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Centuries of domestication has not impaired oviposition site-selection function in the silkmoth, Bombyx mori.

Damodaram KJ, Kempraj V, Aurade RM, Rajasekhar SB, Venkataramanappa RK, Nandagopal B, Verghese A - Sci Rep (2014)

Bottom Line: Silkmoths significantly preferred mulberry leaves to filter paper as oviposition sites.Moreover, we show that generalist egg-parasitoids are strongly repelled by valencene and α-humulene.Our results demonstrate that IRTs tuned to cues that aid crucial functions like oviposition site-selection are less likely to be impaired even after centuries of domestication.

View Article: PubMed Central - PubMed

Affiliation: National Fellow Lab, Division of Entomology and Nematology, Indian Institute of Horticultural Research, Hesserghatta Lake Post, Bangalore 560 089, India.

ABSTRACT
Oviposition site-selection in insects is mediated through innate recognition templates (IRTs) tuned to specific chemical cues. These cues aid gravid insects in choosing suitable oviposition sites and may even enhance the fitness of their offspring by warding off predators and parasitoids. However, studies on the evolution of oviposition site-selection and cues instigating oviposition in domesticated insects remain elusive. Using the interaction between the silkmoth, Bombyx mori, and its host plant mulberry, Morus alba, as a model system, we demonstrate that centuries of domestication of silkmoth has not impaired its oviposition site-selection function. Silkmoths significantly preferred mulberry leaves to filter paper as oviposition sites. Oviposition assays with filter paper, filter paper treated with leaf volatiles and leaf alone proved that surface texture was not a significant criterion for oviposition site-selection, but volatile cues were. Oviposition assays with electrophysiologically active compounds from mulberry revealed that two of the volatiles, valencene and α-humulene, aided moths in choosing suitable oviposition sites and enhanced egg-laying significantly. Moreover, we show that generalist egg-parasitoids are strongly repelled by valencene and α-humulene. Our results demonstrate that IRTs tuned to cues that aid crucial functions like oviposition site-selection are less likely to be impaired even after centuries of domestication.

Show MeSH
Specific mulberry leaf volatiles repel egg parasitoids.Two oviposition stimulants (α-humulene and valencene) repelled T. chilonis. Mulberry leaf volatiles and two oviposition stimulants (α-humulene and valencene) were presented to egg parasitoids in a 4-arm olfactometer assay. Egg parasitoids were repelled by these oviposition stimulants. Significant difference was analyzed by paired t-test (see Results). Leaf image was taken by V.K.
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f3: Specific mulberry leaf volatiles repel egg parasitoids.Two oviposition stimulants (α-humulene and valencene) repelled T. chilonis. Mulberry leaf volatiles and two oviposition stimulants (α-humulene and valencene) were presented to egg parasitoids in a 4-arm olfactometer assay. Egg parasitoids were repelled by these oviposition stimulants. Significant difference was analyzed by paired t-test (see Results). Leaf image was taken by V.K.

Mentions: Why do silkmoths prefer to lay eggs on mulberry? In evolutionary time, the mulberry silkmoth, B. mori, has specialized on mulberry as its host. Mulberry is detrimental to other insects and mammals that feed on it because they contain polyhydroxylated alkaloids that inhibit alpha-glucosidase14. Alkaloids are sequestered in the body of insect larvae and presumably help them to ward off potential predators15. But, what is the fate of the eggs in the wild? In the wild, egg parasitoids are the major cause of insect egg destruction16. A female insect must find a suitable oviposition site that if free from egg or larval parasitoids17. There are no studies that show mulberry plants as a barrier against egg parasitoids. To investigate if there was any possible barrier in mulberry leaves against egg-parasitoids, we next examined the olfactory behavior of Trichogrammachilonis, a general parasitoid that infests most insects' eggs, using a 4-arm olfactometer assay. First, we provided wasps with mulberry leaf volatiles; surprisingly, wasps were repelled by the leaf volatiles. Wasps spent 0.7 ± 0.2 (mean ± s.e.m) mins and 3.8 ± 0.4 mins in treatment and control arms respectively. Paired t-tests revealed that time spent by wasps in treatment and control was significantly different (t = 5.173, df = 5, P = 0.0035, n = 6). The olfactometer assay results indicated that mulberry leaf volatiles contain compounds that are repulsive to parasitoid wasps (Figure 3). We next wondered whether the repellent activity caused by leaf volatiles was mediated through valencene and α-humulene that triggered oviposition in silkmoths. Provided with a choice between control and the selected volatiles (valencene or α-humulene), wasps spent more time in the control than the treatment arm of the olfactometer. In the olfactometer assay with valencene as the treatment, wasps spent 0.3 ± 0.1 min (mean ± s.e.m) and 2.9 ± 0.1 min in treatment and control arm respectively. A paired t-test showed that the mean time spent was significantly different (t = 8.995, df = 5, P = 0.0003, n = 6). A similar trend was observed in assays with α-humulene as treatment. Wasps spent 0.6 ± 0.4 min in treatment and 2.9 ± 0.1 min in control arms and this difference was significant (t = 3.492, df = 5, P = 0.0174, n = 6). Together the results of the olfactometer assay indicate that wasps clearly avoided valencene and α-humulene (Figure 3). Although these volatiles decrease the probability that T. chilonis parasitises silkmoth eggs when laid in the presence of these volatiles it is possible that other egg parasitoids are less repelled. Using a binary choice assay, we tested this notion. We placed silkmoth eggs (n = 450 for each treatment) on filter paper treated with valencene (100 μl of 0.21 mg/ml), α-humulene (100 μl of 1.73 mg/ml) or control (100 μl of redistilled diethyl ether). The solvent was allowed to evaporate and the eggs were placed on the treated or control filter paper and were immediately exposed to gravid wasps (n = 1000) for 24 h, after which we transferred the exposed eggs into plastic vials. Parasitized eggs turned reddish in color, whereas non-parasitized eggs turned black. Eggs were separated based on the coloration and were held in plastic vials for either parasitoids or silkmoth larvae to emerge. As expected eggs placed on filter paper treated with valencene were significantly less parasitized (Paired t-test, t = 26.97, df = 9, P < 0.0001, n = 10) (8.70 ± 1.82% eggs; mean ± s.e.m) as compared to the control (88.0 ± 2.77% eggs). A similar trend was observed in assays with α-humulene where 9.80 ± 0.74% eggs in treatment and 82.80 ± 2.48% eggs (Paired t-test, t = 25.63, df = 9, P < 0.0001, n = 10) in control were parasitized (Figure 4a & 4b). This experiment demonstrated that parasitization (Figure 4c) in treatments was significantly different from the parasitization in control. A heatmap of EAG response of silkmoths to electrophysiologically active compounds at different concentrations proved that valencene and α-humulene triggered more response than other compounds even at lower concentrations (Figure 4d). Previously, we showed consistent EAG response towards oviposition stimulants at varied concentration in Bactrocera dorsalis9. It seems silkmoths also show a similar response towards crucial oviposition site selection cues. In short, the silkmoth's preference for mulberry leaves as oviposition site is supposedly a repercussion of lowered parasitization risk conferred by specific volatiles present in mulberry leaf volatiles.


Centuries of domestication has not impaired oviposition site-selection function in the silkmoth, Bombyx mori.

Damodaram KJ, Kempraj V, Aurade RM, Rajasekhar SB, Venkataramanappa RK, Nandagopal B, Verghese A - Sci Rep (2014)

Specific mulberry leaf volatiles repel egg parasitoids.Two oviposition stimulants (α-humulene and valencene) repelled T. chilonis. Mulberry leaf volatiles and two oviposition stimulants (α-humulene and valencene) were presented to egg parasitoids in a 4-arm olfactometer assay. Egg parasitoids were repelled by these oviposition stimulants. Significant difference was analyzed by paired t-test (see Results). Leaf image was taken by V.K.
© Copyright Policy - open-access
Related In: Results  -  Collection

License
Show All Figures
getmorefigures.php?uid=PMC4265781&req=5

f3: Specific mulberry leaf volatiles repel egg parasitoids.Two oviposition stimulants (α-humulene and valencene) repelled T. chilonis. Mulberry leaf volatiles and two oviposition stimulants (α-humulene and valencene) were presented to egg parasitoids in a 4-arm olfactometer assay. Egg parasitoids were repelled by these oviposition stimulants. Significant difference was analyzed by paired t-test (see Results). Leaf image was taken by V.K.
Mentions: Why do silkmoths prefer to lay eggs on mulberry? In evolutionary time, the mulberry silkmoth, B. mori, has specialized on mulberry as its host. Mulberry is detrimental to other insects and mammals that feed on it because they contain polyhydroxylated alkaloids that inhibit alpha-glucosidase14. Alkaloids are sequestered in the body of insect larvae and presumably help them to ward off potential predators15. But, what is the fate of the eggs in the wild? In the wild, egg parasitoids are the major cause of insect egg destruction16. A female insect must find a suitable oviposition site that if free from egg or larval parasitoids17. There are no studies that show mulberry plants as a barrier against egg parasitoids. To investigate if there was any possible barrier in mulberry leaves against egg-parasitoids, we next examined the olfactory behavior of Trichogrammachilonis, a general parasitoid that infests most insects' eggs, using a 4-arm olfactometer assay. First, we provided wasps with mulberry leaf volatiles; surprisingly, wasps were repelled by the leaf volatiles. Wasps spent 0.7 ± 0.2 (mean ± s.e.m) mins and 3.8 ± 0.4 mins in treatment and control arms respectively. Paired t-tests revealed that time spent by wasps in treatment and control was significantly different (t = 5.173, df = 5, P = 0.0035, n = 6). The olfactometer assay results indicated that mulberry leaf volatiles contain compounds that are repulsive to parasitoid wasps (Figure 3). We next wondered whether the repellent activity caused by leaf volatiles was mediated through valencene and α-humulene that triggered oviposition in silkmoths. Provided with a choice between control and the selected volatiles (valencene or α-humulene), wasps spent more time in the control than the treatment arm of the olfactometer. In the olfactometer assay with valencene as the treatment, wasps spent 0.3 ± 0.1 min (mean ± s.e.m) and 2.9 ± 0.1 min in treatment and control arm respectively. A paired t-test showed that the mean time spent was significantly different (t = 8.995, df = 5, P = 0.0003, n = 6). A similar trend was observed in assays with α-humulene as treatment. Wasps spent 0.6 ± 0.4 min in treatment and 2.9 ± 0.1 min in control arms and this difference was significant (t = 3.492, df = 5, P = 0.0174, n = 6). Together the results of the olfactometer assay indicate that wasps clearly avoided valencene and α-humulene (Figure 3). Although these volatiles decrease the probability that T. chilonis parasitises silkmoth eggs when laid in the presence of these volatiles it is possible that other egg parasitoids are less repelled. Using a binary choice assay, we tested this notion. We placed silkmoth eggs (n = 450 for each treatment) on filter paper treated with valencene (100 μl of 0.21 mg/ml), α-humulene (100 μl of 1.73 mg/ml) or control (100 μl of redistilled diethyl ether). The solvent was allowed to evaporate and the eggs were placed on the treated or control filter paper and were immediately exposed to gravid wasps (n = 1000) for 24 h, after which we transferred the exposed eggs into plastic vials. Parasitized eggs turned reddish in color, whereas non-parasitized eggs turned black. Eggs were separated based on the coloration and were held in plastic vials for either parasitoids or silkmoth larvae to emerge. As expected eggs placed on filter paper treated with valencene were significantly less parasitized (Paired t-test, t = 26.97, df = 9, P < 0.0001, n = 10) (8.70 ± 1.82% eggs; mean ± s.e.m) as compared to the control (88.0 ± 2.77% eggs). A similar trend was observed in assays with α-humulene where 9.80 ± 0.74% eggs in treatment and 82.80 ± 2.48% eggs (Paired t-test, t = 25.63, df = 9, P < 0.0001, n = 10) in control were parasitized (Figure 4a & 4b). This experiment demonstrated that parasitization (Figure 4c) in treatments was significantly different from the parasitization in control. A heatmap of EAG response of silkmoths to electrophysiologically active compounds at different concentrations proved that valencene and α-humulene triggered more response than other compounds even at lower concentrations (Figure 4d). Previously, we showed consistent EAG response towards oviposition stimulants at varied concentration in Bactrocera dorsalis9. It seems silkmoths also show a similar response towards crucial oviposition site selection cues. In short, the silkmoth's preference for mulberry leaves as oviposition site is supposedly a repercussion of lowered parasitization risk conferred by specific volatiles present in mulberry leaf volatiles.

Bottom Line: Silkmoths significantly preferred mulberry leaves to filter paper as oviposition sites.Moreover, we show that generalist egg-parasitoids are strongly repelled by valencene and α-humulene.Our results demonstrate that IRTs tuned to cues that aid crucial functions like oviposition site-selection are less likely to be impaired even after centuries of domestication.

View Article: PubMed Central - PubMed

Affiliation: National Fellow Lab, Division of Entomology and Nematology, Indian Institute of Horticultural Research, Hesserghatta Lake Post, Bangalore 560 089, India.

ABSTRACT
Oviposition site-selection in insects is mediated through innate recognition templates (IRTs) tuned to specific chemical cues. These cues aid gravid insects in choosing suitable oviposition sites and may even enhance the fitness of their offspring by warding off predators and parasitoids. However, studies on the evolution of oviposition site-selection and cues instigating oviposition in domesticated insects remain elusive. Using the interaction between the silkmoth, Bombyx mori, and its host plant mulberry, Morus alba, as a model system, we demonstrate that centuries of domestication of silkmoth has not impaired its oviposition site-selection function. Silkmoths significantly preferred mulberry leaves to filter paper as oviposition sites. Oviposition assays with filter paper, filter paper treated with leaf volatiles and leaf alone proved that surface texture was not a significant criterion for oviposition site-selection, but volatile cues were. Oviposition assays with electrophysiologically active compounds from mulberry revealed that two of the volatiles, valencene and α-humulene, aided moths in choosing suitable oviposition sites and enhanced egg-laying significantly. Moreover, we show that generalist egg-parasitoids are strongly repelled by valencene and α-humulene. Our results demonstrate that IRTs tuned to cues that aid crucial functions like oviposition site-selection are less likely to be impaired even after centuries of domestication.

Show MeSH