Figure 4

Phylogeny and operon structure in nematodes. A: Phylogenetic tree of the phylum Nematoda after [24]. Species mentioned in this study are indicated and mapped to their respective clade. A split between basal (Enoplea) and derived (Chromadorea) nematodes is well supported (see [25, 26] and Discussion) and highlighted by color. B, C: Proposed model for a reorganisation of gene expression during nematode evolution. The presence of operons (references in parentheses) and CTCF (this study) is mapped onto the nematode phylogeny. Their mutually exclusive correlation suggests a switch during nematode evolution from transcriptional units defined by CTCF to those defined by operons (see discussion for details). With increasing formation of operons, CTCF and distant regulatory elements became obsolete. *: our data from BLAST and HMMer searches in the corresponding genome sequence. n. d.: no data. P1, P2: promoter. R1, R2: regulatory element (e. g. enhancer, silencer). A, B, C, X, Y: genes.