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Why does the maintenance of outcrossing require populations to be continuously exposed to changes in their environment? How does the Red Queen hypothesis fulfill this requirement?

Why does the maintenance of outcrossing require populations to be continuously exposed to changes in their environment? How does the Red Queen hypothesis fulfill this requirement?

Human Anatomy & Physiology (11th Edition)
Human Anatomy & Physiology (11th Edition)
11th Edition
ISBN:9780134580999
Author:Elaine N. Marieb, Katja N. Hoehn
Publisher:Elaine N. Marieb, Katja N. Hoehn
Chapter1: The Human Body: An Orientation
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Why does the maintenance of outcrossing require populations to be continuously exposed to changes in their environment? How does the Red Queen hypothesis fulfill this requirement?
6 9:24 < Back *To whom correspondence should be addressed. E-mail: Imorran@indiana.edu in response to S. marcescens exposure (5), and S. marcescens can evolve greater infectivity when successful infection of C. elegans is its only means of proliferation. Selection for increased infectiv- ity can be imposed by propagating only those bacterial cells that have been harvested from the carcasses of hosts, which were killed by the bacte- ria within 24 hours of exposure. Therefore, the C. elegans/S. marcescens system can be used to generate antagonistic coevolution when a host pop- ulation and a pathogen population are repeatedly passaged under selection together, thus permitting a direct test of the Red Queen hypothesis. We used experimental coevolution in the C. elegans/S. marcescens system to test the pre- diction that antagonistic coevolution between host and pathogen populations can maintain high levels of outcrossing despite the inherent cost of males. We used obligately selfing, wild-type, and obligately outcrossing populations of C. elegans with a CB4856 genetic background (5). Where- as the reproductive modes of the obligately self- ing and obligately outcrossing populations are genetically fixed, the wild-type populations can 0.8, A Fig. 2. Coevolutionary dynamics of hosts and pathogens. We exposed hosts evolved under the coevolution treatment and their ancestral popu lations (before coevolution) to three pathogen populations: (i) an ancestor strain (ancestral to all S. marcescens used in this study), (ii) a noncoevolv- ing strain (evolved without selection), and (iii) their respective coevolving strain (coevolving with the host pop- ulation). We evaluated host mortal- ity after 24 hours of exposure to the pathogens and present the means across the replicate host populations. (A) Three obligately selfing C. elegans populations persisted beyond 10 host generations in the coevolution treat 0.6 w ment. These populations were assayed before extinction. (B) All five wild- type C. elegans populations in the coevolution treatment and their an- cestors were assayed at the endpoint of the experiment (30 host gener- ations). (C) All five obligately out- crossing C. elegans populations in the coevolution treatment and their an cestors were assayed at the endpoint of the experiment. Error bars, 2 SEM. DORTO N ◄ Previous 8 JULY 2011 VOL 333 SCIENCE www.sciencemag.org 0.4 0.2 Smarcescens 06 Ancestor 0 08, B 0.4 0.2 File Details PCB4674 U02 1231 0 0.8 C 0.6 0.4 0.2 0 Obligately Selfing C. elegans Non-coevolving Coevolving none of the obligately selfing populations went extinct in either the evolution treatment or in the control treatment. In addition, all of the obligately outcrossing and wild-type populations persisted throughout the experiment in all three treatment types (fig. S1). Thus, extinction was only ob- served in obligately selfing hosts when confronted with coevolving pathogens. reproduce by either selfing or outcrossing [the baseline outcrossing rate is-20 to 30% (5)], and the rate of outcrossing can respond to selection (5). Before the experiment, we mutagenized five independent replicate populations of each mating type (obligate selfing, wild-type, and obligate out crossing) by exposing them to ethyl methane sulfonate (EMS) to infuse novel genetic variation in each population. The five replicate populations We also found that the presence of coevolving were then passaged under three different para- S. marcescens selected for and maintained high site treatments (table S1): (i) control (no exposure levels of outcrossing in wild-type C. elegans pop- to S. marcescens), (ii) evolution (repeated expo-ulations (Fig. 1). Over the first eight generations sure to a fixed, nonevolving strain of S. marcescens), of the experiment, outcrossing rates increased and (i) coevolution. The coevolution treatment in from 20% to more than 70% in both the evo- volved repeated exposure (30 host generations) to lution and coevolution treatments (Fig. 1) (F211- a potentially coevolving population of Smarcescens, 8.26; P=0.006). However, the wild-type popu- which was under selection for increased infectiv- lations consistently exposed to a fixed population ity. S. marcescens Sm2170 served as the ancestral of S. marcescens (evolution treatment) exhibited strain in the coevolution tr strain in the coevolution treatment, as well as the a steady decline in outcrossing rates after this ini- fixed strain in the evolution treatment. tial increase, eventually returning to control levels of outcrossing (Fig. 1), as previously observed (5). In contrast, populations in the coevolution treat- ment consistently maintained high levels of out- crossing throughout the experiment, relative to the control treatment (Fig. 1) (F₁.12-209.5; P< 0.0001). Coevolution with S. marcescens, there- fore, favored the evolution and long-term main- tenance of higher rates of outcrossing. The results were consistent with the Red Queen hypothesis. In the coevolution treatment, all of the obligately selfing populations became extinct within 20 generations (fig. S1). However, Ancestral Populations Generation Generation 10 "Coevolution" Populations Wildtype (Mixed Mating) C. elegans Ancestral Populations Generation 30 "Coevolution" Populations Obligately Outcrossing C. elegans m Ancestral Populations Dashboard Calendar Generation 30 "Coevolution" Populations www.sciencemag.org SCIENCE VOL 333 8 JULY 2011 5 To Do 5GE B As also predicted by the Red Queen hypoth- esis, outcrossing hosts adapted to changes in the pathogen population, whereas selfing apparently prevented an adaptive counter-response. The an- cestral strain of the obligately selfing hosts suffered higher mortality rates when exposed to bacteria from the coevolution treatment than when ex- posed to either the ancestral bacteria (Fig. 2A) (c> a: F₁1-21.2; P<0.0001) or to the nonco- evolving control bacteria (Fig. 2A) (e>b: F 31.9, P<0.0001). Therefore, the bacteria in the Foun coevolution treatment evolved greater infectivity in response to selection. Further, the obligately selfing hosts did not adapt to the evolution of increased infectivity in the bacteria, because the bacteria from the coevolution treatment in- duced greater levels of mortality against the worms after 10 generations of coevolution than against the ancestral hosts (Fig. 2A) (f>c: F171-69.2; P<0.0001). Finally, an increase in mortality by more than a factor of 3 was observed in the obligately selfing hosts in only 10 generations (Fig. 2A) (f> a: F₁173.7; P<0.0001), which could explain why these hosts were driven to extinction. O REPORTS The pathogens that coevolved with the wild- type and obligate outcrossing populations also evolved greater infectivity (Fig. 2, B and C) (i>h: F1,304 69.5; P<0.0001; />g: F1,104-32.9, P< 0.0001; on: F160-141.1; P<0.0001; o> m: F160 50.9; P<0.0001). However, the wild-type and obligately outcrossing populations adapted to the changes in their respective coevolving path- ogen populations. Specifically, both the wild-type and obligately outcrossing populations exhibited lower mortality rates against the pathogens with which they were currently evolving than did their Notifications Next ► 217 62 Inbox
Transcribed Image Text:6 9:24 < Back *To whom correspondence should be addressed. E-mail: Imorran@indiana.edu in response to S. marcescens exposure (5), and S. marcescens can evolve greater infectivity when successful infection of C. elegans is its only means of proliferation. Selection for increased infectiv- ity can be imposed by propagating only those bacterial cells that have been harvested from the carcasses of hosts, which were killed by the bacte- ria within 24 hours of exposure. Therefore, the C. elegans/S. marcescens system can be used to generate antagonistic coevolution when a host pop- ulation and a pathogen population are repeatedly passaged under selection together, thus permitting a direct test of the Red Queen hypothesis. We used experimental coevolution in the C. elegans/S. marcescens system to test the pre- diction that antagonistic coevolution between host and pathogen populations can maintain high levels of outcrossing despite the inherent cost of males. We used obligately selfing, wild-type, and obligately outcrossing populations of C. elegans with a CB4856 genetic background (5). Where- as the reproductive modes of the obligately self- ing and obligately outcrossing populations are genetically fixed, the wild-type populations can 0.8, A Fig. 2. Coevolutionary dynamics of hosts and pathogens. We exposed hosts evolved under the coevolution treatment and their ancestral popu lations (before coevolution) to three pathogen populations: (i) an ancestor strain (ancestral to all S. marcescens used in this study), (ii) a noncoevolv- ing strain (evolved without selection), and (iii) their respective coevolving strain (coevolving with the host pop- ulation). We evaluated host mortal- ity after 24 hours of exposure to the pathogens and present the means across the replicate host populations. (A) Three obligately selfing C. elegans populations persisted beyond 10 host generations in the coevolution treat 0.6 w ment. These populations were assayed before extinction. (B) All five wild- type C. elegans populations in the coevolution treatment and their an- cestors were assayed at the endpoint of the experiment (30 host gener- ations). (C) All five obligately out- crossing C. elegans populations in the coevolution treatment and their an cestors were assayed at the endpoint of the experiment. Error bars, 2 SEM. DORTO N ◄ Previous 8 JULY 2011 VOL 333 SCIENCE www.sciencemag.org 0.4 0.2 Smarcescens 06 Ancestor 0 08, B 0.4 0.2 File Details PCB4674 U02 1231 0 0.8 C 0.6 0.4 0.2 0 Obligately Selfing C. elegans Non-coevolving Coevolving none of the obligately selfing populations went extinct in either the evolution treatment or in the control treatment. In addition, all of the obligately outcrossing and wild-type populations persisted throughout the experiment in all three treatment types (fig. S1). Thus, extinction was only ob- served in obligately selfing hosts when confronted with coevolving pathogens. reproduce by either selfing or outcrossing [the baseline outcrossing rate is-20 to 30% (5)], and the rate of outcrossing can respond to selection (5). Before the experiment, we mutagenized five independent replicate populations of each mating type (obligate selfing, wild-type, and obligate out crossing) by exposing them to ethyl methane sulfonate (EMS) to infuse novel genetic variation in each population. The five replicate populations We also found that the presence of coevolving were then passaged under three different para- S. marcescens selected for and maintained high site treatments (table S1): (i) control (no exposure levels of outcrossing in wild-type C. elegans pop- to S. marcescens), (ii) evolution (repeated expo-ulations (Fig. 1). Over the first eight generations sure to a fixed, nonevolving strain of S. marcescens), of the experiment, outcrossing rates increased and (i) coevolution. The coevolution treatment in from 20% to more than 70% in both the evo- volved repeated exposure (30 host generations) to lution and coevolution treatments (Fig. 1) (F211- a potentially coevolving population of Smarcescens, 8.26; P=0.006). However, the wild-type popu- which was under selection for increased infectiv- lations consistently exposed to a fixed population ity. S. marcescens Sm2170 served as the ancestral of S. marcescens (evolution treatment) exhibited strain in the coevolution tr strain in the coevolution treatment, as well as the a steady decline in outcrossing rates after this ini- fixed strain in the evolution treatment. tial increase, eventually returning to control levels of outcrossing (Fig. 1), as previously observed (5). In contrast, populations in the coevolution treat- ment consistently maintained high levels of out- crossing throughout the experiment, relative to the control treatment (Fig. 1) (F₁.12-209.5; P< 0.0001). Coevolution with S. marcescens, there- fore, favored the evolution and long-term main- tenance of higher rates of outcrossing. The results were consistent with the Red Queen hypothesis. In the coevolution treatment, all of the obligately selfing populations became extinct within 20 generations (fig. S1). However, Ancestral Populations Generation Generation 10 "Coevolution" Populations Wildtype (Mixed Mating) C. elegans Ancestral Populations Generation 30 "Coevolution" Populations Obligately Outcrossing C. elegans m Ancestral Populations Dashboard Calendar Generation 30 "Coevolution" Populations www.sciencemag.org SCIENCE VOL 333 8 JULY 2011 5 To Do 5GE B As also predicted by the Red Queen hypoth- esis, outcrossing hosts adapted to changes in the pathogen population, whereas selfing apparently prevented an adaptive counter-response. The an- cestral strain of the obligately selfing hosts suffered higher mortality rates when exposed to bacteria from the coevolution treatment than when ex- posed to either the ancestral bacteria (Fig. 2A) (c> a: F₁1-21.2; P<0.0001) or to the nonco- evolving control bacteria (Fig. 2A) (e>b: F 31.9, P<0.0001). Therefore, the bacteria in the Foun coevolution treatment evolved greater infectivity in response to selection. Further, the obligately selfing hosts did not adapt to the evolution of increased infectivity in the bacteria, because the bacteria from the coevolution treatment in- duced greater levels of mortality against the worms after 10 generations of coevolution than against the ancestral hosts (Fig. 2A) (f>c: F171-69.2; P<0.0001). Finally, an increase in mortality by more than a factor of 3 was observed in the obligately selfing hosts in only 10 generations (Fig. 2A) (f> a: F₁173.7; P<0.0001), which could explain why these hosts were driven to extinction. O REPORTS The pathogens that coevolved with the wild- type and obligate outcrossing populations also evolved greater infectivity (Fig. 2, B and C) (i>h: F1,304 69.5; P<0.0001; />g: F1,104-32.9, P< 0.0001; on: F160-141.1; P<0.0001; o> m: F160 50.9; P<0.0001). However, the wild-type and obligately outcrossing populations adapted to the changes in their respective coevolving path- ogen populations. Specifically, both the wild-type and obligately outcrossing populations exhibited lower mortality rates against the pathogens with which they were currently evolving than did their Notifications Next ► 217 62 Inbox
EPORTS 6 9:24 < Back File Details PCB4674 U02 1231 Running with the Red Queen: Host-Parasite for Biparental Sex Levi T. Morran, Olivia G. Schmidt, Ian A. Gelarden, Raymond C. Parrish II, Curtis M. Lively Coevolution Selects Most organisms reproduce through outcrossing, even though it comes with substantial costs. The Red Queen hypothesis proposes that selection from coevolving pathogens facilitates the persistence of outcrossing despite these costs. We used experimental coevolution to test the Red Queen hypothesis and found that coevolution with a bacterial pathogen (Serratia marcescens) resulted in significantly more outcrossing in mixed mating experimental populations of the nematode Caenorhabditis elegans. Furthermore, we found that coevolution with the pathogen rapidly drove obligately selfing populations to extinction, whereas outcrossing populations persisted through reciprocal coevolution. Thus, consistent with the Red Queen hypothesis, coevolving pathogens can select for biparental sex. utcrossing (mating between different in dividuals) is the most prevalent mode of The maintenance of outcrossing on such a large scale strongly suggests that there is a selective ad- vantage for outcrossing relative to self-fertilization or asexual reproduction. Nonetheless, the preva- lence of outcrossing is puzzling, because it often incurs costs that are not associated with uni- parental modes of reproduction (1-3). For exam- ple, many outcrossing species produce males that facilitate outcrossing but are incapable of bearing offspring themselves, resulting in the "cost of males." Every male takes the place of an offspring-bearing progeny (female or hermaph- rodite) that could have been produced (2). The systematic loss of offspring-bearing progeny can reduce the numerical contribution of a lineage by as much 50% (2). Therefore, the selective ben- efits of outcrossing must more than compensate for this fitness deficit to achieve a high frequency in nature. One selective benefit of outcrossing, relative to self-fertilization, is the capability to produce offspring with greater fitness under novel envi- ronmental conditions (4, 5). Outcrossing can in- crease fitness and accelerate a population's rate of adaptation to novel conditions by permitting genetic exchange between diverse lineages, pro- moting genetic variation among offspring, and allowing beneficial alleles to be quickly assembled into the same genome (6, 7). In contrast, obligate selfing can impede adaptation by preventing ge- netic exchange, which results in the loss of within- lineage genetic variation and ultimately confines. beneficial alleles to a single lineage (8,9). Under novel environmental conditions, the benefits of outcrossing can compensate for the cost of male production, but these benefits may be short-lived (5). Outcrossing is less likely to be favored after Department of Biology, Indiana University, 1001 East Third Street, Bloomington, IN 47405, USA. "To whom correspondence should be addressed. E-mail: Imorran@indiana.edu in response to S. marcescens exposure (5), and S. marcescens can evolve greater infectivity when successful infection of C. elegans is its only means of proliferation. Selection for increased infectiv ity can be imposed by propagating only those ◄ Previous populations adapt to a novel environment, as ge- netic exchange becomes less imperative or per- haps even deleterious (8,9). Hence, the long-term maintenance of outcrossing would seem to require that populations are constantly exposed to novel environmental conditions. The Red Queen hypothesis provides a pos- sible explanation for the long-term maintenance of outcrossing. Specifically, under the Red Queen hypothesis, coevolutionary interactions between hosts and pathogens might generate ever-changing environmental conditions and thus favor the long- term maintenance of outcrossing relative to self- fertilization (10) or asexual reproduction (11, 12). The reason is that hosts are under selection to evade infection by the pathogen, whereas the pathogen is selected to infect the hosts. Assuming that some form of genetic matching between host and pathogen determines the outcome of inter- actions, pathogen genotypes that infect the most common host genotypes will be favored by natu- ral selection (11, 13). This may produce substan- tial and frequent change in pathogen populations, thus rapidly changing the environment for the host population. Under these conditions, outcross- ing can facilitate rapid adaptation by generating 0.8 0.6 ME 12 16 20 Generation 0.4 0 a 0 Dashboard Calendar 4 8 reproduce by either selfing or outcrossing [the baseline outcrossing rate is-20 to 30% (5)], and the rate of outcrossing can respond to selection (5). Before the experiment, we mutagenized five independent replicate populations of each mating 8 JULY 2011 VOL 333 SCIENCE www.sciencemag.org 5 To Do offspring with rare or novel genotypes, which are more likely to escape infection by coevolving path- ogens (10-13), Conversely, selfing and asexual reproduction generate offspring with little or no genetic diversity, thus impeding the adaptive pro- cess and leaving them highly susceptible to infec- tion by coevolving pathogens (10-13). The Red Queen hypothesis has been empir- ically supported in studies of natural snail popu- lations, which show that sexual reproduction is more common where parasites are common and adapted to infect the local host population (14, 15). Outcrossing also seems to reduce the degree of infection relative to biparental inbreeding and asexual reproduction in fish (16). Finally, the capability of antagonistic interactions to drive rap- id evolutionary change has also been determined for several different systems (17-20). Nonetheless, direct controlled tests for the effect of coevolution on the maintenance of sex have proven difficult, because they require biological systems in which host and pathogen populations can coevolve for multiple generations in a manner that selects for increased infectivity by a pathogen as well as in- creased resistance (or enhanced avoidance) by the host. Further, the host species should exhibit genetic variation in its degree of outcrossing. Thus, we chose to examine the nematode Caenorhabditis elegans and its pathogenic bacteria Serratia marcescens, which exhibit these desired properties. Populations of the host species, C. elegans, are composed of males and hermaphrodites. The hermaphrodites can reproduce through either self-fertilization or by outcrossing with males (27) Although usually low (<1% to 30%) (22), out- crossing rates can be genetically manipulated to produce either obligately selfing (5, 23) or ob- ligately outcrossing (5, 24) populations. The path- ogen, S. marcescens 2170, is highly virulent and capable of exerting strong selection on C. elegares. When consumed, live S. marcescens can produce a systemic infection that kills the nematode with in 24 hours (25). This interaction has a heritable genetic basis (26), which allows for a potential response to selection. Moreover, C. elegans pop- ulations are capable of evolving greater fitness 24 ...Control Evolution Coevolution 5GE 28 A 32 Fig. 1. Wild-type outcross ing rates over time. Out- crossing rates in wild-type populations were not ma nipulated and free to evolve during the experiment. The wild-type populations were exposed to three dif- ferent treatments: control (no S. marcescens; dotted line), evolution (fixed strain of S. marcescens; dashed line), and coevolution (co- evolving S. marcescens; solid line) for 30 gener- ations. Error bars, 2 SEM none of the obligately selfing populations went extinct in either the evolution treatment or in the control treatment. In addition, all of the obligately outcrossing and wild-type populations persisted throughout the experiment in all three treatment Next ► Notifications REPORTS 62 Inbox
Transcribed Image Text:EPORTS 6 9:24 < Back File Details PCB4674 U02 1231 Running with the Red Queen: Host-Parasite for Biparental Sex Levi T. Morran, Olivia G. Schmidt, Ian A. Gelarden, Raymond C. Parrish II, Curtis M. Lively Coevolution Selects Most organisms reproduce through outcrossing, even though it comes with substantial costs. The Red Queen hypothesis proposes that selection from coevolving pathogens facilitates the persistence of outcrossing despite these costs. We used experimental coevolution to test the Red Queen hypothesis and found that coevolution with a bacterial pathogen (Serratia marcescens) resulted in significantly more outcrossing in mixed mating experimental populations of the nematode Caenorhabditis elegans. Furthermore, we found that coevolution with the pathogen rapidly drove obligately selfing populations to extinction, whereas outcrossing populations persisted through reciprocal coevolution. Thus, consistent with the Red Queen hypothesis, coevolving pathogens can select for biparental sex. utcrossing (mating between different in dividuals) is the most prevalent mode of The maintenance of outcrossing on such a large scale strongly suggests that there is a selective ad- vantage for outcrossing relative to self-fertilization or asexual reproduction. Nonetheless, the preva- lence of outcrossing is puzzling, because it often incurs costs that are not associated with uni- parental modes of reproduction (1-3). For exam- ple, many outcrossing species produce males that facilitate outcrossing but are incapable of bearing offspring themselves, resulting in the "cost of males." Every male takes the place of an offspring-bearing progeny (female or hermaph- rodite) that could have been produced (2). The systematic loss of offspring-bearing progeny can reduce the numerical contribution of a lineage by as much 50% (2). Therefore, the selective ben- efits of outcrossing must more than compensate for this fitness deficit to achieve a high frequency in nature. One selective benefit of outcrossing, relative to self-fertilization, is the capability to produce offspring with greater fitness under novel envi- ronmental conditions (4, 5). Outcrossing can in- crease fitness and accelerate a population's rate of adaptation to novel conditions by permitting genetic exchange between diverse lineages, pro- moting genetic variation among offspring, and allowing beneficial alleles to be quickly assembled into the same genome (6, 7). In contrast, obligate selfing can impede adaptation by preventing ge- netic exchange, which results in the loss of within- lineage genetic variation and ultimately confines. beneficial alleles to a single lineage (8,9). Under novel environmental conditions, the benefits of outcrossing can compensate for the cost of male production, but these benefits may be short-lived (5). Outcrossing is less likely to be favored after Department of Biology, Indiana University, 1001 East Third Street, Bloomington, IN 47405, USA. "To whom correspondence should be addressed. E-mail: Imorran@indiana.edu in response to S. marcescens exposure (5), and S. marcescens can evolve greater infectivity when successful infection of C. elegans is its only means of proliferation. Selection for increased infectiv ity can be imposed by propagating only those ◄ Previous populations adapt to a novel environment, as ge- netic exchange becomes less imperative or per- haps even deleterious (8,9). Hence, the long-term maintenance of outcrossing would seem to require that populations are constantly exposed to novel environmental conditions. The Red Queen hypothesis provides a pos- sible explanation for the long-term maintenance of outcrossing. Specifically, under the Red Queen hypothesis, coevolutionary interactions between hosts and pathogens might generate ever-changing environmental conditions and thus favor the long- term maintenance of outcrossing relative to self- fertilization (10) or asexual reproduction (11, 12). The reason is that hosts are under selection to evade infection by the pathogen, whereas the pathogen is selected to infect the hosts. Assuming that some form of genetic matching between host and pathogen determines the outcome of inter- actions, pathogen genotypes that infect the most common host genotypes will be favored by natu- ral selection (11, 13). This may produce substan- tial and frequent change in pathogen populations, thus rapidly changing the environment for the host population. Under these conditions, outcross- ing can facilitate rapid adaptation by generating 0.8 0.6 ME 12 16 20 Generation 0.4 0 a 0 Dashboard Calendar 4 8 reproduce by either selfing or outcrossing [the baseline outcrossing rate is-20 to 30% (5)], and the rate of outcrossing can respond to selection (5). Before the experiment, we mutagenized five independent replicate populations of each mating 8 JULY 2011 VOL 333 SCIENCE www.sciencemag.org 5 To Do offspring with rare or novel genotypes, which are more likely to escape infection by coevolving path- ogens (10-13), Conversely, selfing and asexual reproduction generate offspring with little or no genetic diversity, thus impeding the adaptive pro- cess and leaving them highly susceptible to infec- tion by coevolving pathogens (10-13). The Red Queen hypothesis has been empir- ically supported in studies of natural snail popu- lations, which show that sexual reproduction is more common where parasites are common and adapted to infect the local host population (14, 15). Outcrossing also seems to reduce the degree of infection relative to biparental inbreeding and asexual reproduction in fish (16). Finally, the capability of antagonistic interactions to drive rap- id evolutionary change has also been determined for several different systems (17-20). Nonetheless, direct controlled tests for the effect of coevolution on the maintenance of sex have proven difficult, because they require biological systems in which host and pathogen populations can coevolve for multiple generations in a manner that selects for increased infectivity by a pathogen as well as in- creased resistance (or enhanced avoidance) by the host. Further, the host species should exhibit genetic variation in its degree of outcrossing. Thus, we chose to examine the nematode Caenorhabditis elegans and its pathogenic bacteria Serratia marcescens, which exhibit these desired properties. Populations of the host species, C. elegans, are composed of males and hermaphrodites. The hermaphrodites can reproduce through either self-fertilization or by outcrossing with males (27) Although usually low (<1% to 30%) (22), out- crossing rates can be genetically manipulated to produce either obligately selfing (5, 23) or ob- ligately outcrossing (5, 24) populations. The path- ogen, S. marcescens 2170, is highly virulent and capable of exerting strong selection on C. elegares. When consumed, live S. marcescens can produce a systemic infection that kills the nematode with in 24 hours (25). This interaction has a heritable genetic basis (26), which allows for a potential response to selection. Moreover, C. elegans pop- ulations are capable of evolving greater fitness 24 ...Control Evolution Coevolution 5GE 28 A 32 Fig. 1. Wild-type outcross ing rates over time. Out- crossing rates in wild-type populations were not ma nipulated and free to evolve during the experiment. The wild-type populations were exposed to three dif- ferent treatments: control (no S. marcescens; dotted line), evolution (fixed strain of S. marcescens; dashed line), and coevolution (co- evolving S. marcescens; solid line) for 30 gener- ations. Error bars, 2 SEM none of the obligately selfing populations went extinct in either the evolution treatment or in the control treatment. In addition, all of the obligately outcrossing and wild-type populations persisted throughout the experiment in all three treatment Next ► Notifications REPORTS 62 Inbox
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