User:Stephenoff.2/sandbox

Sandbox for Kevin

Kevin Stephenoff EEOB 3310 15 Sept 2014

Wikipedia Topic: Coevolution of Plants and Fungal Parasites

1.	Kaltz, O. and Shykoff, J. A. Sept 2002. Within- and among-population variation in infectivity, latency and spore production in a host-pathogen system. Journal of Evolutionary Biology 15(5): 850-860. (doi=10.1046/j.1420-9101.2002.00433.x)Accessed 14 Sept 2014. The authors of this article analyzed the potential for coevolution between a plant and its fungal parasite. Specifically, the researchers studied Silene latifolia and Microbotryum violaceum with regards to variance in the populations, changes over generation in genotype, fungal spore production, and plant resistivity. The authors found that plants may be selected based on their interaction with the parasite showing that the two species influence each other evolutionarily.

2.	Zhan, J., Mundt, C. C., Hoffer, M. E., and McDonald, B. A. July 2002. Local adaptation and effect of host genotype on the rate of pathogen evolution: an experimental test in a plant pathosystem. Journal of Evolutionary Biology 15(4): 634-647. (doi= 10.1046/j.1420-9101.2002.00428.x)Accessed 14 Sept 2014. The authors of this article studied the evolution of fungal pathogens through analysis of the genome. Theoretically modeling was employed to see how the host plants can affect the evolution of the pathogen. The main feature of the fungal parasite that was studied was its virulence and how that affected the evolution of both species. In some cases, the reproductive fitness of the pathogen was not closely related to the virulence.

3.	Capelle, J. and Neema, C. Nov 2005. Local adaptation and population structure at a micro-geographical scale of a fungal parasite on its host plant. Journal of Evolutionary Biology 18(6): 1445-1454. (doi= 10.1111/j.1420-9101.2005.00951.x)Accessed 14 Sept 2014. In this study, changes in genes were investigated for a plant pathogen in response to its environment of differing plants. Colletotrichum lindemuthianum strains were analyzed as they evolved in response to their local environment which in this study differed by plants and plant interactions in the environment. The article also discussed how the population diversity and size contributed to evolutionary outcomes. The variance in the population can be a major factor in whether plants and fungi are able to coevolve.

4.	Desprez-Loustau, M.-L., Vitasse, Y., Delzon, S., Capdevielle, X., Marcais, B., and Kremer, A. Jan 2010. Are plant pathogen populations adapted for encounter with their host? A case study of phenological synchrony between oak and an obligate fungal parasite along an altitudinal gradient. Journal of Evolutionary Biology 23(1): 87-97. (doi= 10.1111/j.1420-9101.2009.01881.x)Accessed 14 Sept 2014. This article investigated how fungal plant parasites continue to adapt and change to maximize its infectivity. The study also tested the effect that altitude had on the evolution of both the fungus and the plants. Climate can cause differences in adaptations and genetic changes, especially the rate of genetic change and the sensitivity of species to evolutionary changes. The article also observed the changes in phenotype in response to fungus and elevation.

5.	Saikkonen, Kari, Wali, Piippa., Helander, Jarjo.,and Faeth, Stanley H. June 2004. Evolution of endophyte-plant symbioses. Trends in Plant Science 9(6): 275-280. (doi= 10.1016/j.tplants.2004.04.005)Accessed 14 Sept 2014. The authors of this article studied how the life cycle of parasitic fungi and their plant hosts are affected by each other and can be changed evolutionarily. In the study, the researchers found evidence that the two species try to mutually exploit each other which leads to periods of seemingly mutualistic behavior while other periods appear parasitic. The study also begins to show that the host plant and fungi may react together to selection pressure from the surrounding environment which is a valuable indicator of closely related coevolution.

OCT 1 Assignment

edit made https://en.wikipedia.org/wiki/Evolution_of_plants#Mechanisms_and_players_in_evolution_of_plant_form

An additional contributing factor in some plants leading to evolutionary change is the force due to coevolution with fungal parasites. In an environment with a fungal parasite, which is common in nature, the plants must make adaptation in an attempt to evade the harmful effects of the parasite.[139]

talk page https://en.wikipedia.org/wiki/Talk:Host%E2%80%93parasite_coevolution

Oak - Powdery Mildew Example

Oak trees and powdery mildew are a common host-parasite interaction in nature. I think adding this example would be beneficial to the page. Some research of this interaction has used methods testing a third variable such as altitude which can be described as a method for studying the third variable effect of coevolution. Due to the varying environment, the response to selection differs, and therefore the phenological changes in the population of both the plants and fungi will differ.c[1]

Stephenoff.2 (talk) 20:36, 1 October 2014 (UTC)

Desprez-Loustau, M.; Vitasse, Y.; Delzon, S.; Capdevielle, X.; Marcais, B.; Kremer, A. (January 2010). "Are plant pathogen populations adapted for encounter with their host? A case study of phenological synchrony between oak and an obligate fungal parasite along an altitudinal gradient". Journal of Evolutionary Biology 23 (1): 87-97. doi:10.1111/j.1420-9101.2009.01881.x.

talk page https://en.wikipedia.org/wiki/Talk:Plant_disease_resistance

Coevolution Section

I think that it would be beneficial if information about coevolution between host plants and pathogens were included. It is important to recognize that when there is genetic variance in a plant population and selection due to a pathogen (such as a fungus), there will be an evolutionary response and the plant will continue to evolve. Coevolution with pathogens is a major factor in evolutionary change because both the pathogen and the host plant continuously change to avoid the others' defenses. — Preceding unsigned comment added by Stephenoff.2 (talk • contribs) 12:46, 1 October 2014 (UTC) Stephenoff.2 (talk) 20:37, 1 October 2014 (UTC)

talk page https://en.wikipedia.org/wiki/Talk:Evolution_of_plants

Evolution Due to other Organisms

The mechanisms and players section of the article does a great job explaining many factors that lead to evolutionary change in plants. However, I feel that it leaves out one important aspect: interaction with other organisms. In plants, a common driver of evolutionary change is fungi. As a specific example, parasitic fungi have been found to cause evolutionary change in plants as plants with greater defenses against the parasite are selected for in a population. The fungal species continues to evolve alongside the plant species in an attempt to gain an advantage over another species. Stephenoff.2 (talk) 20:55, 1 October 2014 (UTC)

10/15/14 Assignment No responses on any commented talk page

11/10/14 Initial Posting of Full Paper in sandbox

Coevolution of Plants and Fungal Parasites Plants and some fungi are frequently closely linked species involved in vital symbioses. Some fungi have a parasitic relationship with the plants they interact with. Most parasitic fungi exploit plants for the nutrients they produce by converting energy from the sun. These relationships have led to a coevolution process involving both plant and fungi (Burdon 2009). Whenever a parasitic fungus is siphoning limited resources away from a plant, there is selective pressure for a phenotype that is better able to prevent parasitic attack from fungi. At the same time, fungi that are better equipped to evade the defenses of the plant will have greater fitness level. The combination of these two factors leads to an endless cycle of evolutionary change in the host-pathogen system (Burdon 2009). Because each species in the relationship is influenced by a constantly changing symbiont, evolutionary change usually occurs at a faster pace than if the other species was not present. This is true of most instances of coevolution. This makes the ability of a population to quickly evolve vital to its survival. Also, if the pathogenic species is too successful and threatens the survival and reproductive success of the host plants, the pathogenic fungi risk losing their nutrient source for future generations. These factors create a dynamic that shapes the evolutionary changes in both species generation after generation (Burdon 2009). Much of the current research regarding the coevolution of plants and fungal parasites concerns the potential for evolutionary change and how the two organisms can influence each other evolutionarily. Studies that investigate this aspect of coevolution usually have a specific plant growing in controlled environment. One group is exposed to a fungal parasite while another group of plants is not. Either genotypic or phenotypic changes in the plants are tested to determine the effect of presence of the parasite (Kaltz 2002). For example, Silene latifolia and Microbotryum violaceum were tested in this regard and the key finding of the study was S. latifolia individual plants were selected for with regard to specific traits only when exposed to the parasite (Kaltz 2002). These results support the hypothesis that the plants and fungi in a host-pathogen relationship have an impact on each other which forces evolutionary change in the population. This specific study quantified the amount of evolutionary change and compared it to a known heritability (Kaltz 2002). An alternative to a controlled study like this is to perform field experiments and observe how a species changes in the environment if plants of the same species are present in two places and are only exposed to the fungal pathogen in one location. The changes in fungi can also be observed with respect to presence of the plants (Delmotte 1999). Some researchers are studying how different defense mechanisms in plant work and how they are continuously evolving. Plants have been shown to release higher levels of chitinases which prevent the pathogenic fungus from attaching to vital parts of the plant (Sahai 1992). Chitinase release is just one of many complex changes that occur in plants in response to a fungal attack. A great deal of research is involved in mapping genetic changes in genes that code for defense mechanisms such as the genes for chitinases (Sahai 1992). Individual nucleotide changes can show that evolution has occurred in a population after it was exposed to the selective pressures of an invading fungal parasite (Sahai 1992). These genes must keep changing to keep up with the parasite that constantly works to evade the defenses. Genes that code for attachment mechanisms are the most dynamic and are directly related to the evading ability of the fungi (Sahai 1992). The greater the changes in these genes, the more change in the attachment mechanism. After selective forces on the resulting phenotypes, evolutionary change that promotes evasion of host defenses occurs. Fungi not only evolve to avoid the defenses of the plants, but they also attempt to prevent the plant from enacting the mechanisms to improve its defenses. Anything the fungi can do to slow the evolution process of the host plants will improve the fitness of future generations because the plant will not be able to keep up with the evolutionary changes of the parasite. One of the main processes by which plants quickly evolve in response to the environment is sexual reproduction (Clay 1991). Without sexual reproduction, advantageous traits could not be spread through the plant population as quickly allowing the fungi to gain a competitive advantage (Clay 1991). For this reason, the sexual reproductive organs of plants are targets for attacks by fungi. Studies have shown that many different current types of obligate parasitic plant fungi have developed mechanisms to disable or otherwise affect the sexual reproduction of the plants (Clay 1991). If successful, the sexual reproduction process slows for the plant, thus slowing down evolutionary change or in extreme cases, the fungi can render the plant sterile creating an advantage for the pathogens (Clay 1991). It is unknown exactly how this adaptive trait developed in fungi, but it is clear that the relationship to the plant forced the development of the process (Clay 1991). Some researchers are also studying how a range of factors affect the rate of evolutionary change and the outcomes of change in different environments. For example, as with most evolution, increases in heritability in a population allow for greater a greater evolutionary response in the presence of selective pressure. For traits specific to the plant-fungi coevolution, researchers have studied how the virulence of the invading pathogen affects the coevolution (Zhan 2002). Studies involving Mycosphaerella graminicola have consistently showed that virulence of a pathogen does not have a significant impact on the evolutionary track of the host plant (Zhan 2002). It is believed by many researchers that the level of virulence is actually an adaptation that the fungal pathogen develops and changes in response to the plant (Zhan 2002). Contrarily, the genetic structure of the host plant for genes relating to defense mechanisms has a much more drastic influence on evolution of the fungi. Small changes in the host defense genome can cause drastically different response rates for the fungi (Zhan 2002). Many other factors need to be tested to fully understand the process of coevolution for this interaction. There can be other factors in that can affect the process of coevolution. For example, in small populations, selection is a relatively weaker force on the population due to genetic drift (Capelle 2005). Genetic drift increases the likelihood of having fixed alleles which decreases the genetic variance in the population. Therefore, if there is only a small population of plants in an area with the ability to reproduce together, genetic drift may counteract the effects of selection putting the plant in a disadvantageous position to fungi which can evolve at a normal rate (Capelle 2005). The variance in both the host and pathogen population is a major determinant of evolutionary success compared to the other species (Capelle 2005). The greater the genetic variance, the faster the species can evolve to counteract the other organism’s avoidance or defensive mechanisms (Capelle 2005). Due to the process of pollination for plants, the effective population size is normally larger than for fungi because pollinators can link isolated populations in a way that the fungus is not able (Delmotte 1999). This means positive traits that evolve in non-adjacent but close areas can be passed to nearby areas. Fungi must individually evolve to evade host defenses in each area (Delmotte 1999). This is obviously a clear competitive advantage for the host plants. Sexual reproduction with a broad, high variance population leads to fast evolutionary change and higher reproductive success of offspring (Delmotte 1999). Environment and climate patterns also play a role in evolutionary outcomes. Studies with oak trees and an obligate fungal parasite at different altitudes clearly show this distinction (Desprez-Loustau 2010). For the same species, different altitudinal positions had drastically different rates of evolution and changes in the response to the pathogens due to the organism also in a selective environment due to their surroundings (Desprez-Loustau 2010). Further research studies have explored the current behavioral coevolution of host plant – fungal parasite. In the case of some interactions for some species, the plant and fungi interaction appears to be more mutualistic before reverting to host-parasite rolls (Saikkonen 2004). This shows that in certain instances, the two species feel that their health and reproductive success will be better when they work in a mutualistic relationship. At some point, it is likely the case that one of the species attempts to exploit the positive relationship for further increased fitness leading to the evolutionary arms rate before the cycle returns to a mutualistic form if that is the most beneficial state (Saikkonen 2004). The organism that is able to evolve at a faster rate has a better chance of being able to exploit the other organism. Research has been performed that shows a parasitic fungus and host plant will react in a unified manner and can even evolve in a mutualistic manner in response to an external stimulus including environmental challenges and other species affecting the relationship (Saikkonen 2004). Coevolution is process that is related to the red queen hypothesis. Both the host plant and parasitic fungi have to continue to survive to stay in their ecological niche. If one of the two species in the relationship evolves at a significantly faster rate than the other, the slower species will be at a competitive disadvantage and risk the loss of nutrients. Because the two species in the system are so closely linked, they respond to external environment factors together and each species affects the evolutionary outcome of the other. In other words, each species exerts selective pressure on the other. Population size is also a major factor in the outcome because differences in gene flow and genetic drift could cause evolutionary changes that do not match the direction of selection expected by forces due to the other organism. Coevolution is an important phenomenon necessary for understanding the vital relationship between plants and their fungal parasites. References 1.	Burdon, J. J., and P. H. Thrall. 2009. Co-evolution of plants and their pathogen in natural habitats. Science 324(5928): 755-756. 2.	Capelle, J. and C. Neema. 2005. Local adaptation and population structure at a micro-geographical scale of a fungal parasite on its host plant. Journal of Evolutionary Biology 18(6): 1445-1454. 3.	Clay, Keith. 1991. Parasitic castration of plants by fungi. Trends in Ecology & Evolution 6(5): 162-166. 4.	Delmotte, F., E. Bucheli, and J. A. Shykoff. 1999. Host and parasite population structure in a natural plant-pathogen system. Heredity 82: 300-308. 5.	Desprez-Loustau, M.-L., Y. Vitasse, S. Delzon, X. Capdevielle, B. Marcais, and A. Kremer. 2010. Are plant pathogen populations adapted for encounter with their host? A case study of phenological synchrony between oak and an obligate fungal parasite along an altitudinal gradient. Journal of Evolutionary Biology 23(1): 87-97. 6.	Kaltz, O. and J. A. Shykoff. 2002. Within- and among-population variation in infectivity, latency and spore production in a host-pathogen system. Journal of Evolutionary Biology 15(5): 850-860. 7.	Sahai, A. S., and M. S. Manocha. 1992. Chitinases of fungi and plants: their involvement in morphogenesis and host-parasite interaction. FEMS Microbiology Reviews 11(4): 317-338. 8.	Saikkonen, K., P. Wali, J. Helander, and S. H. Faeth. 2004. Evolution of endophyte-plant symbioses. Trends in Plant Science 9(6): 275-280. 9.	Zhan, J., C. C. Mundt, M. E. Hoffer, and B. A. McDonald. 2002. Local adaptation and effect of host genotype on the rate of pathogen evolution: an experimental test in a plant pathosystem. Journal of Evolutionary Biology 15(4): 634-647.

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FINAL PAPER POSTED BELOW: 17 November 2014

Kevin Stephenoff

Recitation: Tuesday 9:10

14 November 2014

Coevolution of Plants and Fungal Parasites

Plants and some fungi are frequently closely linked species involved in vital symbioses. Some fungi have a parasitic relationship with the plants they interact with. Most parasitic fungi exploit plants for the nutrients they produce via photosynthesis. These relationships have led to a coevolution process involving both plant and fungi (Burdon 2009).

Whenever a parasitic fungus is siphoning limited resources away from a plant, there is selective pressure for a phenotype that is better able to prevent parasitic attack from fungi. At the same time, fungi that are better equipped to evade the defenses of the plant will have greater fitness level. The combination of these two factors leads to an endless cycle of evolutionary change in the host-pathogen system (Burdon 2009).

Because each species in the relationship is influenced by a constantly changing symbiont, evolutionary change usually occurs at a faster pace than if the other species was not present. This is true of most instances of coevolution. This makes the ability of a population to quickly evolve vital to its survival. Also, if the pathogenic species is too successful and threatens the survival and reproductive success of the host plants, the pathogenic fungi risk losing their nutrient source for future generations. These factors create a dynamic that shapes the evolutionary changes in both species generation after generation (Burdon 2009).

Much of the current research regarding the coevolution of plants and fungal parasites concerns the potential for evolutionary change and how the two organisms can influence each other evolutionarily. Studies that investigate this aspect of coevolution usually have a specific plant growing in controlled environment. One group is exposed to a fungal parasite while another group of plants is not. Either genotypic or phenotypic changes in the plants are tested to determine the effect of presence of the parasite (Kaltz 2002). For example, Silene latifolia (a flowering plant) and Microbotryum violaceum (a host specific fungus) were tested in this regard and the key finding of the study was S. latifolia individual plants were selected for with regard to specific traits only when exposed to the parasite (Kaltz 2002). These results support the hypothesis that the plants and fungi in a host-pathogen relationship have an impact on each other which forces evolutionary change in the population. This specific study quantified the amount of evolutionary change and compared it to a known heritability (Kaltz 2002). An alternative to a controlled study like this is to perform field experiments and observe how a species changes in the environment if plants of the same species are present in two places and are only exposed to the fungal pathogen in one location. The changes in fungi can also be observed with respect to presence of the plants (Delmotte 1999).

Some researchers are studying how different defense mechanisms in plant work and how they are continuously evolving. Plants have been shown to release higher levels of chitinases which prevent the pathogenic fungus from attaching to vital parts of the plant (Sahai 1992). Chitinase release is just one of many complex changes that occur in plants in response to a fungal attack. A great deal of research is involved in mapping genetic changes in genes that code for defense mechanisms such as the genes for chitinases (Sahai 1992). Individual nucleotide changes can show that evolution has occurred in a population after it was exposed to the selective pressures of an invading fungal parasite (Sahai 1992).

These genes must keep changing to keep up with the parasite that constantly works to evade the defenses. Genes that code for attachment mechanisms are the most dynamic and are directly related to the evading ability of the fungi (Sahai 1992). The greater the changes in these genes, the more change in the attachment mechanism. After selective forces on the resulting phenotypes, evolutionary change that promotes evasion of host defenses occurs.

Fungi not only evolve to avoid the defenses of the plants, but they also attempt to prevent the plant from enacting the mechanisms to improve its defenses. Anything the fungi can do to slow the evolution process of the host plants will improve the fitness of future generations because the plant will not be able to keep up with the evolutionary changes of the parasite. One of the main processes by which plants quickly evolve in response to the environment is sexual reproduction (Clay 1991). Without sexual reproduction, advantageous traits could not be spread through the plant population as quickly allowing the fungi to gain a competitive advantage (Clay 1991). For this reason, the sexual reproductive organs of plants are targets for attacks by fungi. Studies have shown that many different current types of obligate parasitic plant fungi have developed mechanisms to disable or otherwise affect the sexual reproduction of the plants (Clay 1991). If successful, the sexual reproduction process slows for the plant, thus slowing down evolutionary change or in extreme cases, the fungi can render the plant sterile creating an advantage for the pathogens (Clay 1991). It is unknown exactly how this adaptive trait developed in fungi, but it is clear that the relationship to the plant forced the development of the process (Clay 1991).

Some researchers are also studying how a range of factors affect the rate of evolutionary change and the outcomes of change in different environments. For example, as with most evolution, increases in heritability in a population allow for a greater evolutionary response in the presence of selective pressure. For traits specific to the plant-fungi coevolution, researchers have studied how the virulence of the invading pathogen affects the coevolution (Zhan 2002). Studies involving Mycosphaerella graminicola have consistently showed that virulence of a pathogen does not have a significant impact on the evolutionary track of the host plant (Zhan 2002). It is believed by many researchers that the level of virulence is actually an adaptation that the fungal pathogen develops and changes in response to the plant (Zhan 2002). Contrarily, the genetic structure of the host plant for genes relating to defense mechanisms has a much more drastic influence on evolution of the fungi. Small changes in the host defense genome can cause drastically different response rates for the fungi (Zhan 2002). Many other factors need to be tested to fully understand the process of coevolution for this interaction.

There can be other factors in that can affect the process of coevolution. For example, in small populations, selection is a relatively weaker force on the population due to genetic drift (Capelle 2005). Genetic drift increases the likelihood of having fixed alleles which decreases the genetic variance in the population. Therefore, if there is only a small population of plants in an area with the ability to reproduce together, genetic drift may counteract the effects of selection putting the plant in a disadvantageous position to fungi which can evolve at a normal rate (Capelle 2005). The variance in both the host and pathogen population is a major determinant of evolutionary success compared to the other species (Capelle 2005). The greater the genetic variance, the faster the species can evolve to counteract the other organism’s avoidance or defensive mechanisms (Capelle 2005).

Due to the process of pollination for plants, the effective population size is normally larger than for fungi because pollinators can link isolated populations in a way that the fungus is not able (Delmotte 1999). This means positive traits that evolve in non-adjacent but close areas can be passed to nearby areas. Fungi must individually evolve to evade host defenses in each area (Delmotte 1999). This is obviously a clear competitive advantage for the host plants. Sexual reproduction with a broad, high variance population leads to fast evolutionary change and higher reproductive success of offspring (Delmotte 1999).

Environment and climate patterns also play a role in evolutionary outcomes. Studies with oak trees and an obligate fungal parasite at different altitudes clearly show this distinction (Desprez-Loustau 2010). For the same species, different altitudinal positions had drastically different rates of evolution and changes in the response to the pathogens due to the organism also in a selective environment due to their surroundings (Desprez-Loustau 2010).

Further research studies have explored the current behavioral coevolution of host plant – fungal parasite. In the case of some interactions for some species, the plant and fungi interaction appears to be more mutualistic before reverting to host-parasite rolls (Saikkonen 2004). This shows that in certain instances, the two species feel that their health and reproductive success will be better when they work in a mutualistic relationship. At some point, it is likely the case that one of the species attempts to exploit the positive relationship for further increased fitness leading to the evolutionary arms rate before the cycle returns to a mutualistic form if that is the most beneficial state (Saikkonen 2004). The organism that is able to evolve at a faster rate has a better chance of being able to exploit the other organism. Research has been performed that shows a parasitic fungus and host plant will react in a unified manner and can even evolve in a mutualistic manner in response to an external stimulus including environmental challenges and other species affecting the relationship (Saikkonen 2004).

Coevolution is a process that is related to the red queen hypothesis. Both the host plant and parasitic fungi have to continue to survive to stay in their ecological niche. If one of the two species in the relationship evolves at a significantly faster rate than the other, the slower species will be at a competitive disadvantage and risk the loss of nutrients. Because the two species in the system are so closely linked, they respond to external environment factors together and each species affects the evolutionary outcome of the other. In other words, each species exerts selective pressure on the other. Population size is also a major factor in the outcome because differences in gene flow and genetic drift could cause evolutionary changes that do not match the direction of selection expected by forces due to the other organism. Coevolution is an important phenomenon necessary for understanding the vital relationship between plants and their fungal parasites.

References

Burdon, J. J., and P. H. Thrall. 2009. Co-evolution of plants and their pathogen in natural habitats. Science 324(5928): 755-756.

Capelle, J. and C. Neema. 2005. Local adaptation and population structure at a micro-geographical scale of a fungal parasite on its host plant. Journal of Evolutionary Biology 18(6): 1445-1454.

Clay, Keith. 1991. Parasitic castration of plants by fungi. Trends in Ecology & Evolution 6(5): 162-166.

Delmotte, F., E. Bucheli, and J. A. Shykoff. 1999. Host and parasite population structure in a natural plant-pathogen system. Heredity 82: 300-308.

Desprez-Loustau, M.-L., Y. Vitasse, S. Delzon, X. Capdevielle, B. Marcais, and A. Kremer. 2010. Are plant pathogen populations adapted for encounter with their host? A case study of phenological synchrony between oak and an obligate fungal parasite along an altitudinal gradient. Journal of Evolutionary Biology 23(1): 87-97.

Kaltz, O. and J. A. Shykoff. 2002. Within- and among-population variation in infectivity, latency and spore production in a host-pathogen system. Journal of Evolutionary Biology 15(5): 850-860.

Sahai, A. S., and M. S. Manocha. 1992. Chitinases of fungi and plants: their involvement in morphogenesis and host-parasite interaction. FEMS Microbiology Reviews 11(4): 317-338.

Saikkonen, K., P. Wali, J. Helander, and S. H. Faeth. 2004. Evolution of endophyte-plant symbioses. Trends in Plant Science 9(6): 275-280.

Zhan, J., C. C. Mundt, M. E. Hoffer, and B. A. McDonald. 2002. Local adaptation and effect of host genotype on the rate of pathogen evolution: an experimental test in a plant pathosystem. Journal of Evolutionary Biology 15(4): 634-647.

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======================================================================================================================= 17 November 2014 Wikipedia Edits https://en.wikipedia.org/wiki/Evolution_of_plants#Mechanisms_and_players_in_evolution_of_plant_form

Coevolution of Plants and Fungal Parasites
An additional contributing factor in some plants leading to evolutionary change is the force due to coevolution with fungal parasites. In an environment with a fungal parasite, which is common in nature, the plants must make adaptation in an attempt to evade the harmful effects of the parasite.

Whenever a parasitic fungus is siphoning limited resources away from a plant, there is selective pressure for a phenotype that is better able to prevent parasitic attack from fungi. At the same time, fungi that are better equipped to evade the defenses of the plant will have greater fitness level. The combination of these two factors leads to an endless cycle of evolutionary change in the host-pathogen system.

Because each species in the relationship is influenced by a constantly changing symbiont, evolutionary change usually occurs at a faster pace than if the other species was not present. This is true of most instances of coevolution. This makes the ability of a population to quickly evolve vital to its survival. Also, if the pathogenic species is too successful and threatens the survival and reproductive success of the host plants, the pathogenic fungi risk losing their nutrient source for future generations. These factors create a dynamic that shapes the evolutionary changes in both species generation after generation.

Genes that code for defense mechanisms in plants must keep changing to keep up with the parasite that constantly works to evade the defenses. Genes that code for attachment mechanisms are the most dynamic and are directly related to the evading ability of the fungi. The greater the changes in these genes, the more change in the attachment mechanism. After selective forces on the resulting phenotypes, evolutionary change that promotes evasion of host defenses occurs.

Fungi not only evolve to avoid the defenses of the plants, but they also attempt to prevent the plant from enacting the mechanisms to improve its defenses. Anything the fungi can do to slow the evolution process of the host plants will improve the fitness of future generations because the plant will not be able to keep up with the evolutionary changes of the parasite. One of the main processes by which plants quickly evolve in response to the environment is sexual reproduction. Without sexual reproduction, advantageous traits could not be spread through the plant population as quickly allowing the fungi to gain a competitive advantage. For this reason, the sexual reproductive organs of plants are targets for attacks by fungi. Studies have shown that many different current types of obligate parasitic plant fungi have developed mechanisms to disable or otherwise affect the sexual reproduction of the plants. If successful, the sexual reproduction process slows for the plant, thus slowing down evolutionary change or in extreme cases, the fungi can render the plant sterile creating an advantage for the pathogens. It is unknown exactly how this adaptive trait developed in fungi, but it is clear that the relationship to the plant forced the development of the process.

Some researchers are also studying how a range of factors affect the rate of evolutionary change and the outcomes of change in different environments. For example, as with most evolution, increases in heritability in a population allow for a greater evolutionary response in the presence of selective pressure. For traits specific to the plant-fungi coevolution, researchers have studied how the virulence of the invading pathogen affects the coevolution. Studies involving Mycosphaerella graminicola have consistently showed that virulence of a pathogen does not have a significant impact on the evolutionary track of the host plant.

There can be other factors in that can affect the process of coevolution. For example, in small populations, selection is a relatively weaker force on the population due to genetic drift. Genetic drift increases the likelihood of having fixed alleles which decreases the genetic variance in the population. Therefore, if there is only a small population of plants in an area with the ability to reproduce together, genetic drift may counteract the effects of selection putting the plant in a disadvantageous position to fungi which can evolve at a normal rate. The variance in both the host and pathogen population is a major determinant of evolutionary success compared to the other species. The greater the genetic variance, the faster the species can evolve to counteract the other organism’s avoidance or defensive mechanisms.

Due to the process of pollination for plants, the effective population size is normally larger than for fungi because pollinators can link isolated populations in a way that the fungus is not able. This means positive traits that evolve in non-adjacent but close areas can be passed to nearby areas. Fungi must individually evolve to evade host defenses in each area. This is obviously a clear competitive advantage for the host plants. Sexual reproduction with a broad, high variance population leads to fast evolutionary change and higher reproductive success of offspring.

Environment and climate patterns also play a role in evolutionary outcomes. Studies with oak trees and an obligate fungal parasite at different altitudes clearly show this distinction. For the same species, different altitudinal positions had drastically different rates of evolution and changes in the response to the pathogens due to the organism also in a selective environment due to their surroundings.

Coevolution is a process that is related to the red queen hypothesis. Both the host plant and parasitic fungi have to continue to survive to stay in their ecological niche. If one of the two species in the relationship evolves at a significantly faster rate than the other, the slower species will be at a competitive disadvantage and risk the loss of nutrients. Because the two species in the system are so closely linked, they respond to external environment factors together and each species affects the evolutionary outcome of the other. In other words, each species exerts selective pressure on the other. Population size is also a major factor in the outcome because differences in gene flow and genetic drift could cause evolutionary changes that do not match the direction of selection expected by forces due to the other organism. Coevolution is an important phenomenon necessary for understanding the vital relationship between plants and their fungal parasites.