Outcrossing increases resistance against coevolving parasites.

Despite substantial costs, biparental sex is the dominant mode of reproduction across plant and animal taxa. The Red Queen hypothesis (RQH) posits that coevolutionary interactions with parasites can favor biparental sex in hosts, despite the costs. In support of the RQH, previous studies found that coevolutionary interactions with virulent bacterial parasites maintained high outcrossing rates in populations of the androdioecious nematode host Caenorhabditis elegans . Here we test three non-mutually exclusive mechanisms that could explain how coevolving parasites maintain outcrossing rates in C. elegans hosts: 1) short-term parasite exposure induces plastic increases in the hosts' propensity to outcross, 2) hosts evolve increased outcrossing propensity in response to selection imposed by coevolving parasites, and 3) outcrossed offspring incur less parasite-mediated fitness loss than selfed offspring, increasing host male frequencies and opportunities for outcrossing. We find no evidence that parasites cause plastic or evolved changes in host outcrossing propensity. However, parental outcrossing significantly increases survival of host offspring in the F2 generation when exposed to a coevolving parasite. Hence, coevolving parasites maintain outcrossing in host populations by selecting against selfed offspring, rather than by inducing changes in the propensity to outcross.

High parasite virulence necessary for the maintenance of host outcrossing via parasite-mediated selection.

Biparental sex is widespread in nature, yet costly relative to uniparental reproduction. It is generally unclear why self-fertilizing or asexual lineages do not readily invade outcrossing populations. The Red Queen hypothesis predicts that coevolving parasites can prevent self-fertilizing or asexual lineages from invading outcrossing host populations. However, only highly virulent parasites are predicted to maintain outcrossing, which may limit the general applicability of the Red Queen hypothesis. Here, we tested whether the ability of coevolving parasites to prevent invasion of self-fertilization within outcrossing host populations was dependent on parasite virulence. We introduced wild-type Caenorhabditis elegans hermaphrodites, capable of both self-fertilization and outcrossing, into C. elegans populations fixed for a mutant allele conferring obligate outcrossing. Replicate C. elegans populations were exposed for 24 host generations to one of four strains of Serratia marcescens parasites that varied in virulence, under three treatments: a heat-killed (control, noninfectious) parasite treatment, a fixed-genotype (nonevolving) parasite treatment, and a copassaged (potentially coevolving) parasite treatment. As predicted, self-fertilization invaded C. elegans host populations in the control and fixed-parasite treatments, regardless of parasite virulence. In the copassaged treatment, selfing invaded host populations coevolving with low- to mid-virulence strains, but remained rare in hosts coevolving with highly virulent bacterial strains. Therefore, we found that only highly virulent coevolving parasites can impede the invasion of selfing.

Gene flow accelerates adaptation to a parasite.

Gene flow into populations can increase additive genetic variation and introduce novel beneficial alleles, thus facilitating adaptation. However, gene flow may also impede adaptation by disrupting beneficial genotypes, introducing deleterious alleles, or creating novel dominant negative interactions. While theory and fieldwork have provided insight into the effects of gene flow, direct experimental tests are rare. Here, we evaluated the effects of gene flow on adaptation in the nematode Caenorhabditis elegans during exposure to the bacterial parasite, Serratia marcescens. We evolved hosts against nonevolving parasites for 10 passages while controlling host gene flow and source population. We used source nematode populations with three different genetic backgrounds (one similar to the sink population and two different) and two evolutionary histories (previously adapted to S. marcescens or naive). We found that populations with gene flow exhibited greater increases in parasite resistance than those without gene flow. Additionally, gene flow from adapted populations resulted in greater increases in resistance than gene flow from naive populations, particularly with gene flow from novel genetic backgrounds. Overall, this work demonstrates that gene flow can facilitate adaptation and suggests that the genetic architecture and evolutionary history of source populations can alter the sink population's response to selection.

Associations of Evolutionary-Concordance Diet and Lifestyle Pattern Scores with Incident, Sporadic Colorectal Adenoma in a Pooled Case-Control Study.

Differences in diet and lifestyle relative to those of our Paleolithic-era ancestors may explain current high incidences of chronic diseases, including colorectal cancer (CRC), in Westernized countries. Previously reported evolutionary-concordance diet and lifestyle pattern scores, reflecting closeness of diet and lifestyle patterns to those of Paleolithic-era humans, were associated with lower CRC incidence. Separate and joint associations of the scores with colorectal adenoma among men and women are unknown. To address this, we pooled data from three case-control studies of incident, sporadic colorectal adenomas (n = 771 cases, 1,990 controls), used participants' responses to food frequency and lifestyle questionnaires to calculate evolutionary-concordance diet and lifestyle pattern scores, and estimated the scores' associations with adenomas using multivariable unconditional logistic regression. The multivariable-adjusted odds ratios comparing those in the highest relative to the lowest diet and lifestyle score quintiles were 0.84 (95% confidence interval [CI] 0.62, 1.12; Ptrend:0.03) and 0.41 (95% CI 0.29, 0.59; Ptrend:<0.0001), respectively. The inverse associations were stronger for high-risk adenomas, and among those with both high relative to those with both low diet and lifestyle scores. These results suggest that more evolutionary-concordant diet and lifestyle patterns, separately and jointly, may be associated with lower risk for incident, sporadic colorectal adenoma.Supplemental data for this article is available online at https://doi.org/10.1080/01635581.2021.2002919 .

The evolution of parasite host range in heterogeneous host populations.

Theory on the evolution of niche width argues that resource heterogeneity selects for niche breadth. For parasites, this theory predicts that parasite populations will evolve, or maintain, broader host ranges when selected in genetically diverse host populations relative to homogeneous host populations. To test this prediction, we selected the bacterial parasite Serratia marcescens to kill Caenorhabditis elegans in populations that were genetically heterogeneous (50% mix of two experimental genotypes) or homogeneous (100% of either genotype). After 20 rounds of selection, we compared the host range of selected parasites by measuring parasite fitness (i.e. virulence, the selected fitness trait) on the two focal host genotypes and on a novel host genotype. As predicted, heterogeneous host populations selected for parasites with a broader host range: these parasite populations gained or maintained virulence on all host genotypes. This result contrasted with selection in homogeneous populations of one host genotype. Here, host range contracted, with parasite populations gaining virulence on the focal host genotype and losing virulence on the novel host genotype. This pattern was not, however, repeated with selection in homogeneous populations of the second host genotype: these parasite populations did not gain virulence on the focal host genotype, nor did they lose virulence on the novel host genotype. Our results indicate that host heterogeneity can maintain broader host ranges in parasite populations. Individual host genotypes, however, vary in the degree to which they select for specialization in parasite populations.

Dauer life stage of Caenorhabditis elegans induces elevated levels of defense against the parasite Serratia marcescens.

Host-parasite research often focuses on a single host life stage, yet different life stages may exhibit different defenses. The nematode Caenorhabditis elegans has an alternate dispersal life stage, dauer. Despite dauer's importance in nature, we know little of how it responds to parasites. Previous research indicates that non-dauer C. elegans prefer to consume the virulent bacterial parasite, Serratia marcescens, when given a choice between the parasite and benign Escherichia coli. Here, we compared the preferences of dauer individuals from six strains of C. elegans to the preferences of other life stages. We found that dauer individuals exhibited reduced preference for S. marcescens, and dauers from some strains preferred E. coli to S. marcescens. In addition to testing food preference, a mechanism of parasite avoidance, we also measured host mortality rates after direct parasite exposure to determine if life stage also altered host survival. Overall, dauer individuals exhibited reduced mortality rates. However, dauer versus non-dauer larvae mortality rates also varied significantly by host strain. Collectively, we found evidence of dauer-induced parasite avoidance and reduced mortality in the presence of a parasite, but these effects were strain-specific. These results demonstrate the importance of host life stage and genotype when assessing infection dynamics.

Sex and Mitonuclear Adaptation in Experimental Caenorhabditis elegans Populations.

To reveal phenotypic and functional genomic patterns of mitonuclear adaptation, a laboratory adaptation study with Caenorhabditis elegans nematodes was conducted in which independently evolving lines were initiated from a low-fitness mitochondrial electron transport chain (ETC) mutant, gas-1 Following 60 generations of evolution in large population sizes with competition for food resources, two distinct classes of lines representing different degrees of adaptive response emerged: a low-fitness class that exhibited minimal or no improvement compared to the gas-1 mutant ancestor, and a high-fitness class containing lines that exhibited partial recovery of wild-type fitness. Many lines that achieved higher reproductive and competitive fitness levels were also noted to evolve high frequencies of males during the experiment, consistent with adaptation in these lines having been facilitated by outcrossing. Whole-genome sequencing and analysis revealed an enrichment of mutations in loci that occur in a gas-1-centric region of the C. elegans interactome and could be classified into a small number of functional genomic categories. A highly nonrandom pattern of mitochondrial DNA mutation was observed within high-fitness gas-1 lines, with parallel fixations of nonsynonymous base substitutions within genes encoding NADH dehydrogenase subunits I and VI. These mitochondrial gene products reside within ETC complex I alongside the nuclear-encoded GAS-1 protein, suggesting that rapid adaptation of select gas-1 recovery lines was driven by fixation of compensatory mitochondrial mutations.

No measurable fitness cost to experimentally evolved host defence in the Caenorhabditis elegans-Serratia marcescens host-parasite system.

Host susceptibility to parasites can vary over space and time. Costs associated with the maintenance of host defence are thought to account for a portion of this variation. Specifically, trade-offs wherein elevated defence is maintained at the cost of fitness in the absence of the parasite may cause levels of host defence to change over time and differ between populations. In previous studies, we found that populations of the host nematode, Caenorhabditis elegans, evolved greater levels of parasite avoidance and resistance against the bacterial parasite, Serratia marcescens. Here, we passaged these host populations either in the presence or absence of the parasite to test for a cost of elevated host defences. After 16 generations, we found that elevated levels of host defence were maintained during evolution in both the presence and absence of the parasite. Further, this maintenance of defence was not the result of limited standing genetic variation, but rather the absence of a measurable cost associated with defence. Therefore, costs associated with host defence may not broadly account for differences in host susceptibility across space and time.

Turnover in local parasite populations temporarily favors host outcrossing over self-fertilization during experimental evolution.

The ubiquity of outcrossing in plants and animals is difficult to explain given its costs relative to self-fertilization. Despite these costs, exposure to changing environmental conditions can temporarily favor outcrossing over selfing. Therefore, recurring episodes of environmental change are predicted to favor the maintenance of outcrossing. Studies of host-parasite coevolution have provided strong support for this hypothesis. However, it is unclear whether multiple exposures to novel parasite genotypes in the absence of coevolution are sufficient to favor outcrossing. Using the nematode Caenorhabditis elegans and the bacterial parasite Serratia marcescens, we studied host responses to parasite turnover. We passaged several replicates of a host population that was well-adapted to the S. marcescens strain Sm2170 with either Sm2170 or one of three novel S. marcescens strains, each derived from Sm2170, for 18 generations. We found that hosts exposed to novel parasites maintained higher outcrossing rates than hosts exposed to Sm2170. Nonetheless, host outcrossing rates declined over time against all but the most virulent novel parasite strain. Hosts exposed to the most virulent novel strain exhibited increased outcrossing rates for approximately 12 generations, but did not maintain elevated levels of outcrossing throughout the experiment. Thus, parasite turnover can transiently increase host outcrossing. These results suggest that recurring episodes of parasite turnover have the potential to favor the maintenance of host outcrossing. However, such maintenance may require frequent exposure to novel virulent parasites, rapid rates of parasite turnover, and substantial host gene flow.

Assessment of Caenorhabditis elegans Competitive Fitness in the Presence of a Bacterial Parasite.

Accurate measurements of an organism's fitness are crucial for measuring evolutionary change. Methods of fitness measurement are most accurate when incorporating an individual's survival and fecundity, as well as accounting for any ecological interactions or environmental effects experienced by the organism. Here, we describe a protocol for measuring the relative mean fitness of Caenorhabditis elegans populations, or strains, through an assay that accounts for individual survival, fecundity, and intraspecific competitive ability in the presence of a bacterial parasite. In this competitive fitness assay nematodes from a focal population or strain are mixed with a GFP-marked tester strain in equal proportions, the mixture of nematodes are then exposed to a parasite, and the relative competitive fitness of the focal strain is determined by measuring the change in the ratio of focal nematodes to GFP-marked nematodes after one generation. Specifically, this protocol can be implemented to measure changes in nematode host fitness after experimental evolution by determining the relative competitive fitness of evolved versus ancestral nematode populations.

Host mating system and coevolutionary dynamics shape the evolution of parasite avoidance in Caenorhabditis elegans host populations.

Hosts exhibit a variety of defence mechanisms against parasites, including avoidance. Both host-parasite coevolutionary dynamics and the host mating system can alter the evolutionary trajectories of populations. Does the nature of host-parasite interactions and the host mating system affect the mechanisms that evolve to confer host defence? In a previous experimental evolution study, mixed mating and obligately outcrossing Caenorhabditis elegans host populations adapted to either coevolving or static Serratia marcescens parasite populations. Here, we assessed parasite avoidance as a mechanism underlying host adaptation. We measured host feeding preference for the coevolved and static parasites vs preference for Escherichia coli, to assess the evolution of avoidance behaviour within our experiment. We found that mixed mating host populations evolved a preference for E. coli relative to the static parasite strain; therefore, the hosts evolved parasite avoidance as a defence. However, mixed mating hosts did not exhibit E. coli preference when exposed to coevolved parasites, so avoidance cannot account for host adaptation to coevolving parasites. Further, the obligately outcrossing host populations did not exhibit parasite avoidance in the presence of either static or coevolved parasites. Therefore, both the nature of host-parasite interactions and the host mating system shaped the evolution of host defence.

Evolution of Caenorhabditis elegans host defense under selection by the bacterial parasite Serratia marcescens.

Parasites can impose strong selection on hosts. In response, some host populations have adapted via the evolution of defenses that prevent or impede infection by parasites. However, host populations have also evolved life history shifts that maximize host fitness despite infection. Outcrossing and self-fertilization can have contrasting effects on evolutionary trajectories of host populations. While selfing and outcrossing are known to affect the rate at which host populations adapt in response to parasites, these mating systems may also influence the specific traits that underlie adaptation to parasites. Here, we determined the role of evolved host defense versus altered life history,in mixed mating (selfing and outcrossing) and obligately outcrossing C. elegans host populations after experimental evolution with the bacterial parasite, S. marcescens. Similar to previous studies, we found that both mixed mating and obligately outcrossing host populations adapted to S. marcescens exposure, and that the obligately outcrossing populations exhibited the greatest rates of adaptation. Regardless of the host population mating system, exposure to parasites did not significantly alter reproductive timing or total fecundity over the course of experimental evolution. However, both mixed mating and obligately outcrossing host populations exhibited significantly reduced mortality rates in the presence of the parasite after experimental evolution. Therefore, adaptation in both the mixed mating and obligately outcrossing populations was driven, at least in part, by the evolution of increased host defense and not changes in host life history. Thus, the host mating system altered the rate of adaptation, but not the nature of adaptive change in the host populations.

The Integral Role of Genetic Variation in the Evolution of Outcrossing in the Caenorhabditis elegans-Serratia marcescens Host-Parasite System.

Outcrossing is predicted to facilitate more rapid adaptation than self-fertilization as a result of genetic exchange between genetically variable individuals. Such genetic exchange may increase the efficacy of selection by breaking down Hill-Robertson interference, as well as promoting the maintenance of within-lineage genetic diversity. Experimental studies have demonstrated the selective advantage of outcrossing in novel environments. Here, we assess the specific role of genetic variation in the evolution of outcrossing. We experimentally evolved genetically variable and inbred populations of mixed mating (outcrossing and self-fertilizing) Caenorhabditis elegans nematodes under novel ecological conditions-specifically the presence of the virulent parasite Serratia marcescens. Outcrossing rates increased in genetically variable host populations evolved in the presence of the parasite, whereas parasite exposure in inbred populations resulted in reduced rates of host outcrossing. The host populations with genetic variation also exhibited increased fitness in the presence of the parasite over eight generations, whereas inbred populations did not. This increase in fitness was primarily the result of adaptation to the parasite, rather than recovery from initial inbreeding depression. Therefore, the benefits of outcrossing were only manifested in the presence of genetic variation, and outcrossing was favored over self-fertilization as a result. As predicted, the benefits of outcrossing under novel ecological conditions are a product of genetic exchange between genetically diverse lineages.

Nematode-bacteria mutualism: Selection within the mutualism supersedes selection outside of the mutualism.

The coevolution of interacting species can lead to codependent mutualists. Little is known about the effect of selection on partners within verses apart from the association. Here, we determined the effect of selection on bacteria (Xenorhabdus nematophila) both within and apart from its mutualistic partner (a nematode, Steinernema carpocapsae). In nature, the two species cooperatively infect and kill arthropods. We passaged the bacteria either together with (M+), or isolated from (M-), nematodes under two different selection regimes: random selection (S-) and selection for increased virulence against arthropod hosts (S+). We found that the isolated bacteria evolved greater virulence under selection for greater virulence (M-S+) than under random selection (M-S-). In addition, the response to selection in the isolated bacteria (M-S+) caused a breakdown of the mutualism following reintroduction to the nematode. Finally, selection for greater virulence did not alter the evolutionary trajectories of bacteria passaged within the mutualism (M+S+ = M+S-), indicating that selection for the maintenance of the mutualism was stronger than selection for increased virulence. The results show that selection on isolated mutualists can rapidly breakdown beneficial interactions between species, but that selection within a mutualism can supersede external selection, potentially generating codependence over time.