Plants make galls to accommodate foreigners: some are friends, most are foes.

At the colonization site of a foreign entity, plant cells alter their trajectory of growth and development. The resulting structure - a plant gall - accommodates various needs of the foreigner, which are phylogenetically diverse: viruses, bacteria, protozoa, oomycetes, true fungi, parasitic plants, and many types of animals, including rotifers, nematodes, insects, and mites. The plant species that make galls also are diverse. We assume gall production costs the plant. All is well if the foreigner provides a gift that makes up for the cost. Nitrogen-fixing nodule-inducing bacteria provide nutritional services. Gall wasps pollinate fig trees. Unfortunately for plants, most galls are made for foes, some of which are deeply studied pathogens and pests: Agrobacterium tumefaciens, Rhodococcus fascians, Xanthomonas citri, Pseudomonas savastanoi, Pantoea agglomerans, 'Candidatus' phytoplasma, rust fungi, Ustilago smuts, root knot and cyst nematodes, and gall midges. Galls are an understudied phenomenon in plant developmental biology. We propose gall inception for discovering unifying features of the galls that plants make for friends and foes, talk about molecules that plants and gall-inducers use to get what they want from each other, raise the question of whether plants colonized by arbuscular mycorrhizal fungi respond in a gall-like manner, and present a research agenda.

Sex- and tissue-specific profiles of chemosensory gene expression in a herbivorous gall-inducing fly (Diptera: Cecidomyiidae).

BACKGROUND: The chemical senses of insects mediate behaviors that are closely linked to survival and reproduction. The order Diptera contains two model organisms, the vinegar fly Drosophila melanogaster and the mosquito Anopheles gambiae, whose chemosensory genes have been extensively studied. Representing a third dipteran lineage with an interesting phylogenetic position, and being ecologically distinct by feeding on plants, the Hessian fly (Mayetiola destructor Say, Diptera: Cecidomyiidae) genome sequence has recently become available. Among plant-feeding insects, the Hessian fly is unusual in 'reprogramming' the plant to create a superior food and in being the target of plant resistance genes, a feature shared by plant pathogens. Chemoreception is essential for reproductive success, including detection of sex pheromone and plant-produced chemicals by males and females, respectively. RESULTS: We identified genes encoding 122 odorant receptors (OR), 28 gustatory receptors (GR), 39 ionotropic receptors (IR), 32 odorant binding proteins, and 7 sensory neuron membrane proteins in the Hessian fly genome. We then mapped Illumina-sequenced transcriptome reads to the genome to explore gene expression in male and female antennae and terminal abdominal segments. Our results reveal that a large number of chemosensory genes have up-regulated expression in the antennae, and the expression is in many cases sex-specific. Sex-specific expression is particularly evident among the Or genes, consistent with the sex-divergent olfactory-mediated behaviors of the adults. In addition, the large number of Ors in the genome but the reduced set of Grs and divergent Irs suggest that the short-lived adults rely more on long-range olfaction than on short-range gustation. We also report up-regulated expression of some genes from all chemosensory gene families in the terminal segments of the abdomen, which play important roles in reproduction. CONCLUSIONS: We show that a large number of the chemosensory genes in the Hessian fly genome have sex- and tissue-specific expression profiles. Our findings provide the first insights into the molecular basis of chemoreception in plant-feeding flies, representing an important advance toward a more complete understanding of olfaction in Diptera and its links to ecological specialization.

Triticum monococcum subsp. monococcum and aegilopoides: new sources of resistance to the dipteran pest, Hessian fly (Diptera: Cecidomyiidae).

The Hessian fly, Mayetiola destructor (Say) belonging to the order Diptera (family: Cecidomyiidae), is a destructive pest of host wheat (Triticum aestivum L.) causing significant economic losses. Although planting resistant wheat cultivars harboring an effective Hessian fly resistance gene (H) is the most economical and environmentally friendly pest management strategy, it imposes selection pressure on the insect populations and can lead to the evolution of Hessian fly virulence. This results in the eventual failure of the deployed H gene. New sources and novel types of resistance are urgently needed to expand the repertoire of H genes and enable strategies that are more effective and durable over the long-term. New sources of Hessian fly resistance have been identified from tetraploid (T. turgidum L., AABB) and hexaploid (T. aestivum, AABBDD) wheat species, as well as from wheat's D-genome donor (Aegilops tauschii Coss., DD). In contrast, diploid einkorn wheat (T. monococcum L., AA) has not been extensively explored for Hessian fly resistance. In this study, we phenotyped 506 T. monococcum accessions belonging to 2 subspecies, T. monococcum L. subsp. monococcum (205 accessions) and T. monococcum subsp. aegilopoides (Link) Thell. (301 accessions), for resistance against 2 predominant Hessian fly biotypes, L and GP (Great Plains). Three and 6 accessions belonging to subsp. monococcum and aegilopoides, respectively, showed > 70% resistance. These accessions provide additional resources for improving wheat cultivars as mitigating strategies for Hessian fly management.

No fitness cost for wheat's H gene-mediated resistance to Hessian fly (Diptera: Cecidomyiidae).

Resistance (R) genes have a proven record for protecting plants against biotic stress. A problem is parasite adaptation via Avirulence (Avr) mutations, which allows the parasite to colonize the R gene plant. Scientists hope to make R genes more durable by stacking them in a single cultivar. However, stacking assumes that R gene-mediated resistance has no fitness cost for the plant. We tested this assumption for wheat's resistance to Hessian fly, Mayetiola destructor (Say) (Diptera: Cecidomyiidae). Our study included ten plant fitness measures and four wheat genotypes, one susceptible, and three expressing either the H6, H9, or H13 resistance gene. Because R gene-mediated resistance has two components, we measured two types of costs: the cost of the constitutively-expressed H gene, which functions in plant surveillance, and the cost of the downstream induced responses, which were triggered by Hessian fly larvae rather than a chemical elicitor. For the constitutively expressed Hgene, some measures indicated costs, but a greater number of measures indicated benefits of simply expressing the H gene. For the induced resistance, instead of costs, resistant plants showed benefits of being attacked. Resistant plants were more likely to survive attack than susceptible plants, and surviving resistant plants produced higher yield and quality. We discuss why resistance to the Hessian fly has little or no cost and propose that tolerance is important, with compensatory growth occurring after H gene-mediated resistance kills the larva. We end with a caution: Given that plants were given good growing conditions, fitness costs may be found under conditions of greater biotic or abiotic stress.

Microbial Inoculants Differentially Influence Plant Growth and Biomass Allocation in Wheat Attacked by Gall-Inducing Hessian Fly (Diptera: Cecidomyiidae).

Beneficial root microbes may mitigate negative effects of crop pests by enhancing plant tolerance or resistance. We used a greenhouse experiment to investigate impacts of commercially available microbial root inoculants on growth and biomass allocation of wheat (Triticum aestivum L. [Cyperales: Poaceae]) and on survival and growth of the gall-inducing wheat pest Hessian fly, Mayetiola destructor (Say). A factorial design was used, with two near-isogenic wheat lines (one susceptible to Hessian fly, the other resistant), two levels of insect infestation (present, absent), and four inoculants containing: 1) Azospirillum brasilense  Tarrand et al. (Rhodospirillales: Azospirillaceae), a plant growth-promoting bacterium, 2) Rhizophagus intraradices (N.C. Schenck & G.S. Sm.) (Glomerales: Glomeraceae), an arbuscular mycorrhizal fungus, 3) A. brasilense + R. intraradices, and 4) control, no inoculant. Larval feeding stunted susceptible wheat shoots and roots. Plants had heavier roots and allocated a greater proportion of biomass to roots when plants received the inoculant with R. intraradices, regardless of wheat genotype or insect infestation. Plants receiving the inoculant containing A. brasilense (alone or with R. intraradices) had comparable numbers of tillers between infested and noninsect-infested plants and, if plants were susceptible, a greater proportion of aboveground biomass was allocated to tillers. However, inoculants did not impact density or performance of Hessian fly immatures or metrics associated with adult fitness. Larvae survived and grew normally on susceptible plants and mortality was 100% on resistant plants irrespective of inoculants. This initial study suggests that by influencing plant biomass allocation, microbial inoculants may offset negative impacts of Hessian flies, with inoculant identity impacting whether tolerance is related to root or tiller growth.

Saturation and comparative mapping of the genomic region harboring Hessian fly resistance gene H26 in wheat.

Resistance gene H26, derived from Aegilops tauschii Coss., is one of the most effective R genes against the Hessian fly [Mayetiola destructor (Say)], an important pest of wheat (Triticum aestivum L.). Using a limited number of PCR-based molecular markers a previous study mapped H26 to the wheat chromosomal deletion bin 3DL3-0.81-1.00. The objectives of this study were to saturate the chromosomal region harboring H26 with newly developed PCR-based markers and to investigate the collinearity of this wheat chromosomal region with rice (Oryza sativa L.) and Brachypodium distachyon genome. A population of 96 F(2) individuals segregating at the H26 gene locus was used for saturation mapping. All wheat ESTs assigned to the deletion bin 3DL3-0.81-1.00 were used to design STS (sequence tagged site) primers. The wheat ESTs mapped near H26 were further used to BLAST rice and B. distachyon genomic sequences for comparative mapping. To date, 26 newly developed STS markers have been mapped to the chromosomal region spanning the H26 locus. Two of them were mapped 1.0 cM away from the H26 locus. Comparative analysis identified genomic regions on rice chromosome 1 and Brachypodium Super contig 13 which are collinear with the genomic region spanning the H26 locus within the distal region of 3DL. The newly developed STS markers closely linked to H26 will be useful for mapped-based cloning of H26 and marker-assisted selection of this gene in wheat breeding. The results will also enhance understanding of this chromosomal region which contains several other Hessian fly resistance genes.

Identification of sex pheromone components of the hessian fly, Mayetiola destructor.

Coupled gas chromatographic (GC)-electroantennographic detection (EAD) analyses of ovipositor extract of calling Hessian fly, Mayetiola destructor, females revealed that seven compounds elicited responses from male antennae. Four of the compounds-(2S)-tridec-2-yl acetate, (2S,10Z)-10-tridecen-2-yl acetate, (2S,10E)-10-tridecen-2-yl acetate, and (2S,10E)-10-tridecen-2-ol-were identified previously in female extracts. Two new EAD-active compounds, (2S,8Z,10E)-8,10-tridecadien-2-yl acetate and (2S,8E,10E)-8,10-tridecadien-2-yl acetate, were identified by GC-mass spectroscopy (MS) and the use of synthetic reference samples. In a Y-tube bioassay, a five-component blend (1 ng (2S)-tridec-2-yl acetate, 10 ng (2S,10E)-10-tridecen-2-yl acetate, 1 ng (2S,10E)-10-tridecen-2-ol, 1 ng (2S,8Z,10E)-8,10-tridecadien-2-yl acetate, and 1 ng (2S,8E,10E)-8,10-tridecadien-2-yl acetate) was as attractive to male Hessian flies as a similar amount of female extract (with respect to the main compound, (2S,10E)-10-tridecen-2-yl acetate). The five-component blend was more attractive to male flies than a three-component blend lacking the two dienes. Furthermore, the five-component blend was more attractive than a blend with the same compounds but that contained one tenth the concentration of (2S,8E,10E)-8,10-tridecadien-2-yl acetate (more accurately mimicking the ratios found in female extract). This suggests that the ratios emitted by females might deviate from those in gland extracts. In a field-trapping experiment, the five-component blend applied to polyethylene cap dispensers in a 100:10 microg ratio between the main component and each of the other blend components attracted a significant number of male Hessian flies. Also, a small-plot field test demonstrated the attractiveness of the five-component blend to male Hessian flies and suggests that this pheromone blend may be useful for monitoring and predicting Hessian fly outbreaks in agricultural systems.

Susceptibility of North Dakota Hessian Fly (Diptera: Cecidomyiidae) to 31 H Genes Mediating Wheat Resistance.

The agricultural landscape of North Dakota is changing. Corn and soybean are now commonplace, but once were rare. Spring sown wheat Triticum aestivum L. and durum wheat Triticum turgidum spp. durum continue to be dominant, but more winter-sown wheat is expected in the future. The presence of wheat in the landscape throughout much of the year will benefit populations of the Hessian fly, Mayetiola destructor (Say), which occurs throughout the state, sometimes in large numbers. Hessian fly is unusual among crop pests for which resources for plant resistance are well developed. On wheat genotypes expressing a single effective H resistance gene, 100% of larvae die before exhibiting any growth. Over 35 H genes in the public domain are available for crossing into elite cultivars. We explored the effectiveness of 31 Hessian fly resistance genes for a North Dakota Hessian fly population. Six genes-H4, H15, H21, H23, H26, and H29-caused 100% larval mortality. Seven others caused at least 80% mortality. Experimental data were used to address three additional questions. Do adult females avoid laying eggs on plants that will kill their offspring: Are neonate larvae able to detect resistance that will end up killing them? Do all 31 genes confer equal protection against larval-induced growth deficits? North Dakota wheat breeders have the necessary tools to create highly resistant wheat cultivars. So far, H genes have been deployed singly in cultivars. Advances in plant breeding will enable gene stacking, a more durable strategy over time.

Shared weapons of blood- and plant-feeding insects: Surprising commonalities for manipulating hosts.

Insects that reprogram host plants during colonization remind us that the insect side of plant-insect story is just as interesting as the plant side. Insect effectors secreted by the salivary glands play an important role in plant reprogramming. Recent discoveries point to large numbers of salivary effectors being produced by a single herbivore species. Since genetic and functional characterization of effectors is an arduous task, narrowing the field of candidates is useful. We present ideas about types and functions of effectors from research on blood-feeding parasites and their mammalian hosts. Because of their importance for human health, blood-feeding parasites have more tools from genomics and other - omics than plant-feeding parasites. Four themes have emerged: (1) mechanical damage resulting from attack by blood-feeding parasites triggers "early danger signals" in mammalian hosts, which are mediated by eATP, calcium, and hydrogen peroxide, (2) mammalian hosts need to modulate their immune responses to the three "early danger signals" and use apyrases, calreticulins, and peroxiredoxins, respectively, to achieve this, (3) blood-feeding parasites, like their mammalian hosts, rely on some of the same "early danger signals" and modulate their immune responses using the same proteins, and (4) blood-feeding parasites deploy apyrases, calreticulins, and peroxiredoxins in their saliva to manipulate the "danger signals" of their mammalian hosts. We review emerging evidence that plant-feeding insects also interfere with "early danger signals" of their hosts by deploying apyrases, calreticulins and peroxiredoxins in saliva. Given emerging links between these molecules, and plant growth and defense, we propose that these effectors interfere with phytohormone signaling, and therefore have a special importance for gall-inducing and leaf-mining insects, which manipulate host-plants to create better food and shelter.

Plant Photosynthetic Responses During Insect Effector-Triggered Plant Susceptibility and Immunity.

Gall-inducing insects are known for altering source-sink relationships within plants. Changes in photosynthesis may contribute to this phenomenon. We investigated photosynthetic responses in wheat [Triticum aestivum L. (Poaceae: Triticeae)] seedlings attacked by the Hessian fly [Mayetiola destructor (Say) (Diptera: Cecidomyiidae], which uses a salivary effector-based strategy to induce a gall nutritive tissue in susceptible plants. Resistant plants have surveillance systems mediated by products of Resistance (R) genes. Detection of a specific salivary effector triggers downstream responses that result in a resistance that kills neonate larvae. A 2 × 2 factorial design was used to study maximum leaf photosynthetic assimilation and stomatal conductance rates. The plant treatments were-resistant or susceptible wheat lines expressing or not expressing the H13 resistance gene. The insect treatments were-no attack (control) or attack by larvae killed by H13 gene-mediated resistance. Photosynthesis was measured for the second and third leaves of the seedling, the latter being the only leaf directly attacked by larvae. We predicted effector-based attack would trigger increases in photosynthetic rates in susceptible but not resistant plants. For susceptible plants, attack was associated with increases (relative to controls) in photosynthesis for the third but not the second leaf. For resistant plants, attack was associated with increases in photosynthesis for both the second and third leaves. Mechanisms underlying the increases appeared to differ. Resistant plants exhibited responses suggesting altered source-sink relationships. Susceptible plants exhibited responses suggesting a mechanism other than altered source-sink relationships, possibly changes in water relations that contributed to increased stomatal conductance.

Avirulence effector discovery in a plant galling and plant parasitic arthropod, the Hessian fly (Mayetiola destructor).

Highly specialized obligate plant-parasites exist within several groups of arthropods (insects and mites). Many of these are important pests, but the molecular basis of their parasitism and its evolution are poorly understood. One hypothesis is that plant parasitic arthropods use effector proteins to defeat basal plant immunity and modulate plant growth. Because avirulence (Avr) gene discovery is a reliable method of effector identification, we tested this hypothesis using high-resolution molecular genetic mapping of an Avr gene (vH13) in the Hessian fly (HF, Mayetiola destructor), an important gall midge pest of wheat (Triticum spp.). Chromosome walking resolved the position of vH13, and revealed alleles that determine whether HF larvae are virulent (survive) or avirulent (die) on wheat seedlings carrying the wheat H13 resistance gene. Association mapping found three independent insertions in vH13 that appear to be responsible for H13-virulence in field populations. We observed vH13 transcription in H13-avirulent larvae and the salivary glands of H13-avirulent larvae, but not in H13-virulent larvae. RNA-interference-knockdown of vH13 transcripts allowed some H13-avirulent larvae to escape H13-directed resistance. vH13 is the first Avr gene identified in an arthropod. It encodes a small modular protein with no sequence similarities to other proteins in GenBank. These data clearly support the hypothesis that an effector-based strategy has evolved in multiple lineages of plant parasites, including arthropods.

The adult head morphology of the hessian fly Mayetiola destructor (Diptera, Cecidomyiidae).

Cover illustration. Mayetiola destructor is a major pest of wheat in Europe, North Africa and North America. In this issue of the Journal of Morphology, Schneeberg et al. (pp. 1299-1311) investigate the adult head structures of the cecidomyiid fly and compared their findings with evolutionarily less successful families within Bibionomorpha. The cover image shows a histological cross section of the head of Mayetiola destructor (Diptera, Cecidomyiidae).

The adult head morphology of the hessian fly Mayetiola destructor (Diptera, Cecidomyiidae).

The adult head of the Hessian fly Mayetiola destructor was examined and described in detail. Morphological features are evaluated with respect to phylogenetic implications and possible effects of miniaturisation. Preserved groundplan features of Diptera are the orthognathous orientation of the head, the vestiture of small microtrichia (possible autapomorphy), filiform antennae inserted frontally between the compound eyes, the presence of a clypeolabral muscle (possible autapomorphy), the presence of labellae (autapomorphy), and the presence of only one premental retractor. Potential synapomorphies of the groups assigned to Bibionomorpha are the origin of M. tentorioscapalis medialis on the frons and the loss of M. craniolacinialis. Further apomorphies of Cecidomyiidae identified in Mayetiola are the unusually massive anterior tentorial arm, the absence of the labro-epipharyngeal food channel, the absence of the lacinia, and the presence of antennal sensilla connected by a seta, a feature not known from any other group of Diptera. The very large size of the compound eyes (in relation to the entire head surface) and the complete loss of ocelli are possible effects of miniaturization. The large size of the brain (in relation to the cephalic lumen), the unusual shape of the optic lobes, and the absence of the frontal ganglion as a separate structure are probably also linked with size reduction.

Why oviposit there? Fitness consequences of a gall midge choosing the plant's youngest leaf.

For animals that lay eggs, a longstanding question is, why do females choose particular oviposition sites? For insects that lay eggs on plants there are three hypotheses: maximizing suitable habitat for juveniles, maximizing female lifespan, and maximizing egg survival. We investigated the function of the oviposition-site choice behavior of a gall midge, the Hessian fly, Mayetiola destructor (Say). In spite of living less than a day and having hundreds of eggs, the ovipositing female is choosy about the placement of eggs. Choosiness makes sense. The tiny gall-making neonate larva has limited movement and strict requirements for colonization. We examined whether offspring benefit from the Hessian fly female's preference for the plant's youngest leaf. To do this we restricted the female's access to the first, second, or third leaf of a seedling (wheat Triticum aestivum L.) plant. Being placed on older leaves did not impact egg survival or larval survival during migration to attack sites at the base of the plant, but did have negative impacts on egg-to-adult survival (reduced by 48%) and reproductive potential (reduced by 30-45%). These negative impacts appear to come from larvae having to search harder to find the limited number of reactive plant cells that can be reprogrammed to form the gall nutritive tissue. We propose that the ability of larvae to find these reactive cells in spite of being placed on an older leaf is important because it creates leeway for female behavior to evolve in the face of other selection pressures, e.g., attack by egg parasitoids.

Gall midges (Hessian flies) as plant pathogens.

Gall midges constitute an important group of plant-parasitic insects. The Hessian fly (HF; Mayetiola destructor), the most investigated gall midge, was the first insect hypothesized to have a gene-for-gene interaction with its host plant, wheat (Triticum spp.). Recent investigations support that hypothesis. The minute larval mandibles appear to act in a manner that is analogous to nematode stylets and the haustoria of filamentous plant pathogens. Putative effector proteins are encoded by hundreds of genes and expressed in the HF larval salivary gland. Cultivar-specific resistance (R) genes mediate a highly localized plant reaction that prevents the survival of avirulent HF larvae. Fine-scale mapping of HF avirulence (Avr) genes provides further evidence of effector-triggered immunity (ETI) against HF in wheat. Taken together, these discoveries suggest that the HF, and other gall midges, may be considered biotrophic, or hemibiotrophic, plant pathogens, and they demonstrate the potential that the wheat-HF interaction has in the study of insect-induced plant gall formation.

Genetic characterization and molecular mapping of Hessian fly resistance genes derived from Aegilops tauschii in synthetic wheat.

Two synthetic hexaploid wheat lines (xAegilotriticum spp., 2n = 6x = 42, genomes AABBDD), SW8 and SW34, developed from the crosses of the durum wheat cultivar Langdon (Triticum turgidum L. var. durum, 2n = 4x = 28, genomes AABB) with two Aegilops tauschii Cosson accessions (2n = 2x = 14, genome DD), were determined to carry Hessian fly [Mayetiola destructor (Say)] resistance genes derived from the Ae. tauschii parents. SW8 was resistant to the Hessian fly biotype Great Plains (GP) and strain vH13 (virulent to H13). SW34 was resistant to biotype GP, but susceptible to strain vH13. Allelism tests indicated that resistance genes in SW8 and SW34 may be allelic to H26 and H13 or correspond to paralogs at both loci, respectively. H26 and H13 were localized to chromosome 4D and 6D, respectively, in previous studies. Molecular mapping in the present study, however, assigned the H26 locus to chromosome 3D rather than 4D. On the other hand, mapping of the resistance gene in SW34 verified the previous assignment of the H13 locus to chromosome 6D. Linkage analysis and physical mapping positioned the H26 locus to the chromosomal deletion bin 3DL3-0.81-1.00. A linkage map for each of these two resistance genes was constructed using simple sequence repeat (SSR) and target region amplification polymorphism (TRAP) markers.