Thomas L. Bultman

A FUNGAL ENDOPHYTE MEDIATES REVERSAL OF WOUND-INDUCED RESISTANCE AND CONSTRAINS TOLERANCE IN A GRASS

Most plants respond to herbivore damage with elevated chemical and/or structural mechanisms of resistance. However, some plants, particularly those with few chemical defenses, may actually become more susceptible to herbivores following damage; that is, damage may induce susceptibility. We tested the hypothesis that a fungal symbiont of tall fescue grass provides induced resistance to aphids by manipulating fungal infection, plant damage (clipping), and level of nutrient fertilizer in greenhouse/laboratory experi- ments. The fungus had a negative effect on aphid reproduction that was most pronounced following damage. This result appears to be caused by elevated levels of alkaloids produced by the fungus after damage. Furthermore, plants lacking fungal infection were more sus- ceptible to aphids following damage. Hence, while plant damage induces susceptibility in uninfected plants, it does the opposite (induced resistance) in infected plants. Additional experiments showed that the reversal of induced susceptibility is influenced by intrinsic (fungal strain) as well as extrinsic (nutrient application to plants) factors. We also found that the induced resistance conferred by the fungus comes at the cost of tolerance (reduced regrowth) in infected plants. As a modulator of both resistance and tolerance, the fungus plays a central role in influencing defense of host plants against herbivores and should have important implications for plant-herbivore interactions in natural and agricultural systems.

“Pollination” of a fungus by a fly

SummaryThe mechanism resulting in fertilization of Epichloë typhina, a heterothallic ascomycete that is an endophytic pathogen of grasses, has now been discovered. Conidia of one mating type are produced in stromata and are then transferred by insects to individuals of the opposite mating type. One insect, Phorbia phrenione, is a particularly important vector of conidia. Once conidia of the opposite mating type have been transferred to a stroma, the life cycle continues with the formation of perithecia.

Effect of a fungal endophyte on the growth and survival of two Euplectrus parasitoids

Presence of the endophytic fungus, Acremonium coenophialum, within tall fescue (Festuca arundinacea) deters many vertebrate and invertebrate herbivores, apparently because of alkaloids produced by the grass-fungus symbiotum. In one set of experiments we tested if A. coenophialum alters the growth and survival of two parasitoids of an insect herbivore feeding on tall fescue. We fed fall armyworm [Spodoptera fruigiperda (Lepidoptera: Noctuidae)] larvae tall fescue either infected or lacking its endophytic fungus and then allowed the parasitoids, Euplectrus comstockii and Euplectrus plathypenae (Hymenoptera: Eulophidae), to parasitize larvae. We found the presence of endophyte-infected plants in the diet of fall armyworm had a negative impact on the pupal mass of parasitoids. In contrast, developmental rate of the parasitoids was generally not affected by the fungal endophyte. Survival of neither parasitoid to the adult stage was reduced by the presence of endophyte-infected grass in the diet of fall armyworm. We also tested if effects of the fungus on E. comstockii were due to the specific alkaloids, N-acetyl and N-formyl loline, that are produced by the grass-fungus symbiotum. When added to artificial diets of fall armyworm, both lolines resulted in reduced survival of parasitoids. Overall, we found the fungus had a moderate negative impact on two natural enemies that mirrors some of the effects the fungus has on insect herbivores and that may be mediated, at least in part, by alkaloids associated with the fungus. We discuss implications of our work for the hypothesis that endophytes defend their host grasses from herbivores.

A new kind of mutualism between fungi and insects

Female flies ( Phorbia phrenione ) cross fertilize Epichloe typhina via a specific behaviour in which they transfer spores among self-incompatible fungi. Flies ingest spores while visiting fungi for egg laying. Only 11% of stromata lacking P. phrenione eggs produced perithecia and these perithecia covered an average of only 8% of the stromatal surfaces. In contrast, 71% of stromata that possessed fly eggs produced perithecia, covering 54% of the stromatal surfaces. Immediately following oviposition, flies stereotypically drag their abdomen across the fungus in a spiral pattern while excreting faeces. Transfer of fly faeces to unfertilized stromata resulted in cross fertilization. We observed spiralling patterns of perithecial development on fungal surfaces, signifying the behaviour of the fly results in cross fertilization of the fungus. This is the first documentation of active fertilization in an insect-fungus mutualism.

Visual Cues Used by Mantids in Learning Aversion to Aposematically Colored Prey

-We experimentally tested visual cues used by the mantid (Tenodera ardifolia) in learning aversion to noxious prey. For 2 wk, we repeatedly presented mantids with milkweed bugs (Oncopeltusfasicatus) that were fed cardenolide-containing seeds of the milkweed, Asclepias syriaca. Milkweed bugs were painted with one of four color patterns: solid black, solid orange, half black and half orange, and alternately striped with orange and black. A fifth treatment included naturally colored (orange and black mottling), unpainted bugs. As mantids gained experience with unpalatable prey, they became significantly more hesitant to strike and struck less frequently. Furthermore, our data suggest mantids presented prey with broken patterns hesitate to strike longer than those presented prey with solid patterns. We conclude that this invertebrate predator can learn to delay attacks on distasteful prey and that pattern in aposematic coloration may influence aversion learning by mantids. INTRODUCTION There is accumulating evidence for the ability of insects, and invertebrates in general, to learn. Flies have been shown to exhibit a wide array of learning phenomena. For example, induced responsiveness to a second water stimulus when preceded by sucrose stimulation was demonstrated in the blowfly Phormia regina (Dethier et al., 1965) and classical conditioning of the proboscis extension reflex was shown with the same species (Nelson, 1971). Vargo and Hirsch (1982) demonstrated induced responsiveness similar to that described by Dethier et al. (1965) but with Drosophila melanogaster. Classical conditioning with regard to the proboscis extension reflex has also been shown for D. melanogaster (Holliday and Hirsch, 1986). Bees also possess a considerable ability to learn; classical conditioning of proboscis extension was demonstrated in Apis mellifera (Bitterman et al., 1983); A. mellifera also exhibited aversion conditioning to odor (Abramson, 1986) and shock (Abramson and Bitterman, 1986). Bees were shown to associate color with odor of food (Couvillon et al., 1991) while Smith et al. (1991) demonstrated proboscis extension conditioning with both aversive and appetitive stimuli. In addition, seed-harvester ants are able to learn a suite of behaviors that enhances foraging efficiency (Johnson, 1991), while cockroaches exhibit probability-learning and habit-reversal (Longo, 1964). Past studies of learning in mantids have dealt with physiological aspects of learning Present address: Department of Biology, University of South Florida, Tampa, Florida 33620

The cost of mutualism in a fly-fungus interaction

Abstract Botanophila flies act as ”pollinating” parasites of the ascomycetous fungus, Epichloë elymi. Flies transfer fungal spermatia (gametes) among fungi as they visit their hosts for oviposition. Fly larvae consume the products of cross-fertilization (ascospores). We tested whether the cost to the fungus of engaging in the obligate mutualism rises as fly visitation increases and whether mechanisms operate to prevent excessive exploitation of the fungus. Fungi and flies were monitored over 3 years. We recorded the reproductive output of fungi, the amount of feeding by fly larvae on host reproductive tissues, and the mortality of fly eggs and larvae. In two of three years, fly eggs were randomly dispersed; eggs were clumped in the remaining year. The reproductive output of fungi did not decrease with increasing egg load; rather, fungal reproductive output tended to increase as more eggs were laid on fungi. Larval feeding was only weakly associated with the number of eggs on fungi. The lack of over-exploitation of the fungus by the fly shows that the interaction was stable during the period of time we conducted our study. Our data suggest the stability was primarily due to high fly egg/larval mortal-ity that increased as egg load increased.

NATURAL RELEASE OF EPICHLOE TYPHINA ASCOSPORES AND ITS TEMPORAL RELATIONSHIP TO FLY PARASITISM

Epichloe typhina (Clavicipitaceae), a fungal endophyte of Elymus virginicus and other grasses, is parasitized by larvae ofthe fly Phorbia phrenione (Anthomyiidae). Ascospores released from E. typhina stromata on intact, uncut E. virginicus culms were collected and counted at two field sites in Missouri to determine when ascospores were naturally discharged from perithecia. Consumption of perithecia by larvae of P. phrenione was also monitored. The time period over which larvae consumed perithecia was compared with the timing of ascospore release in order to assess the impact of larvae on the numbers of ascospores discharged. Ascospores were liberated from stromata on uncut host plants as intact filaments and underwent iterative germination under nonstressful conditions. The number of ascospores collected was highly variable among days, stromata, and sites. Yet, ascospores were not liberated until at least 88% of total larval feeding had occurred. Larvae consumed at least some perithecia on 28% of stromata, removing an average of 22% of the surface area of each stroma. Because larvae consume perithecia before most ascospores are released, P. phrenione should substantially reduce ascospore release by the fungus.

A TEST FOR CONSTITUTIVE AND INDUCED RESISTANCE BY TALL FESCUE (FESTUCA ARUNDINACEA) TO AN INSECT HERBIVORE : IMPACT OF THE FUNGAL ENDOPHYTE, ACREMONIU M COENOPHIALUM

-Endophytic fungi living within some grasses have been shown to increase resistance of their host plants to insect herbivores. We tested the hypothesis that endophytes also mediate induced resistance by a grass to an herbivorous insect. Tall fescue (Festuca arundinacea), both infected and uninfected with the endophyte, Acremonium coenophialum, was artificially damaged by clipping a tiller from each plant four weeks after germination. We used the fall armyworm (Spodoptera frugiperda) as a bioassay to determine the affects of our treatments. Fall armyworm larvae were reared by feeding them shoot and stem material of damaged and undamaged plants in a two factor (infection status and damage) design. Eight-day-old larvae weighed less and took longer to develop into adults when fed endophyteinfected vs. endophyte-free plant material. However, the interaction between status and damage was not statistically significant. In contrast, pupae weighed more when fed infected vs. endophyte-free plant material and the interaction between infection status and damage had a marginally significant effect on pupal mass (F1,177 = 32.2; P = 0.05). Pupae reared from damaged infected plants weighed less than those reared from undamaged infected plants. No pattern with damage was apparent for insects reared on endophyte-free plants. Our results suggest that the clipping damage may result in an induced response in plants infected with the fungal endophyte.