There isn't a commonly recognized abbreviation "UM" with a specific meaning in biology. It's possible that "UM" could be used informally in specific contexts or within a particular research group.

Could you please provide more context about where you encountered "UM"? For example:

* What was the sentence or document where you saw it?

* What field of biology is it related to?

Knowing this information will help me understand what "UM" might stand for in this specific case.