| HOME | HELP | FEEDBACK | SUBSCRIPTIONS | ARCHIVE | SEARCH | TABLE OF CONTENTS |
|
|
|
|||||||
|
|||||||||
Ecology |
Departamento de Ecología y Comportamiento Animal, Instituto de Ecología, A.C., Apdo. Postal 63, Xalapa, Veracruz 91070, Mexico
Received for publication April 9, 2002. Accepted for publication August 16, 2002.
 |
ABSTRACT |
|---|
 TOP ABSTRACT INTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONLITERATURE CITED |
|---|
Key Words: anther-smut disease • flower longevity • flowering phenology • Gesneriaceae • Fusarium moniliforme • Moussonia deppeana • nectar production • pollinator behavior • protandry • seed production • spore deposition
|
INTRODUCTION |
|---|
|
TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONLITERATURE CITED |
|---|
; Pfunder and Roy, 2000
), floral scents (Batra, 1991
; Patt, 1992
), and colors (Batra and Batra, 1985
; Roy, 1993
) induced by fungi to attract pollinators and the transmission of spores to the plants that they visit (Roy, 1993
, and references therein) suggest that some fungi exploit the mutualistic relationships between flowering plants and their pollinators. Studies have shown that some rusts, smuts, and other pathogenic fungi can use pollinators to ferry gametes between different fungal individuals as a way to promote sexual reproduction and/or use their host's pollinators to transfer infectious spores to a new host (Jennersten, 1983
; Batra and Batra, 1985
; Alexander and Maltby, 1990
; Batra, 1991
; Roy, 1996
).
The most studied anther smut, Ustilago violacea Pers (= Microbotryum violaceum Pers) (Basidiomycetes: Ustilaginales), affects members of the Caryophyllaceae (Baker, 1947
; Alexander, 1989
; Biere and Honders, 1996
; Shykoff, Bucheli, and Kaltz, 1997
). It has been documented that these fungi can alter the host flowering phenology, floral morphology, and flower visitation as a way to ensure spore transmission (Lee, 1981
; Biere and Honders, 1996
; Shykoff and Kaltz, 1998
). Although the epidemiology of the Ustilago–Caryophyllaceae systems is well known, pollinator transmission of fungal spores is known for other groups of fungi (Batra and Batra, 1985
; Burdon, Ericson, and Müller, 1995
; Ericson, Burdon, and Müller, 1999
; Pfunder and Roy, 2000
). Insect transmission is important for these fungal diseases (Jennersten, 1983
, 1988
; Bultman and White, 1988
; Batra, 1991
), however vertebrate-pollinated plants are also at risk.
We have discovered that Fusarium moniliforme (Scheld.) Snyder & Ansen (Deuteromycota: section Liseola) infects flowers of Moussonia deppeana (Schlecht. & Cham.) Hanst. (Gesneriaceae), a protandrous, hummingbird-pollinated perennial shrub. Here, we describe (1) disease prevalence and intensity of infection, (2) effects on the host flowering phenology, flower morphology and longevity, and nectar and seed production, and (3) hummingbird visitation patterns and their influence on spore and disease transmission. To our knowledge, this is the first study that has attempted to evaluate hummingbirds as vectors of plant diseases.
|
MATERIALS AND METHODS |
|---|
|
TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONLITERATURE CITED |
|---|
300 plants) growing in a remnant of cloud forest (29 ha), in the Parque Ecológico Francisco Xavier Clavijero near Xalapa, Veracruz, Mexico (19°30' N, 96°57' W; at 1225 m above sea level).
Study system
The pathogen
Fusarium moniliforme (hereafter Fusarium) is a fungal pathogen characterized by its microconidia occurring in chains (Booth, 1971
). The aerial mycelium is generally dense with a powdery appearance. Fusarium is widespread in both humid and subhumid temperate zones and extends into subtropical and tropical zones throughout the world (Booth, 1971
). Host plant families include Asclepiadeaceae, Bromeliaceae, Caryophyllaceae, Colvolvulaceae, Euphorbiaceae, Moraceae, Orchidaceae, Polemoniaceae, Rosaceae, and Rubiaceae (Booth, 1971
). It is a major crop pathogen of several Gramineae in which it may cause seedling blight, scorch, foot rot, stunting, and hypertrophy (Booth, 1971
). Wind and rain dispersal of Fusarium has been confirmed in cornfields (Ooka and Kommedahl, 1977
; Pascoe, 1990
), and European corn borer (Lepidoptera, Pyrallidae) larvae also disseminate spore among leaves causing kernel rot and symptomless infection of maize kernels (Sobek and Munkvold, 1999
). However, little is known about how Fusarium spores are disseminated in the wild.
The host plant
Moussonia deppeana (hereafter Moussonia) is an abundant, 1–3-m-tall perennial subshrub distributed in shaded areas of forests from southern Mexico to Honduras (Wiehler, 1982
). This species flowers from November to mid-March. The axillary inflorescences have pronounced peduncles with compound cymes of four flowers each (Wiehler, 1975
). On average, eight flowers are open on healthy plants at mid-flowering season (mean ± 1 SD, 8.01 ± 3.5, N = 20; Lara and Ornelas, 2001
). The orange-red tubular flowers are solely pollinated by the amethyst-throated hummingbird (Lampornis amethystinus) (Lara and Ornelas, 2001
, 2002
). We have not observed other animals visiting the flowers. Moussonia flowers are protandrous, opening in the morning and staying open 4 d (Lara and Ornelas, 2002
). Each flower passes through a 2-d male period (staminate phase), followed by a 2-d female phase (pistillate phase) (Lara and Ornelas, 2001
). More nectar is secreted on average during the staminate phase (1.98 ± 1.6 µL/flower, N = 20) than in the pistillate phase (1.12 ± 0.5 µL/flower, N = 20). Sugar concentration (BRIX scale) is the same, on average, between flower phases (16% of sucrose; C. Lara, unpublished data).
Fusarium grows endophytically and asymptomatically within host plant tissues and sporulates in the anthers. In our study site, diseased plants commonly produce both infected and uninfected flowers. Undescribed hummingbird flower mites (Tropicoseius Baker & Yunker) inhabit Moussonia flowers (Lara and Ornelas, 2001
), but we have not observed them among flowers infected by Fusarium. Healthy flowers of the diseased plants contain mites.
The vector
Lampornis amethystinus Swainson (Trochilidae) (hereafter Lampornis) is a traplining hummingbird, endemic to the highlands of Central Mexico and Middle America. In our study site, Lampornis hummingbirds are residents and feed from flowers of Palicourea padifolia (Rubiaceae), Lobelia laxiflora (Lobeliaceae), and bromeliads (Contreras and Ornelas, 1999
; Lara and Ornelas, 2001
). During winter, these hummingbirds pollinate and mostly rely on Moussonia flowers for nectar.
Field procedures
Prevalence and intensity of Fusarium infection
In November 1998, 72 plants were randomly selected prior to flowering along the main trail of our study site. Plants were monitored monthly for 3 yr to determine (1) what fraction of flowers are infected per plant, (2) whether the overall level of infection varies among years, and (3) whether individual plants were consistently infected from year to year. We evaluated disease prevalence as the number of plants that produced infected flowers relative to the 72 randomly selected plants and disease intensity as the proportion of infected flowers relative to the total number of open flowers in each diseased plant. The measure of disease prevalence includes healthy plants (i.e., plants with 0% infected flowers).
Association between disease status and plant traits
In December 1999, five diseased flowering plants were randomly selected to determine the effect of Fusarium on flower longevity. We chose six infected buds about to open on each plant. These were identified by the dark-white color of the young anthers. Three buds were bagged with bridal netting and the other three remained unbagged to determine whether flower longevity among infected flowers is influenced by floral visitors. We followed both groups of flowers until wilting. Longevity of uninfected flowers on healthy plants was estimated at the same time, as previously reported (Lara and Ornelas, 2001
, 2002
).
Flowering phenology was followed in detail from November 2000 to March 2001. We randomly selected 33 healthy and 11 diseased plants from the 72 above. For each one, we recorded monthly the number of inflorescences, buds, and open flowers and the number of staminate or pistillate flowers out of the total number of open flowers. We discriminated between uninfected and infected flowers within diseased plants. We also measured plant size (height in centimeters) and flower size (corolla length in millimeters; flower entrance width in millimeters) of both types of plants. In March 2001, we counted and then harvested all fruit capsules produced by healthy (N = 11) and diseased (N = 11) plants in the 2000–2001 flowering season (standing crop). Given that seed number increases linearly with fruit mass in this species (Lara and Ornelas, 2001
, 2002
), we used total fruit mass as an estimate of seed production and female reproductive success.
Nectar production
Twenty flowering plants were randomly selected (10 healthy and 10 diseased) to determine the effect of Fusarium on Moussonia nectar production. From each plant, we chose three buds ready to open and bagged them with bridal netting to exclude floral visitors. Flower buds chosen for sampling of nectar production in diseased plants were infected. Flower mites were excluded by applying Tanglefoot (sticky resin; Tanglefoot Co., Grand Rapids, Michigan, USA) as they may consume up to half the nectar otherwise available to pollinators (Lara and Ornelas, 2001
). Nectar was extracted the following day at 0800 with capillary tubes without removing the flowers from the plants. Nectar volume (in microliters) was measured using graduated micropipettes (5 µL) and a ruler (Kearns and Inouye, 1993
). Nectar secretion pattern was determined for infected and uninfected flowers over their lifetime. Data were taken once every morning (0800) from the same set of flowers, allowing the nectar to accumulate for 24 h between measurements.
Flowers subjected to repeated nectar removals might be stimulated to produce additional nectar and these then might confound real secretion patterns (Castellanos, Wilson, and Thomson, 2002
, and references therein). To explore this possible confounding factor, we conducted additional nectar measurements on different healthy and diseased plants. Three sets of buds were chosen and bagged to exclude floral visitors to let nectar accumulate for 12, 15, and 18 h. The following day, the amount of accumulated nectar was collected only once on 1-d flowers and measured at 0800, 1100, and 1300.
Nectar standing crop
Because pollinators are probably responding to nectar standing crop, we also extracted the nectar available in flowers that had been exposed to floral visitors and measured its volume, concentration, and mass of sugar. Data were collected from individual infected (N = 43) and uninfected staminate (N = 70) and pistillate (N = 54) flowers between 0800 and 1300. Nectar samples from 0800 (N = 61), 1100 (N = 52), and 1300 (N = 54) were taken to evaluate eventual variation in sugar through the peak of hummingbird activity (Lara and Ornelas, 2002
). Nectar volume was measured as described above and sugar concentration (percentage sucrose) with a pocket refractometer (American Optical 10431, Buffalo, New York, USA; range concentration 0°–50°, BRIX units). The amount of sugar produced was expressed as milligrams after Kearns and Inouye (1993)
.
Pollinator behavior
Twenty flowering plants were randomly selected to determine whether hummingbirds discriminate between healthy (N = 10) and diseased plants (N = 10). Behavioral observations were conducted on each individual plant (one plant per day) in November and December 1999 from 0800 to 1200 (hummingbird activity peaks during this period; Lara and Ornelas, 2002
). We recorded (1) number of visits to each focal plant, (2) number of flowers visited per foraging bout, and (3) number of probes to each flower visited. After observation, we recorded the number and sexual phase of open flowers on each focal plant. Plants were observed on separate days as a way to increase the likelihood of flower visitation by different hummingbird individuals. By observing only one plant per day, the treatment (diseased vs. healthy) is confounded by date. Paired observations may have been preferable but impractical under field conditions.
Effectiveness of hummingbirds on spore and disease transmission
We used fluorescent dye as a pollen analogue and direct observations of hummingbird behavior to track pollen and spore movements. In November 1998, pollinators were visually followed and the distance between flower visits estimated, as described by Parra, Vargas, and Eguiarte (1993)
. Behavioral observations were made for 5 h/d during five consecutive days, covering different areas every day that contained 10–20 healthy and diseased plants. In December 1998, fluorescent powder dyes (USR Optonix, Hacketstown, New Jersey, USA) were applied to freshly opened 1-d infected flowers located at the center of a patch containing healthy (N = 22) and diseased plants (N = 11). Fluorescent dye was applied early in the morning before pollinators became active (as described by Waser, 1988
). Flowers were marked on a focal plant with one dye color. Anthers were dusted with dye using a cotton swab. The following day at noon, we collected all open flowers within a 10-m radius of the focal plant and recorded the distances between the source and recipient plants. We confirmed the presence of dyes in flowers with a dissecting microscope.
In January 2000, an inoculation experiment was conducted to evaluate the effectiveness of Lampornis in the transmission of Fusarium spores. Ten healthy flowering plants, each with three buds, were randomly selected (N = 30 flowers) and bagged with bridal netting. Hummingbird inoculation was then conducted on 1-d flowers (staminate; N = 15 flowers) and 3-d flowers (pistillate; N = 15 flowers) by using a stuffed Lampornis hummingbird as spore vector. Inoculations were accomplished by probing the hummingbird's bill three times into the corolla of a donor flower with infected anthers and then once into the corolla of a recipient uninfected flower. We mimicked hummingbird behavior in flowers as closely as possible (Lara and Ornelas, 2002
). Spore loads carried by the stuffed bird are likely different from those carried by live birds because spores from a specific infected flower (donor) may persist on hummingbirds for many flowers, mixing with pollen grains. Instead, we only performed inoculations with spores to simplify our experimental design. Only one plant was used as spore donor to minimize possible confounding factors due to host plant. Most spores adhered to the hummingbird's forehead. As much as possible, we removed the residual spores with a paintbrush prior to performing the next inoculation. After inoculations were performed, staminate and pistillate flowers remained bagged and were daily checked for symptoms of infection until they wilted (flower retention).
Statistical analyses
We used a 2 (infection or not, year 1) x 2 (infection or not, year 2) x 2 (infection or not, year 3) contingency table to determine whether individual plants were consistently infected from year to year. Variation in intensity of infection over time was analyzed using one-way repeated-measures ANOVA. In the model, year was treated as a fixed factor and the monthly intensity of infection was the repeated measure.
Flower production among plants with and without infection over time was analyzed using repeated-measures ANOVAs. In the model, disease status of the plant (healthy vs. diseased) was treated as a fixed effect; the number of inflorescences, buds, and open flowers and the proportion of staminate and pistillate flowers per plant over the flowering period (5 mo) were the repeated measures.
Nectar production among plants with and without infection over time was also evaluated with repeated-measures ANOVA. In the model, disease status of the flower (infected vs. uninfected) was treated as a fixed factor and daily nectar production (4 d) as the repeated measure. Between-plant variation was assessed with disease status as the main factor and plants nested within disease status. Along with the main effects, flower age x disease status, and flower age x plant(disease status) interactions were included in the model. Variation in accumulated nectar and nectar standing crop between groups of plants as a function of time for the nectar to accumulate was assessed with a two-way ANOVA.
Plant and flower size, nectar production over the lifetime, and fruit standing crop among diseased and healthy plants were assessed using one-way ANOVAs. Variation in the number of seeds (fruit mass) among healthy and diseased plants was contrasted with a two-way ANOVA. Between-plant variation was assessed with disease status as the main factor and plant nested within disease status.
Lastly, pollinator behavior on healthy and diseased plants was assessed with ANCOVAs (Zar, 1984
), as plants varied in the number of open flowers and proportion of staminate and pistillate flowers over the observation period. In the model, disease status was a fixed factor, number of open flowers per plant and proportions of each flower phase were the covariates, and number of flowers visited per bout and number of probes per flower were the response variables. Because we observed plants on different days (between-plant variation), the plant factor was not included in our model.
All statistical analyses were done using General Linear Modeling with StatView and SuperANOVA (Abacus Concepts, 1989
, 1996
). Data were arcsine, square-root, or log10 transformed as needed before analysis to correct for nonnormality or heteroscedasticity, but untransformed data are reported in text and figures.
|
RESULTS |
|---|
|
TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONLITERATURE CITED |
|---|
2 = 32.8, df = 3, P < 0.001); all plants infected in year 1 were also infected in year 2 and year 3. The percentage of infected flowers on diseased plants (infection intensity) averaged 24.1 ± 1.0% (range 1–70%) and varied among years (means ± 1 SD; 1998, 21.9 ± 15.8, N = 15 plants; 1999, 25.9 ± 16.8, N = 16; 2000, 18.4 ± 14.5, N = 21). Infection intensity generally decreased over the season, with the exception of March 2000, when the eight flowering infected plants produced numerous infected flowers (Fig. 1B). Results of the repeated-measures ANOVA indicated that the intensity of infection differed over the three years of the study (year effect, F2,196 = 9.13, P < 0.001) and varied within each year (month effect, F4,196 = 45.76, P < 0.001). The significant year x month interaction (F8,196 = 37.32, P < 0.001) was due to the dramatic infection increase in December–January 1998 and March 2000.
|
|
|
|
Fruit standing crop was similar between healthy (27.5 ± 1.2, N = 33) and diseased plants (25.5 ± 1.1, N = 11) (F1,42 = 0.63, P = 0.43). However, healthy plants produced significantly heavier fruits (more seeds) (0.217 ± 0.002 g, N = 11) than diseased plants (0.195 ± 0.002 g, N = 11) (disease status effect, F1,534 = 42.77, P < 0.001). Individual plants varied in fruit mass [plant(disease status) effect, F20,534 = 2.16, P < 0.01].
Nectar production
Nectar production during the first 4 d did not differ between infected and uninfected flowers (repeated-measures ANOVA; disease status effect, F1,120 = 0.63, P = 0.43). Nectar production increased over time (flower age effect, F3,120 = 183.04, P < 0.001; Fig. 4) similarly for diseased and healthy flowers (flower age x disease status interaction, F3,120 = 0.77, P = 0.51). Plants varied for nectar production [plant(disease status) effect, F18,120 = 2.01, P = 0.03] and individual plants varied in how their nectar production changed with flower age [flower age x plant(disease status) interaction, F54,120 = 1.79, P < 0.01]. Total nectar volume produced by infected flowers over their lifetimes (6 d) (39.95 ± 0.11 µL/flower, N = 30) was significantly higher than the total nectar produced by uninfected flowers over shorter life spans (4 d) (26.88 ± 0.09 µL/flower, N = 30; t test = 90.7, df = 58, P < 0.001).
|
Nectar standing crop
The amount of nectar available to floral visitors did not differ with disease status or time of day (disease status effect, F2,158 = 0.72, P = 0.48; time-of-day effect, F2,158 = 0.34, P = 0.71; disease status x time-of-day interaction, F4,158 = 0.44, P = 0.77). Infected flowers had less sugar-concentrated nectar (14.9 ± 0.5 BRIX, N = 29) than uninfected flowers (staminate, 16.5 ± 0.5, N = 40; pistillate, 16.2 ± 0.4, N = 32), but these differences were marginally significant (F2,92 = 2.64, P = 0.076) and were consistent over time (disease status x time-of-day interaction, F4,92 = 1.98, P = 0.10). On the other hand, sugar concentration varied almost significantly over time, decreasing over the course of the day (time-of-day effect, F2,92 = 3.06, P = 0.051). Lastly, there were no significant differences between infected and uninfected flowers in the total number of milligrams of sugar per volume over time (disease status effect, F2,92 = 0.07, P = 0.93; time-of-day effect, F2,92 = 0.26, P = 0.77; disease status x time-of-day interaction, F4,92 = 0.76, P = 0.55).
Pollinator behavior
Neither the number of open flowers nor the proportion of pistillate and staminate flowers affected pollinator behavior on Moussonia individual plants. Because the F values for the disease status x covariates interactions were not significant (results not shown), we removed them from the ANCOVAs. After removing such effects from the ANCOVAs, the F value for disease status was significant only for the number of flowers visited per bout (Table 2).
|
Hummingbird visitation rate (number of visits a plant received during the observation period) was significantly higher among diseased plants (3.0 ± 0.2, N = 10) than healthy plants (1.9 ± 0.3, N = 10; t test = 2.8, P = 0.011). However, on a per flower basis, visitation rates were similar among flowers regardless of the plant disease status (means ± 1 SE; flowers of diseased plants, 0.5 ± 3.9 visits · flower–1 · bout–1, N = 30; flowers of healthy plants, 0.4 ± 6.0 visits · flower–1 · bout–1, N = 18; t test = –1.58, P = 0.12).
Effectiveness of hummingbirds on spore and disease transmission
Pollen dispersal distances were limited despite the typical traplining behavior of the pollinator. The average flight distances between flowers by Lampornis was 143.7 ± 40.8 cm (mean ± 1 SE) (N = 89), with 90% of the visits within less than 100 cm (Fig. 5A). The distance of dye transfers from the dye source to recipient plant ranged from 0 to 100 cm (Fig. 5B). The mean distance (± 1 SE) of dye transfer was 237.2 ± 65.8 cm with a median of 90.5 cm. The longest distance was 8 m, but 80% of the collected flowers with fluorescent particles were also recovered between 0 and 100 cm from the focal infected flower. The distributions for dye transfer distance and pollinator flight distance were leptokurtic. Both estimates suggest that most pollen (or spores) removed from an infected flower can be deposited on the next visited flower.
|
|
DISCUSSION |
|---|
|
TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONLITERATURE CITED |
|---|
). We found that in Moussonia plants infected by Fusarium, infected flowers were retained longer than uninfected ones, both in the presence and absence of hummingbird visitation, as has been observed for other species of pollinator-transmitted fungal diseases (Baker, 1947
; Alexander, 1990
; Shykoff, 1997
). Such extended floral life spans of infected flowers may enhance fungal transmission. Also, diseased plants produced more flowers overall, and more healthy flowers, than do healthy ones. It is not clear if this is a consequence of phenotypic manipulation by the fungus to attract vectors (Alexander, 1989
; Elmqvist et al., 1993
) or whether plants with high flower production or higher attractiveness to vectors were more likely to become infected (Shykoff, Bucheli, and Kaltz, 1997
). This latter process could lead to pathogen-mediated selection on flower production, and we can test for this by comparing flower production and other floral traits of healthy plants that subsequently became infected over our study with those that did not. Unfortunately, flower production data are not yet available for plants from the season before they became infected, so additional observations are necessary to distinguish between these two processes.
Despite the larger, healthy floral display of diseased plants fruit crops were similar in size, though they contained fewer seeds. It is possible that the large floral display led to more geitonogamy (Harder and Barrett, 1995
) and loss of seeds through inbreeding depression. The systemic fungal infection could also have direct negative effects on seed production. In addition, spore deposition on healthy flowers can reduce seed set (Alexander, 1987
; Carlsson-Granér et al., 1998
; Marr, 1998
) in part by impeding pollen germination (Marr, 1998
). We also found that spore deposition alters phenological pattern, preventing the normal transition from staminate to pistillate flowers. Indeed, on diseased plants healthy pistillate flowers were underrepresented, compared with their healthy counterparts in the population. Local spore deposition within plants could have led to this relative lack of pistillate flowers.
Flower life span should evolve in the presence of pollinator-borne diseases, with high disease pressure reducing optimal life span of healthy flowers to reduce their risk of contracting disease (Shykoff, Bucheli, and Kaltz, 1996
). Optimal mating strategies of male and female flowers in the presence of sexually transmitted disease may not necessarily be those that minimize the risk of infection and disease spread in a population (Guldbrandtsen, 1997
; Thrall, Antonovics, and Bever, 1997
; Kaltz and Shykoff, 2001
). Male-biased infection rates have been documented for dioecious Silene alba (Caryophyllaceae) (Alexander, 1989
; Biere and Antonovics, 1996
; Biere and Honders, 1996
; Carlsson et al., 1990
), possibly because male plants are selected to be more promiscuous. To our knowledge, there is virtually no information on how disease risk would operate in dichogamous, protandrous plants. The increased flower retention by 1-d experimentally inoculated flowers excluded from pollinators implies that the extended flower retention is in reaction to fungal contamination. For the fungus it may be advantageous to extend flower retention by a few days if the infection process requires some time. By contrast, pistillate flowers inoculated with Fusarium spores were not retained longer. Plastic responses in flower longevity to pollination events, particularly to pollen removal, have been documented for protandrous plants (Devlin and Stephenson, 1985
; Richardson and Stephenson, 1989
), but whether Fusarium is taking advantage of this plasticity in Moussonia staminate flowers remains to be investigated. Experiments of removal and deposition of pollen combined with spore inoculation should help to discern whether the increase in flower longevity is a response to pollen removal and/or fungal contamination.
A reduction in nectar production among plants diseased by fungi has been reported for Viscaria vulgaris (Jennersten, 1988
) and Silene latifolia (Shykoff and Bucheli, 1995
), and it has been suggested that this reduction in attractiveness can lower insect visitation (Alexander, 1990
; Jennersten and Kwak, 1991
; Shykoff and Bucheli, 1995
) and influence disease transmission. We found no difference in nectar production between infected and uninfected flowers of Moussonia during the first 4 d of their lifetimes nor nectar produced or available on a daily basis (standing crop and accumulated nectar data). However, infected flowers produced nectar over a longer period of time (2 mg/µL of additional sugar). This additional nectar secretion may entail an energy cost to diseased plants (Pyke, 1991
; M. Ordano and J. F. Ornelas, Instituto de Ecología, AC, unpublished data) and could explain in part their observed reduction in seed production. Alternatively, lower seed set in diseased plants could be due to more pollen limitation in diseased than healthy plants. That is, pollen losses to infected flowers in diseased plants may reduce pollen deposition on uninfected flowers. In addition to the costs of added flowers, flower retention, and sugar production, it may be that there is an energy cost to being infected (i.e., the infection is systemic), not necessarily related to reproductive traits.
Hummingbirds as vectors of fungal spores
We believe that the deposition of Fusarium spores on flowers is primarily due to the action of hummingbirds. We acknowledge the limitation of our study in terms of assuming that hummingbirds are the only or most important disease vectors. Here, we have experimentally demonstrated successful spore deposition through hummingbird inoculation, and pollen, and presumably, spore deposition mainly occurred to the nearest neighboring flowers. Similar results of pollen deposition have been documented for several hummingbird-pollinated plants (Waser, 1982
; Wolf and Hainsworth, 1990
; Parra, Vargas, and Eguiarte, 1993
). Although our two estimates of pollen (and spore) deposition were similar, the fluorescent dyes reached the longest distance. This was expected, as the flight distance method considers the unrealistic supposition of no pollen carryover (Levin and Kerster, 1974
). Thus, the fluorescent dye method may be a more realistic estimate (Marr et al., 2000
; but see Waser, 1988
). Fluorescent dye was moved by hummingbirds primarily within plants and between plants in close proximity to each other (80% of the dye transfers occurred within an 8-m radius of the source plant). The consequences of this behavioral outcome were not evaluated in our study; however, flights within Moussonia individuals should promote geitonogamy and limit fungal spore mixing if we assume that only a single fungal strain infects a plant.
Hummingbirds visited more flowers on diseased than on healthy plants. This is not surprising, because diseased plants were more conspicuous, bearing 7–9 times as many combined healthy and infected flowers, and pollinators are known to respond to display size. However, on a per flower basis, hummingbird visitation rates did not differ between diseased and healthy plants (Schmid-Hempel and Speiser, 1988
). Nor was flower number a good predictor of visitation rate within the healthy or the infected groups, as indicated by our ANCOVA results. Our hummingbird behavioral observations and distances of dye deposition suggest that much pollen and spore transmission will occur within rather than between plants. Indeed, the larger number of open flowers and longer visitation bouts of hummingbirds on diseased plants suggests that more geitonogamy might occur (Harder and Barrett, 1995
). Further, much spore deposition will be expected to occur within a plant because, despite the relatively small number of flowers on a diseased plant that are infected (about 26%), these flowers remain open and provide nectar rewards for longer. Nevertheless, the appearance of new diseased individuals over the years shows that successful long-distance transmission events occur.
In summary, Fusarium appears to modify Moussonia flowering phenology, increasing allocation of resources to pollinator attraction and reward. Increased floral display enhanced pollinator visitation by hummingbirds, which are effective vectors of Fusarium spores, thus increasing the dissemination of this fungal disease. These results suggest that pollinator preference might affect the dynamics of disease transmission and the outcomes of the plant–hummingbird mutualistic interaction.
|
FOOTNOTES |
|---|
2 Current address: Laboratorio de Ecología del Comportamiento, Centro de Investigaciones Fisiológicas (CIF), Universidad Autónoma de Tlaxcala, Carretera Tlaxcala-Puebla, Km 1.5 s/n Tlaxcala, Tlaxcala 90070, Mexico
3 Author for reprint requests (ornelasj{at}ecologia.edu.mx
)
|
LITERATURE CITED |
|---|
|
TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONLITERATURE CITED |
|---|
Abacus Concepts. 1996 Abacus Concepts, StatView Reference. Abacus Concepts, Berkeley, California, USA
Alexander H. M. 1987 Pollination limitation in a population of Silene alba infected by the anther-smut fungus, Ustilago violacea. Journal of Ecology 75: 771-780[CrossRef]
Alexander H. M. 1989 An experimental field study of anther-smut disease of Silene alba caused by Ustilago violaceae: genotypic variation and disease incidence. Evolution 43: 835-847[CrossRef][ISI]
Alexander H. M. 1990 Epidemiology of anther-smut infection of Silene alba caused by Ustilago violaceae: patterns of spore deposition and disease incidence. Journal of Ecology 78: 166-179[CrossRef]
Alexander H. M. A. Maltby 1990 Anther-smut infection of Silene alba caused by Ustilago violaceae: factors determining fungal reproduction. Oecologia 84: 249-253
Baker H. G. 1947 Infection of species of Melandrium by Ustilago violaceae (Pers.) Fuckel and the transmission of the resultant disease. Annals of Botany 11: 333-348
Batra L. R. S. W. T. Batra 1985 Floral mimicry induced by mummy-berry fungus exploits host's pollinators as vectors. Science 228: 1011-1013
Batra S. W. T. 1991 Floral mimicry and insects as vectors of conidia. In L. R. Battra [ed.], World species of Monilinia (fungi): their ecology, biosystematics and control, 93–97. J. Cramer, Berlin, Germany
Biere A. J. Antonovics 1996 Sex-specific costs of resistance to the fungal pathogen Ustilago violaceae (Microbotryum violaceum) in Silene alba. Evolution 50: 1098-1110[CrossRef][ISI]
Biere A. S. J. Honders 1996 Impact of flowering phenology of Silene alba and S. dioca on susceptibility to fungal infection and seed predation. Oikos 77: 467-480[CrossRef][ISI]
Booth C. 1971 The genus Fusarium. Commonwealth Mycological Institute, Kew, Surrey, UK
Bultman T. L. J. F. White 1988 "Pollination" of a fungus by a fly. Oecologia 73: 317-319[ISI]
Burdon J. J. L. Ericson W. J. Müller 1995 Temporal and spatial changes in a metapopulation of the rust pathogen Triphragmium ulmariae and its host, Filipendula ulmaria. Journal of Ecology 83: 979-989[CrossRef]
Carlsson U. T. Elmqvist A. Wennströn L. Ericson 1990 Infection by pathogens and population age of host plants. Journal of Ecology 78: 1094-1105[CrossRef]
Carlsson-Granér U. T. Elmqvist J. Ågren H. Gardfjell P. Ingvarsson 1998 Floral sex ratios and seed set in dioecious Silene dioica. Journal of Ecology 86: 79-91[CrossRef]
Castellanos M. C. P. Wilson J. D. Thomson 2002 Dynamic nectar replenishment in flowers of Penstemon (Scrophulariaceae). American Journal of Botany 89: 111-118
Contreras P. S. J. F. Ornelas 1999 Reproductive conflicts of Palicourea padifolia (Rubiaceae) a distylous shrub of a tropical cloud forest in Mexico. Plant Systematics and Evolution 219: 225-241[CrossRef][ISI]
Devlin B. A. G. Stephenson 1985 Sex differential floral longevity, nectar secretion, and pollinator foraging in a protandrous species. American Journal of Botany 72: 303-310[CrossRef][ISI]
Elmqvist T. D. Liu U. Carlsson B. E. Giles 1993 Anther-smut infection in Silene dioica: variation in floral morphology and patterns of spore deposition. Oikos 68: 207-216[CrossRef][ISI]
Ericson L. L. L. Burdon J. Müller 1999 Spatial and temporal dynamics of epidemics of the rust fungus Uromyces valerianae on populations of its host Valeriana salina. Journal of Ecology 87: 649-658[CrossRef][ISI]
Guldbrandtsen B. 1997 Maintenance of variation in mating success in hermaphrodites with a sexually transmitted disease. American Naturalist 149: 693-705[CrossRef][ISI]
Harder L. D. S. C. H. Barrett 1995 Mating cost of large floral displays in hermaphrodite plants. Nature 373: 512-515[CrossRef]
Jennersten O. 1983 Butterfly visitors as vectors of Ustilago violaceae spores between caryophyllaceous plants. Oikos 40: 125-130[CrossRef][ISI]
Jennersten O. 1988 Insect dispersal of fungal disease: effects of Ustilago infection on pollinator attraction in Viscaria vulgaris. Oikos 51: 163-170[CrossRef][ISI]
Jennersten O. M. M. Kwak 1991 Competition for bumblebee visitation between Melampyrum pratense and Viscaria vulgaris with healthy and Ustilago-infected flowers. Oecologia 86: 88-98[CrossRef][ISI]
Kaltz O. J. A. Shykoff 2001 Male and female Silene latifolia plants differ in per-contact risk of infection by a sexually transmitted disease. Journal of Ecology 89: 99-109[CrossRef]
Kearns C. N. D. W. Inouye 1993 Techniques for pollination biologists. University Press of Colorado, Niwot, Colorado, USA
Lara C. J. F. Ornelas 2001 Nectar "theft" by hummingbird flower mites and its consequences for seed set in Moussonia deppeana. Functional Ecology 15: 78-84[CrossRef][ISI]
Lara C. J. F. Ornelas 2002 Effects of nectar theft by flower mites on hummingbird behavior and the reproductive success of their host plant, Moussonia deppeana (Gesneriaceae). Oikos 96: 470-480[CrossRef][ISI]
Lee J. A. 1981 Variation in the infection of Silene dioica L. Clairv. by Ustilago violaceae (Pers) Fuckel in northwest England. New Phytologist 87: 81-89[CrossRef][ISI]
Levin D. A. H. W. Kerster 1974 Gene flow in seed plants. Evolutionary Biology 7: 139-220
Marr D. L. 1998 The effect of Microbotryum violaceum spores on pollen germination in Silene acaulis. International Journal of Plant Sciences 159: 221-227[CrossRef]
Marr D. L. J. Leebens-Mack L. Elms O. Pellmyr 2000 Pollen dispersal in Yucca filamentosa (Agavaceae): the paradox of self-pollination behavior by Tegeticula yuccasella (Prodoxidae). American Journal of Botany 87: 670-677
Ooka J. J. T. Kommedahl 1977 Wind and rain dispersal of Fusarium moniliforme in cornfields. Phytopathology 67: 1023-1026[ISI]
Parra V. C. F. Vargas L. E. Eguiarte 1993 Reproductive biology, pollen and seed dispersal, and neighborhood size in the hummingbird-pollinated Echeveria gibbiflora (Crassulaceae). American Journal of Botany 80: 153-159[CrossRef][ISI]
Pascoe I. G. 1990 Fusarium morphology II: experiments on growing conditions and dispersal of mesoconidia. Mycotaxon 37: 161-172[ISI]
Patt J. M. 1992 Floral odors as gender recognition cues in an aroid-fly mutualism. Ph.D. dissertation, Rutgers University, Piscataway, New Jersey, USA
Pfunder M. B. A. Roy 2000 Pollinator-mediated interactions between a pathogenic fungus, Uromyces pisi (Pucciniaceae), and its host plant, Euphorbia cyparissias (Euphorbiaceae). American Journal of Botany 87: 48-55
Pyke G. H. 1991 How much does floral nectar cost?. Nature 350: 58-59[CrossRef]
Richardson T. E. A. G. Stephenson 1989 Pollen removal and pollen deposition affect the duration of the staminate and pistillate phases in Campanula rapunculoides. American Journal of Botany 76: 532-538[CrossRef][ISI]
Roy B. A. 1993 Floral mimicry by a plant pathogen. Nature 362: 56-58[CrossRef]
Roy B. A. 1994 The use and abuse of pollinators by fungi. Trends in Ecology and Evolution 9: 335-339[CrossRef]
Roy B. A. 1996 A plant pathogen influences pollinator behavior and may influence reproduction of nonhosts. Ecology 77: 2445-2457[CrossRef][ISI]
Schmid-Hempel P. B. Speiser 1988 Effects of inflorescence size on pollination in Epilobium angustifolium. Oikos 53: 98-104[CrossRef][ISI]
Shykoff J. A. 1997 Sex differences in floral nectar production by Silene latifolia (Caryophyllaceae), with reference to susceptibility to a pollinator-borne fungal disease. Canadian Journal of Botany 75: 1407-1414[CrossRef]
Shykoff J. A. E. Bucheli 1995 Pollinator visitation patterns, floral rewards, and the probability of transmission of Microbotryum violaceum, a venereal disease of plants. Journal of Ecology 83: 189-198[CrossRef]
Shykoff J. A. E. Bucheli O. Kaltz 1996 Flower lifespan and disease risk. Nature 379: 779.
Shykoff J. A. E. Bucheli O. Kaltz 1997 Anther smut disease in Dianthus silvester (Caryophyllaceae): natural selection on floral traits. Evolution 51: 383-392[CrossRef][ISI]
Shykoff J. A. O. Kaltz 1998 Phenotypic changes in hosts plants diseased by Microbotryum violaceum: parasite manipulation, side-effects, and trade-offs. International Journal of Plant Sciences 159: 236-243[CrossRef]
Sobek E. A. P. Munkvold 1999 European corn borer (Lepidoptera: Pyralidae) larvae as vectors of Fusarium moniliforme, causing kernel rot and symptomless infection of maize kernels. Journal of Economic Entomology 92: 503-509[ISI]
Thrall P. H. J. Antonovics J. D. Bever 1997 Sexual transmission of disease and host mating systems; within-season reproductive success. American Naturalist 149: 485-506[CrossRef][ISI]
Waser N. M. 1982 A comparison of distances flown by different visitors to flowers of the same species. Oecologia 62: 251-257
Waser N. M. 1988 Comparative pollen and dye transfer by pollinators of Delphinium nelsonii. Functional Ecology 2: 41-48
Wiehler H. 1975 The re-establishment of Moussonia Regel (Gesneriaceae). Selbyana 1: 22-31
Wiehler H. 1982 A synopsis of the Neotropical Gesneriaceae. Selbyana 6: 1-219
Wolf L. L. F. R. Hainsworth 1990 Non-random foraging by hummingbirds: patterns of movement between Ipomopsis aggregata (Pursch) V. Grant inflorescences. Functional Ecology 4: 149-157
Zar J. H. 1984 Biostatistical analysis. Simon & Schuster, Englewood Cliffs, New Jersey, USA
| ||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||
