Academic journal article Genetics

Limits to Adaptation in Partially Selfing Species

Academic journal article Genetics

Limits to Adaptation in Partially Selfing Species

Article excerpt

(ProQuest: ... denotes formulae omitted.)

SELF-FERTILIZATION-reproduction where both gametes arise from the same parent-frequently evolves from outcrossing species in nature. Self-fertilization is widespread in angiosperms (Igic and Kohn 2006), some groups of animals (Jarne and Auld 2006), and fungi (Billiard et al. 2011; Gioti et al. 2012). It confers an initial benefit to an individual's fecundity, including up to a 50% transmission advantage (Fisher 1941) and reproductive assurance under mate limitation (Baker 1955, 1967; Pannell et al. 2015). Both factors should allowselfing organisms to rapidly spread upon invasion of new habitats, unless countered by high levels of inbreeding depression (Lande and Schemske 1985). However, empirical studies usually find that selfing lineages are a "dead end," since back transitions to outcrossing are rare, and high extinction rates have been inferred from comparative studies of related selfing-outcrossing taxa (Igic et al. 2008; Goldberg et al. 2010; Wright and Barrett 2010; Wright et al. 2013).

Self-fertilization has therefore been posited to be detrimental in the long term. For an organism with selfing rate s, the population has an inbreeding rate F ¼ s=ð22sÞ; equivalent to Wright's (1951) FIS statistic. The effective population size Ne is reduced by a factor of at least 1=ð1 þ FÞ (Pollak 1987; Caballero and Hill 1992; Charlesworth 1992). Furthermore, the effective recombination rate is reduced in proportion to the inbreeding rate (Golding and Strobeck 1980; Nordborg 2000; Roze 2009). This joint reduction in both effective population size and recombination can lead to a decrease in the efficacy of selection. Deleterious mutations can therefore accumulate more rapidly in selfing organisms, leading to population extinction (Heller and Maynard Smith 1978; Lynch et al. 1995).

Whether this mechanism is a major cause of extinction of self-fertilizing species is still under debate (reviewed in Glémin and Galtier 2012; Igic and Busch 2013; Hartfield 2016). Some sister-species comparisons of selfing and outcrossing taxa reveal evidence of increased mutation accumulation in selfers, as demonstrated with either increased nonsynonymous-to-synonymous polymorphism ratio (pn=ps) or weaker codon usage bias. Conversely, analyses of divergence rates generally do not show evidence for relaxed selection. Part of the reason for this lack of evidence could arise due to recent transitions to selfing in most of these species, as explicitly demonstrated in Capsella rubella by Brandvain et al. (2013), thus leaving little time for mutation accumulation to act

Less investigated is the idea that selfing reduces the ability of a species to adapt, especially to new environmental conditions, although it was the one initially formulated by Stebbins (1957). For adaptation at a single locus, selfing organisms are more likely than outcrossers to fix new recessive adaptive mutations (Haldane 1927; Charlesworth 1992) but are generally less efficient in adapting from standing variation (Glémin and Ronfort 2013). Yet the effect of adaptation at multiple loci in partially selfing organisms has received much less attention. Of particular interest is howthe reduction in recombination efficacy in highly selfing organisms impedes the overall adaptation rate. A well-established phenomenon in low-recombining genomes is the "Hill-Robertson effect," where the efficacy of selection acting on a focal mutation is reduced, due to simultaneous selection acting on linked loci (Hill and Robertson 1966; Charlesworth et al. 2009). Outcrossing can therefore break down these effects and unite beneficial mutations from different individuals into the same genome, greatly increasing the adaptation rate (Fisher 1930; Muller 1932; Felsenstein 1974; Otto and Barton 1997).

Historically, the effect of advantageous mutations on mating system evolution has been neglected, since most observable spontaneous mutations are deleterious in partial selfers (Slotte 2014), and the inbreeding depression they cause plays a central role in mating system evolution. …

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