Trying to think of all the possible combinations, and if they exist in reality:
Starting with same-kingdom:
- We now know about virophages, the viruses that infect other viruses
- Archaea do not seem to be parasitic on anything
- I have not, however, found references to bacteria infecting other bacteria
- Nor to protists parasitic on other protists
- Fungi parasitic on other fungi: the “lobster mushroom” is an example
- Plants parasitic on other plants: all of the mistletoes, broomrape family, and balanophores
- Animals parasitic on other animals: roundworms, tapeworms, flukes, etc.
Now, considering cross-kingdom:
- Viruses can infect all other kingdoms
- Bacteria can infect fungi, plants, and animals
- Protists can infect plants (downy mildew) and animals (malaria).
- Fungi can be parasitic on plants and animals
- Plants can be parasitic on fungi (saprophytes)
- Animals can be parasitic on plants (gall-makers, nematodes)
So it seems that some possible combinations do not exist in reality: I cannot find any reference to a protist or an animal parasitic on a fungus, nor to a plant parasitic on an animal. If I am correct, is there some general explanation for these gaps?