Table 1. The number of synonymous (dS) and non-synonymous (dN) substitutions per site for ABS, non-ABS and all sites in exon 2 of MHC class IIB (mean±SE); the value of Z-test of positive selection and probability (P) that positive selection acts on these sites.

	dN±SE	dS±SE	dN/dS	Z-test	P
ABS	0.303±0.075	0.134±0.058	2.261	2.156	0.017
non-ABS	0.066±0.017	0.056±0.025	1.179	0.401	0.344
all sites	0.120±0.020	0.078±0.023	1.538	1.610	0.055

The analysis was conducted using the Nei-Gojobori model [93] with Jukes-Cantor correction. Standard errors were estimated by bootstrap method with 1000 replications. Z-test of positive selection was conducted in MEGA v5.03 [65]. P is considered significant at 5% level.