First animal without a centrosome

Planarians are a favorite lab animal.  Among their talents, these cross-eyed little flatworms display an amazing ability to regenerate, even when carved into pieces.  And that’s not all that’s remarkable about these tiny creatures.  Apparently, they also lack centrosomes.

The freshwater flatworm Schmidtea mediterranea lives in southern Europe and Northern Africa is the first animal ever discovered without a crucial structure inside its cells known as the centrosome.

Image credit: UCSF/J.Azimzadeh

Here’s an abbreviated explanation of the way animal cells usually divide:  Prior to cell division, each chromosome  (DNA package) divides in two.  The centrosome, an organelle composed of two microtubule bundles called centrioles, also divides.  The two centrosomes migrate to opposite ends of the cell, each drawing a full complement of chromosomes toward that end of the cell.  Once this process is complete, the cell cleaves in two. Without centrosomes, DNA cannot be split evenly between the resulting daughter cells.

Or so we thought.  According to Juliette Azimzadeh from the University of California, San Francisco and her colleagues, the planarian Schmidtea mediterranea does not have centrosomes. The researchers discovered this odd fact when trying to see what effect eliminating centrosomes would have on the millimeter long flatworms’ ability to regenerate. Answer:  no effect at all, given that S. mediterranea never had any centrosomes to begin with. This makes planarians the only known animals to lack centrosomes.

Obviously, this changes much of what we thought we knew about cell division.  Author Sánchez Alvarado also wonders why centrosomes are found in all other types of animals, if they aren’t necessary:

There may be another function for centrosomes that is still obscured.