Maximum likelihood phylogenetic trees of the putative diatom GPCRs in relation to known class C GPCRs and microeukaryotic homologs. (A) Rooted and (B) unrooted trees. The unrooted tree highlights the overall genetic distance and relationships between branches. Diatom GPCR sequences identified in this study that contain the N-terminal ligand binding domain are highlighted in brown, those not containing the binding domain are highlighted in yellow. Class C GPCR subfamilies and putative diatom and microeukaryote GPCRs are color-coded: diatoms, dark green; microeukaryotes, light green; GABAB, red; metabotropic glutamate, purple; calcium-sensing and vomeronasal, blue; hypothetical proteins, black. Bootstrap values ≥50% were included in the tree.