Reduced selection leads to accelerated gene loss in Shigella

Table 2 Genes under lower evolutionary constraint are lost more readily

No. of times lost along the E. coli/Shigella phylogenetic tree	No. of genes*	Average normalized dN/dS	P value^†
0	1,510	-0.1704	NA
1	403	0.0278	1e-14
2	252	0.0967	0.0454
3	125	0.2041	0.0223
4-5^‡	33	0.8347	0.0144

*A normalized dN/dS could not be calculated for 73 out of the 2,394 genes that were used in this study. Of these 73 genes, 45 are absent from all of the pathogenic strains studied and the additional 28 could not be fully aligned to the E. coli K12 sequence for any of the studied strains. For this reason the total number of genes summarized in this table is 2,321. ^†Genes were divided based on the number of times they are lost and each group was compared to the preceding group. The P value shown is the P value with which the following null hypothesis can be rejected: the genes in this group have average dN/dS that are smaller or equal to the dN/dS values of the preceding group. ^‡Genes lost four or five times were grouped together due to their small numbers

ISSN: 1474-760X