Planarian flatworms have rid their cells of centrosomes, organelles that are found in nearly every animal cell. The structures anchor microtubule filaments that control cell division, cell migration and cell orientation.

Juliette Azimzadeh and Wallace Marshall at the University of California, San Francisco, and their team noticed that the flatworm Schmidtea mediterranea lacks centrioles — which make up the centrosomes — in dividing cells. They found centrioles only in certain cells that help the worms to glide across surfaces, feed and sense their environment.

The S. mediterranea genome is missing several genes needed to form centrioles; most flatworms have a full suite of centrosome genes. The authors suggest that planarians shed their centrosomes because, unlike many other animals, they rely on a form of embryonic development that does not depend on a cell's orientation.

Science http://dx.doi.org/10.1126/science.1214457 (2012)