Can multi-cellular organisms transform into single-cellular organisms and conversely?

Question

If we have a multicellular organism A, can it transform into an organism B in its lifespan, so that B is single-cellular?

And conversely given a single-cellular organism B, can it become a multi-cellular organism C?

Does any organism which satisfies both of those properties in some way exist?

I don't have background in biology and this question was mostly curiosity, so this might be a bit vague.

Answer 1

The unicellular protists called choanoflagellates, ichthyosporeans and filastereans are the sister groups of metazoa (animals). They are single-celled but can live in multicellular versions. Either through aggregates (genetically different cells coming together) or by clonal division (like an animal - cells are genetically identical). So right now this is a huge field of research, in which scientists try to understand the evolution of multicellularity in respect to animals (multicellularity also arose multiple times in other species, e.g., fungi, plants etc.). Sponges are very basal animals and have very unstable cell states. Meaning, many cells can differentiate into other cell types and if you fragment a sponge (into single cells) given that a few specific cell types are present, it can simply regenerate. But as far as I know, a single cell alone can not regenerate a sponge. So to your questions:

1. If you talk about reproductive scenarios then a multicellular organism does convert itself into a single-celled organism (think about sperm, oocytes or about spores etc.).
2. As explained before, many unicellular organisms have multicellular states (mostly triggered by some kind of stress response, e.g., starvation).
3. The organisms I described before switch between these states, but they are not as complex as typical animals.