Trypanothione reductase family: trypanosomatids vs their hosts
The pyridine nucleotide-disulphide oxidoreductases (Pfam PF07992) are in every proteome, so the family alone separates nothing. Continue to high-resolution profiling to split it into subclades: two are carried by all five parasites and by none of the hosts.
Ionotropic glutamate receptors: mites and ticks vs bees and vertebrates
An acaricide target must be in the mite and absent from the bee. The iGluR domains are in every animal here, so the answer lies one level down, in the paralogous subclades of the high-resolution profile.
Nitrogenase across nitrogen-fixing bacteria
The nitrogenase iron protein (NifH), the MoFe protein subunits (NifD/K) and the NifU cluster scaffold in rhizobia, Azotobacter, Frankia and a heterocystous cyanobacterium, against three bacteria that do not fix nitrogen. NifU also has a housekeeping role, so expect it more widely.
Cutinase and PETase-like esterases in plastic-degrading bacteria
Cutinase and esterase domains in bacteria known to degrade polyester plastics (Ideonella, Thermobifida, Pseudomonas putida), next to two laboratory bacteria for contrast.
Choline and carnitine acyltransferases across animals
The acyltransferase family (Pfam PF00755) that spans choline acetyltransferase (ChAT) and the carnitine acyltransferases (CrAT, CPT), across vertebrates, insects and nematodes.