1. Needle monoterpene content gradually decreases from west to east
To further explore the relationship between longitude and monoterpene concentration, I generated a line plot to observe the linear relationships between total monoterpene concentration and each individual monoterpene with longitude in degrees (Figures 16). The images depict a decreasing trend in the concentration of total monoterpenes and 12 individual monoterpenes as longitude increases from west to east and shows a sudden drop in New Brunswick.
Monoterpenes play a vital role in the chemical defense against the eastern spruce budworm and serve as one of the key defense metabolites in white spruce leaves. The concentration of monoterpenes in the leaves significantly affects the resistance of white spruce to the eastern spruce budworm (Delvas et al., 2011). Consequently, it can be argued that the chemical defense mechanism against the eastern spruce budworm in white spruce populations from southeastern Canada may not have been as robust as that observed in native populations at the test site.
Futuyma and Agrawal (2009) demonstrated the existence of natural variation in the terpene content of white spruce leaves, suggesting that this variability may arise from the evolutionary adaptations of white spruce to its environment. Consequently, I propose that this observed trend is potentially attributed to the different genotypes of different population and greater adaptation of white spruce from the western region to the climate of the experimental site. This adaptability enables it to harmonize metabolic processes and energy allocation while concurrently employing chemical defenses against the eastern spruce budworm. The significant reduction in terpene content observed in species originating from New Brunswick can be attributed to the historically infrequent outbreaks of eastern spruce budworm in this region, leading to comparatively lower levels of terpenes in the needles of local white spruce populations.
Monoterpenes play a vital role in the chemical defense against the eastern spruce budworm and serve as one of the key defense metabolites in white spruce leaves. The concentration of monoterpenes in the leaves significantly affects the resistance of white spruce to the eastern spruce budworm (Delvas et al., 2011). Consequently, it can be argued that the chemical defense mechanism against the eastern spruce budworm in white spruce populations from southeastern Canada may not have been as robust as that observed in native populations at the test site.
Futuyma and Agrawal (2009) demonstrated the existence of natural variation in the terpene content of white spruce leaves, suggesting that this variability may arise from the evolutionary adaptations of white spruce to its environment. Consequently, I propose that this observed trend is potentially attributed to the different genotypes of different population and greater adaptation of white spruce from the western region to the climate of the experimental site. This adaptability enables it to harmonize metabolic processes and energy allocation while concurrently employing chemical defenses against the eastern spruce budworm. The significant reduction in terpene content observed in species originating from New Brunswick can be attributed to the historically infrequent outbreaks of eastern spruce budworm in this region, leading to comparatively lower levels of terpenes in the needles of local white spruce populations.
Figure 15&16: box plot of total monoterpene concentration of samples originated from different genetic origin in the order of longitude (from west to east, Alberta to Saskatchewan) (left), relationship of concentration of log total monoterpene (black) and log concentration individual monoterpene (other colors) and the longitude of the seed origin (right).
2. Needle sesquiterpene content gradually decreases from west to east
Using the same method, I employed box plot and line plot to analyze sesquiterpenes. Figure 15 and 16 depict the concentrations of total sesquiterpene and 12 individual sesquiterpenes ordered by longitude, both of them illustrate a clear pattern: the total sesquiterpene content in the leaves decreases gradually from west to east, mirroring the trend observed for monoterpenes, there's also a sudden drop of sesquiterpene concentration in New Brunswick. Notably, major sesquiterpenes like Germacrene D-5-ol and Germacrene D exhibit a consistent trend with the overall sesquiterpene levels. However, this trend is not observed in sesquiterpenes with lower concentrations, possibly attributed to detection errors arising from their lower quantities.
Sesquiterpenes, while have less concentration in conifer resins, exhibit considerable structural diversity and can contribute significantly to chemical defense (Kolosova & Bohlmann, 2012). As depicted in the images, it is evident that populations from the Southeast, present at the experimental site in Alberta, did not exhibit the same level of proficiency in sesquiterpene-mediated chemical defense as observed in native species. Given the striking similarity in the metabolic pathways of monoterpenes and sesquiterpenes, I posit that the potential cause of this observed phenomenon aligns with the hypothesis presented in the preceding section. It likely stems from the difference in genotypes and the comparatively weaker adaptation of the southeastern populations to the northern environmental conditions, as opposed to the local populations, leading to a diminished efficacy in secondary metabolism.
Sesquiterpenes, while have less concentration in conifer resins, exhibit considerable structural diversity and can contribute significantly to chemical defense (Kolosova & Bohlmann, 2012). As depicted in the images, it is evident that populations from the Southeast, present at the experimental site in Alberta, did not exhibit the same level of proficiency in sesquiterpene-mediated chemical defense as observed in native species. Given the striking similarity in the metabolic pathways of monoterpenes and sesquiterpenes, I posit that the potential cause of this observed phenomenon aligns with the hypothesis presented in the preceding section. It likely stems from the difference in genotypes and the comparatively weaker adaptation of the southeastern populations to the northern environmental conditions, as opposed to the local populations, leading to a diminished efficacy in secondary metabolism.
Figure 17&18: box plot of total sesquiterpene concentration of samples originated from different genetic origin in the order of longitude (from west to east, Alberta to Saskatchewan) (left), relationship of concentration of log total sesquiterpene (black) and individual sesquiterpene (other colors) and the longitude of the seed origin (right).
3. Positive relationship between monoterpene and sesquiterpene concentration
Figure 19: interactions of foliar total monoterpenes and total sesquiterpenes.
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Consequently, it is evident that the foliar monoterpenes and sesquiterpenes of white spruce exhibit similar trends as latitude changes. In light of this, I created a scatter plot and linear regression for both total monoterpenes and total sesquiterpenes (p=1.61e-07). As depicted in Figure 19, there is a positive correlation between the total monoterpenes and total sesquiterpenes content in white spruce leaves. The positive correlation between monoterpene and sesquiterpene content in needles is most likely due to the fact that these two classes of compounds share a very similar synthesis pathway and are regulated by similar regulatory genes. |
4. Positive correlation between leaf terpenoid content and ESB larval weight loss and mortality
I created bar chart and scatter plot to explore the relationship between terpene content and weight loss in eastern spruce budworm larvae, the control representatives were fed eastern spruce budworm equivalents of terpene-free diets. Through figure 20, there's a decrease of weight gain of all the 6 treatment groups in the weight of larvae compared to the control group which feed the larvae with non-terpenoid diet. It can be observed that the extent of larval weight is inversely related to the previously posted terpenoid content. Figure 21 shows a negative association between body weight and the content of pastes in the diets (p=2.15e-05).
Figure 20&21: bar chart of the larvae weight and treatment (left),and scatter plot with linear regression of total terpenoid concentration (left).
The scatterplot of eastern spruce budworm larvae feeding reveals a relationship between the content of different terpenoids in the larval diet and survival rates. An intriguing negative linear relationship exists between the terpenoid concentration and survival rates (p=6.24e-06). The above results illustrate the ability of terpenoids in needles to slow down larval weight gain and lead to larval death.
Figure 22&23: Scatter plot of the eastern spruce budworm survival rate and log terpenoid concentration (left), and its linear regression (right).
Conclusion & Discussion
The objective of this experiment was to assess the terpene-mediated chemical defenses against the eastern spruce budworm and investigate the impact of terpenes on the body weight and mortality of eastern spruce budworm larvae in various populations within the Canadian white spruce range after their migration to Alberta. The findings revealed that white spruce populations originating from the southeastern side of the range exhibited lower levels of foliar terpenes (both monoterpenes and sesquiterpenes) in Alberta. Elevated terpene concentrations in the diets of eastern spruce budworms were correlated with reduced larval weights and increased mortality which suggest that terpenes play a role in inhibiting the growth of eastern spruce budworms, leading to higher mortality rates.
Hence, it can be deduced that the content of leaf monoterpenes and sesquiterpenes significantly influences white spruce's defense against the eastern spruce budworm. Surprisingly, white spruce populations from the eastern population may not exhibit higher resistance to the eastern spruce budworm compared to those from the western regions in Alberta. This finding suggests that the eastern populations might be better equipped to confront the potential escalation of the eastern spruce budworm outbreak, especially in the context of a warming climate.
In 2019, Sinclair proposed the transplantation of white spruce from southeastern populations to Alberta in response to the stresses induced by elevated temperatures and drought, attributed to climate warming. However, the findings of this experiment suggest a potential vulnerability of southeastern populations to chemical defense against the eastern spruce budworm. Eastern populations may be more susceptible to the eastern spruce budworm than native populations and this phenomenon might be due to genotypic differences and inadequate adaptation to Alberta's environmental conditions, rendering them prone to extensive infestations and consequential losses. Therefore, the recommendations from this study advocate a cautious approach, suggesting a comprehensive consideration of population drought tolerance and chemical defense against the eastern spruce budworm when making forestation plans. Additionally, forest management strategies should be implemented to enhance the early detection of eastern spruce budworm infestations in the sites of transplanting white spruce, allowing for timely insecticide use to control damage during the initial stages of an outbreak.
Hence, it can be deduced that the content of leaf monoterpenes and sesquiterpenes significantly influences white spruce's defense against the eastern spruce budworm. Surprisingly, white spruce populations from the eastern population may not exhibit higher resistance to the eastern spruce budworm compared to those from the western regions in Alberta. This finding suggests that the eastern populations might be better equipped to confront the potential escalation of the eastern spruce budworm outbreak, especially in the context of a warming climate.
In 2019, Sinclair proposed the transplantation of white spruce from southeastern populations to Alberta in response to the stresses induced by elevated temperatures and drought, attributed to climate warming. However, the findings of this experiment suggest a potential vulnerability of southeastern populations to chemical defense against the eastern spruce budworm. Eastern populations may be more susceptible to the eastern spruce budworm than native populations and this phenomenon might be due to genotypic differences and inadequate adaptation to Alberta's environmental conditions, rendering them prone to extensive infestations and consequential losses. Therefore, the recommendations from this study advocate a cautious approach, suggesting a comprehensive consideration of population drought tolerance and chemical defense against the eastern spruce budworm when making forestation plans. Additionally, forest management strategies should be implemented to enhance the early detection of eastern spruce budworm infestations in the sites of transplanting white spruce, allowing for timely insecticide use to control damage during the initial stages of an outbreak.