Table 3. Estimates of dN/dS and results of site model tests for adaptive evolution among codon sites for Prochlorococcus and Synechococcus.

Bolded LRT statistic values are chi-square critical values that meet a significance level of <0.001. For all genes, the inclusion of a class of neutral sites (M1) fits the data better than one dN/dS value for all sites (M0). While the inclusion of a class of sites under positive selection may be statistically justified under the M2 and M8 models, all dN/dS values are well below one suggesting that most sites are under purifying or neutral selection.

			log-likelihood of site models for adaptive evolution					likelihood ratio test (LRT) statistic for model pairs (degrees of freedom)
Gene	Taxa	dN/dS	M0	M1	M2	M7	M8	M0 vs. M1 (1)	M1 vs. M2 (2)	M7 vs. M8 (2)
gyrB	226	0.036	−105998	−101367	−101367	−102434	−100472	9263	0	3924
pstB	200	0.045	−39945	−39466	−39466	−38543	−38484	957	0	118
amtB	200	0.066	−70319	−68351	−68234	−68226	−67620	3935	235	1212
glnA	229	0.031	−66948	−65905	−65905	−65462	−65118	2086	0	688
glsF	215	0.103	−267696	−249591	−249591	−253085	−246724	36210	0	12722
napA	76	0.054	−23709	−23028	−23028	−23201	−22961	1363	0	480
narB	78	0.119	−40791	−38790	−38790	−39102	−38498	4001	0	1209
moaA	67	0.206	−24794	−22973	−22931	−23268	−22770	3643	84	996
focA	60	0.078	−18477	−17701	−17701	−17895	−17615	1553	0	562
nirA	115	0.109	−54829	−52361	−52361	−52063	−51470	4937	0	1187