Higher mutation load in the black-faced spoonbill
A consequence of relaxed selection is the accumulation of deleterious mutations in the genome (Kimura, 1962; Kirkpatrick & Jarne, 2000). We found that black-faced spoonbill individuals carried about 3% (8,294.00± 332.66 substitutions) more nonsynonymous substitutions (NS) than royal spoonbill individuals (8,025.78± 211.77 substitutions;t = -1.91, p < 0.035; Fig. 4B; Table S8). Moreover, the mean phenotypic effect, Grantham’s score (Grantham, 1974), for each deleterious NS (Grantham’s score> 50; Chun & Fay, 2009)) was approximately 7% higher for the black-faced spoonbill (56.98± 0.69) than in the royal spoonbill (53.25± 1.83; two-samplet test= 6.5154, p = 3.6e-6; Table S9). As a result, the average accumulated Grantham’s score per individual was higher in the black-faced spoonbill (527,350± 33,698.9) than in the royal spoonbill (502,030± 9,571.9; t = 2.47, p = 0.014; Fig. 4C; Table S9). Moreover, the ratio of homozygous to heterozygous NS in the black-faced spoonbill (0.13± 0.04) was also significantly higher than that in the royal spoonbill (0.11± 0.007; t = -1.86,p < 0.04; Fig. 4D), suggesting that purifying selection has been less effective in removing deleterious homozygotes in the bottlenecked black-faced spoonbill. Therefore, we provide a real-world example of the theory that mutation load, accumulation of deleterious mutations, is insensitive to recent population growth (Simons et al., 2014).