Rapid decline in fitness of mutation accumulation lines of gonochoristic (outcrossing) Caenorhabditis nematodes

Mutation rate varies within and among taxa, including among individuals within the same population (Sturtevant 1937; Drake et al. 1998; Lynch 2008). In principle, mutation rate may vary among groups descended from a common ancestor for three general reasons. First, the respective environments inhabited by the different groups may differ in mutagenicity (e.g., incident UV radiation), in which case the difference in mutation rates is solely due to environmental effects and does not represent evolutionary divergence, except incidentally. Second, non-adaptive processes (mutation, genetic drift, hitchhiking) may have caused the different groups to evolve different mutation rates. Third, natural selection may have favored different mutation rates in the different groups - i.e., the difference represents the adaptive outcome of divergent optimizing selection. It is commonly asserted by evolutionary biologists that the mutation rate is subject to optimizing selection, at least in principle (e.g., Fisher 1930; Sturtevant 1937; Kimura 1967; Leigh 1973; Kondrashov 1995; Dawson 1998; Drake et al. 1998; Johnson 1999; Sniegowski et al. 2000; Andre and Godelle 2006; Baer et al. 2007). The orthodox view, initially articulated by Kimura (1967), (but see Lynch 2008) holds that direct selection on a modifier of mutation rate (almost) always favors a reduction in mutation rate so as to reduce the fitness cost of deleterious mutations, but there is also indirect selection to reduce the pleiotropic fitness costs of increasing the fidelity of genome replication. The pleiotropic “cost of fidelity” may be imposed by speed of replication and/or metabolic resources devoted to genome surveillance. The optimum (equilibrium) mutation rate occurs at the point at which opposing direct and indirect selection exactly counterbalance each other. The extent to which observed variation among groups in mutation rate reflects underlying differences in optimizing selection is unknown, but theoretical principles provide guidance. The two clearest cases concern the role of recombination (~ sexual vs. asexual reproduction) and self-fertilization. Under the assumption that beneficial mutations are trivially rare and can be ignored (which is demonstrably untrue in asexual taxa; Denamur and Matic 2006 and references therein), the strength of direct selection on a modifier allele that reduces the genome-wide deleterious mutation rate (U) by some amount v in (diploid) sexual taxa is vhs‒, where hs‒ represents the average strength of selection against new deleterious mutations in heterozygotes; in asexual organisms the equivalent strength of selection is v. Comparing obligate selfing vs. obligate outcrossing organisms, the strength of selection in the obligate selfer is v/2 (Kondrashov 1995; Drake et al. 1998). If hs‒ is on the order of 1–2% (Lynch et al. 1999), then the strength of selection to reduce mutation rate will be perhaps two orders of magnitude greater in asexual than in sexual taxa, and half that difference in selfers vs. outcrossers. Similar reasoning applies to other situations, e.g. sex-chromosomes vs. autosomes (McVean and Hurst 1997). Although these theoretical predictions are clear, the relevance of the theory to organisms in nature is not. Which is to say: Do asexual and/or self-fertilizing taxa typically evolve lower mutation rates than their sexual and/or outcrossing relatives? If the answer is yes, then optimizing selection on mutation rate must be taken into account in any context in which it is relevant (e.g., genome evolution). If the answer is no, it suggests that optimizing selection is weak relative to other factors potentially underlying variation in mutation rate (e.g., genetic drift, hitchhiking with beneficial mutations), and the cause(s) of variation must be sought elsewhere. There is considerable evidence that predominantly selfing taxa harbor less standing genetic variation than their outcrossing relatives (Hamrick and Godt 1996; Glemin et al. 2006; Ross-Ibarra et al. 2008), consistent with (among other things) selfing taxa having evolved lower mutation rates. However, Ne in a pure selfing population is expected to be half that of the same population under random mating (Pollak 1987). Moreover, populations of self-fertilizing organisms are potentially founded by a single individual propagule, with the concomitant drastic reduction in Ne. Finally, and possibly most importantly, selection at linked loci is likely to reduce Ne in selfing populations below that of equivalently-sized outcrossing populations (Hill and Robertson 1966; Gillespie 2000; Wright et al. 2008). Thus, although the preponderance of indirect empirical evidence is consistent with self-fertilizing taxa having evolved reduced mutation rates, other explanations are equally plausible and (arguably) more parsimonious and cannot be ruled out. Nematodes in the genus Caenorhabditis provide an ideal opportunity to test these theoretical predictions. Self-fertilization has evolved independently in the genus at least three times, in the species Caenorhabditis elegans and C. briggsae (Kiontke et al. 2004) and in the recently discovered C. “species 11” (C. Braendle, personal communication). C. briggsae and C. elegans are both believed to be primarily (but not exclusively) self-fertilizing in nature (Cutter et al. 2006). Several lines of evidence suggest that the mutation rate in C. briggsae is greater than in C. elegans (Baer et al. 2006; Ostrow et al. 2007; Phillips et al. 2009; D. Denver et al., unpublished). Two studies have attempted to compare the genomic deleterious mutation rates (U) of these species with the gonochorisitc (= dioecious) species C. remanei indirectly using base substitution rates at four-fold degenerate sites in coding genes. One (Cutter and Payseur 2003) found no difference between the three species C. elegans, C. briggsae, and C. remanei; a much larger study (Artieri et al. 2008) inferred a modestly (~15%) but significantly lower U in C. remanei than in C. briggsae, contrary to the theoretical prediction. We report here the results of a comparative mutation accumulation (MA) experiment in which mutations were allowed to accumulate under relaxed natural selection for 100 generations in several highly inbred populations of the dioecious taxa C. remanei, C. brenneri, and C. “species 5”, as well as in a population of C. elegans rendered functionally dioecious. We characterize two mutational parameters, the per-generation decline in mean fitness (ΔM) and the per-generation increase in genetic variance in fitness due to new mutations (VM). These parameters are composite functions of the genomic mutation rate for the trait (U) and the average homozygous effect of a mutation on the trait (E[a]); ΔM=UE[a] and VM=UE[a2] (Lynch and Walsh 1998, Ch. 12). MA studies have advantages and disadvantages relative to indirect inferences from comparative genomics. Two of the most important advantages of MA studies are that generation time and population size are known with certainty; the most obvious disadvantage of MA experiments is that inferences are necessarily drawn from a very small number of genotypes (usually one, typically highly homozygous) in a very small number of environmental contexts (usually one), either of which may not accurately reflect the typical properties of the species. MA and comparative studies are complementary and thus each is more informative in light of the other.