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. 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: 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.).As explained before, many unicellular organisms have multicellular states (mostly triggered by some kind of stress response, e.g., starvation).The organisms I described before switch between these states, but they are not as complex as typical animals.

Answer 1

Multicellularity requires production of different types of cells and three-dimensional organization. Some unicellular protists can live in multicellular versions, but they are not as complex as typical animals. Sponges have unstable cell states but cannot regenerate from a single cell alone.

Step-by-step explanation:

Multicellularity requires that an organism produce more than one type of cell and organize them into a three-dimensional pattern to form tissues, organs, and organisms. While some unicellular organisms may exhibit multiple cell types, possessing different-looking or different-behaving cells associated with the reproductive or sexual process is not sufficient to label an organism as multicellular. However, there are certain unicellular protists, such as choanoflagellates, ichthyosporeans, and filastereans, that can live in multicellular versions either through aggregates or by clonal division. These organisms are of interest in the field of research on the evolution of multicellularity in animals. Sponges, which are basal animals, have unstable cell states and can regenerate from fragments, but they cannot regenerate from a single cell alone.