Table 2.

Summary of current knowledge on four V2R clades.

	V2R2	ancV2R	f-V2R	t-V2R
Specific characters
Expression	Broada,32	Sparseb*	Sparse5,35	Sparse7,10
Gene number	One or a few32	One*	Expanded2,13,14	Expanded7,9,15
Ligands	Amino aicds34	Unknown	Amino acids5	Peptides10–12
Habitat (water/land)	Both	Both	Mainly water	Mainly land
Presence (P)/absence (A)
Cartilaginous fish	P	P*	P	A
Basal ray-finned fish	P	P*	P*	P*
Teleost fish	P	A	P	A
Coelacanth	P	A	P	P
Amphibians	P	P*	P*	P
Reptiles	P	A	A	P
Mammals	P	A	A	P

*Newly found in this study.

^abroadly expressed in all vomeronasal neurons and co-expressed with the other V2Rs.

^bsparsely expressed according to one neuron one receptor rule.