on the completion of a number of steps in the mode of action of the toxin. Here, we investigated the role of proteolytic processing in determining the potency of the B. thuringiensis Cry1Ac delta-endotoxin towards Pieris brassicae (family: Pieridae) and Mamestra brassicae (family: Noctuidae). In bioassays, Cry1Ac was over 2,000 times more active against P. brassicae than against M. brassicae larvae. Using gut juice purified from both insects, we processed Cry1Ac to soluble forms that had the same N terminus and the same apparent molecular weight. However, extended proteolysis of Cry1Ac in vitro with proteases from both insects resulted in the formation of an insoluble aggregate. With proteases from P. brassicae, the Cry1Ac-susceptible insect, Cry1Ac was processed to an insoluble product with a molecular mass of approximately 56 kDa, whereas proteases from M. brassicae, the non-susceptible insect, generated products with molecular masses of approximately 58, approximately 40, and approximately 20 kDa. N-terminal sequencing of the insoluble products revealed that both insects cleaved Cry1Ac within domain I, but M. brassicae proteases also cleaved the toxin at Arg423 in domain II. A similar pattern of processing was observed in vivo. When Arg423 was replaced with Gln or Ser, the resulting mutant toxins resisted degradation by M. brassicae proteases. However, this mutation had little effect on toxicity to M. brassicae. Differential processing of membrane-bound Cry1Ac was also observed in qualitative binding experiments performed with brush border membrane vesicles from the two insects and in midguts isolated from toxin-treated insects.

Molecular phylogeny of swallowtail butterflies of the tribe Papilionini (Papilionidae, Lepidoptera)

Swallowtail butterflies of the tribe Papilionini number about 225 species and are currently used as model organisms in several research areas, including genetics, chemical ecology and phylogenetics of host plant utilization and mimicry, mechanisms of speciation, and conservation. We have inferred phylogenetic relationships for a sample of 18 species of the genus Papilio (sensu lato) and five outgroup taxa by sequencing two stretches of mitochondrial DNA that correspond to segments 12886-13370 and 12083-12545 of Drosophila melanogaster mitochondrial DNA and consist of sections of the genes for the large ribosomal RNA and subunit 1 of NADH-dehydrogenase. Our data support the monophyly of Papilio and, within it, of several traditionally recognized subgroups. Species belonging to groups that utilize primarily Rutaceae as larval foodplants form two clusters, corresponding to Old World and American taxa, respectively, while two previously recognized clades-of American and South Asian-Austronesian origin-whose members were known to feed mostly on Lauraceae and Magnoliaceae, are observed to form a clade. The sister group of Papilio is found to be the South Asian genus Meandrusa, which also happens to feed on Lauraceae. The latter plant family is therefore the probable larval host of the ancestor Papilio and the shift to Rutaceae (which four-fifths of extant Papilio species use as foodplants) is more likely to have occurred only after the initial diversification of the genus.