Peter Reddien

My lab studies regeneration, or how animals regrow missing body parts. This process depends on stem cells, which can mature into many types of adult cells. When tissue is lost, stem cells must choose which missing cell types to replace, and in what amounts. We want to understand how these decisions are made and how they’re regulated so the body produces the right cells in the right numbers.

This past year, we looked at how evolution tunes this process using a cave-dwelling species of flatworm called a planarian. Unlike its relatives, this species lacks body pigmentation and seems to have no eyes. But upon looking closely we found something surprising: the genes for making eye cells, and even for detecting light, are still active. The flatworms do have eyes — they are just very small. 

We found that the reason for small eyes in this species lies in decisions made by stem cells during a process of adult tissue maintenance called cell turnover. In this species, adult stem cells are still choosing to make eye cells, but they are doing so at a much lower rate than in other (surface) planarians. And the size of the eye depends upon the rate at which stem cells make the choice to become eye cells. 

This means evolution has simply shifted the probability of stem cells making the choice to become eye cells versus other cell types. And this subtle change had led to these flatworms having tiny eyes.