Temporal dynamics of outcrossing and host mortality rates in host-pathogen experimental coevolution.

The nematode Caenorhabditis elegans has long been recognized as a model organism in genetics and developmental biology. Sydney Brenner first demonstrated the nematode’s utility in genetics to conduct mutant screens and map unprecedented numbers of phenotypes, by capitalizing on the general ease of maintaining large populations of the worm in the laboratory, their rapid generation time (~4 days at 20°C), and their ability to self-fertilize (Brenner 1974). Though perhaps a bit under-utilized in evolutionary biology for the first couple decades of its use as a model in genetics, C. elegans has quickly become a multi-purpose model system for use in experimental evolution studies over the last several years for many of the same reasons Sydney Brenner originally chose the species for his work. Although it might seem that nematodes lack interesting phenotypes, evolutionary biologists have recently capitalized on the wealth of genetic tools available for manipulating phenotypes (from mating systems to mutation repair), and the ease of maintaining large populations of C. elegans, to test fundamental questions in evolutionary biology via experimental evolution. The nematode has been utilized in the laboratory to: (1) determine mutation rates (Vassilieva and Lynch 1999; Vassilieva et al. 2000; Estes and Lynch 2003; Denver et al. 2004; Estes et al. 2004; Ajie et al. 2005; Baer et al. 2005; Cutter 2005; Baer et al. 2006; Manoel et al. 2007; Ostrow et al. 2007; Morran et al. 2009; Morran et al. 2010; Estes et al. 2011), (2) to study life-history evolution (Anderson et al. 2011; Murray and Cutter 2011), (3) to examine adaptation to novel environments (Lopes et al. 2008; Morran et al. 2009; Wegewitz et al. 2009; Anderson et al. 2010; Teotonio et al. 2012), (4) to examine mating-system evolution (Chasnov and Chow 2002; Stewart and Phillips 2002; Cutter 2005; Manoel et al. 2007; Katju et al. 2008; Wegewitz et al. 2008; Morran et al. 2009; Wegewitz et al. 2009; Chandler et al. 2011; Morran et al. 2011; Teotonio et al. 2012), and finally (5) to experimentally evaluate host-pathogen coevolution (Schulte et al. 2010; Morran et al. 2011). Experimental evolution is a particularly valuable tool for investigating host-pathogen coevolutionary dynamics, because it permits the assessment of interactions between hosts and pathogens as they coevolve. This dissection of host-pathogen coevolutionary dynamics, rather than just the ultimate outcome of coevolution, is crucial for understanding many of the evolutionary implications of host-pathogen coevolution (Hall et al. 2011). The C. elegans system may be especially valuable as an experimental model to understand the role of host-parasite coevolution on the evolution and maintenance of high levels of outcrossing. Where pathogens are common, highly virulent, and impose frequency-dependent selection, hosts are predicted to evolve greater levels of recombination and/or outcrossing (Hamilton 1980; Peters and Lively 1999; Gandon and Otto 2007). Recent empirical studies are consistent with these basic ideas on host-pathogen coevolutionary dynamics (e.g., Fischer and Schmid-Hempel 2005; Jokela et al. 2009; King et al. 2009; Schulte et al. 2010; Morran et al. 2011; Kerstes et al. 2012), which form the conceptual foundation of the Red Queen hypothesis as it relates to the maintenance of outcrossing (Jaenike 1978; Hamilton 1980; Bell 1982). Van Valen’s macroevolutionary formulation of the Red Queen hypothesis did not specifically address mating-system evolution; however his view of the Red Queen hypothesis is still relevant to host-pathogen coevolutionary dynamics and perhaps the maintenance of outcrossing. The premise of Van Valen’s Red Queen hypothesis was that species engaging in coevolution must rapidly evolve or face the threat of extinction (Van Valen 1973). Under this scenario, host populations must continue to produce resistant types to evade the pathogens, and the pathogens must evolve to infect the newly resistant types. Outcrossing could be the most effective mating system for enabling the persistence of hosts under both the microevolutionary and macroevolutionary variants of the Red Queen hypothesis, whether by producing offspring with novel genotypes that confer greater resistance, or by generating offspring with rare genotypes that avoid infection. Outcrossing has been shown to facilitate more rapid adaptation than self-fertilization to a novel environment. Presumably, mating systems that constrain alleles to specific lineages, like self-fertilization, ultimately slow host adaptation (Stebbins 1957; Maynard Smith 1978; Lande and Schemske 1985), and thus increase the probability of infection and potentially extinction. Therefore, the mating system of the host population should strongly influence the outcome of host-pathogen coevolution when the interaction between the two species generates strong selection. In a previous study, we exposed obligate selfing, mixed mating (i.e., a mixture of selfing and outcrossing individuals), and obligately outcrossing Caenorhabditis elegans hosts to coevolving populations of the bacterial pathogen, Serratia marcescens, for thirty generations. We found that the obligately outcrossing populations evolved reduced host mortality rates, and that the mixed mating populations maintained equivalent levels of host mortality, relative to their respective ancestral populations. Conversely, the obligately self-fertilizing populations exhibited substantial increases in host mortality rates. Both the mixed mating and obligately outcrossing hosts persisted throughout the experiment, but the obligately selfing hosts went extinct, which is consistent with the macroevolutionary version of the Red Queen. Further, we found that exposure to the coevolving pathogens selected for greater rates of outcrossing in the mixed mating hosts, which is consistent with the microevolutionary version of the Red Queen (Morran et al. 2011). In the present study, we capitalized on the ability to freeze and revive C. elegans to assess the temporal dynamics of host-pathogen coevolution in host and pathogen populations periodically preserved during our previous study (Morran et al. 2011). We compared the infectivity of the coevolving bacterial pathogens over time on host populations with different mating systems: mixed mating vs. obligately outcrossing. In particular, we documented the change in pathogen infectivity as outcrossing rates increased in the mixed mating populations over time. The results demonstrate that host-pathogen coevolution generates rapid evolutionary change, and provide direct empirical support for the idea that outcrossing permits hosts to keep pace with their coevolving pathogens, while predominant self-fertilization leads to greater infection.