In a previous post, I discussed the phenomenon of background selection, which results in rapid expungement of neutral alleles linked to loci under purifying or negative selection, and conversely, the rapid fixation of neutral variants that are linked to loci of high fitness (hitchhiking during a selective sweep, positive selection). Both processes lead to an overall reduction in genomic diversity at neutral sites (eg. Charlesworth et al. 1993).
In highly inbred populations, theory predicts the efficacy of selection to be lower, due to a fall in the effective population size. This fits well into the theory of sexual system evolution – particularly the transition from cross- to self-fertilization, which is often seen as an “evolutionary dead-end” (accumulation of deleterious mutations, small population sizes, limited ability to adapt to changing environments). Evolutionary persistence of selfers is thus hypothesized due to the phenomenon of “purging”, or strong negative selection against deleterious mutations. This seems contradictory – we expect lower efficacy of selection in smaller selfing populations, and expect greater efficacy of selection to purge recessive deleterious mutations to persist nonetheless.
Eichornia paniculata; Panel C (Top) shows an outcrossing flower, versus (Bottom), a selfer. Image courtesy: The Barrett Lab, http://labs.eeb.utoronto.ca/BarrettLab/Research.html
In a recent publication, Arunkumar et al. (2015) analyze the distribution of fitness effects (DFE’s) across different selective classes of new mutations in outcrossing and selfing populations of the aquatic flowering plant, Eichhornia paniculata. In short, they (1) simulate genomic datasets under varying outcrossing rates, population sizes, recombination rates, dominance, and compare the DFE’s of outcrossing versus selfing populations, and (2) sequence E. paniculata transcriptomes from selfing and outcrossing populations, identify variants, and compare strengths of selection at each variant site.
Results from their study (a) show more power to detect purging in selfing populations with increasingly recessive mutations (lower dominance), (b) with increasing dominance, greater proportion of nearly neutral mutations, compared to outcrossing populations, and (c) with variability in both dominance, and selection coefficients, they detect the presence of both strongly deleterious (purging) variants, and variants under relaxed purifying selection (weakly deleterious). Also, the sequencing, and the simulation studies found an overall decline in fitness of selfers, and that selfers accumulated more non-synonymous mutations, than outcrossers.
In conclusion, both simulated and the empirical data show evidence of purging (due to purifying selection against recessive deleterious mutations), reduced efficacy of selection (due to reduced effective population sizes) in selfing populations, and that these patterns are distributed across a variety of dominance and selection coefficients across selfing E. paniculata genomes. In yet another standstill, the neutralist-selectionist debate continues.
Arunkumar, Ramesh, et al. “The Evolution of Selfing Is Accompanied by Reduced Efficacy of Selection and Purging of Deleterious Mutations.” Genetics(2014): genetics-114. DOI: http://dx.doi.org/10.1534/genetics.114.172809
Charlesworth, Brian, M. T. Morgan, and D. Charlesworth. “The effect of deleterious mutations on neutral molecular variation.” Genetics 134.4 (1993): 1289-1303.