MUCH of the genetic tool kit needed to build multicellular animals was already present in their primitive ancestors. This discovery solves a key part of the puzzle of how animals evolved from single-celled organisms. It even gives a glimpse of the role these genes originally played.
All multicellular organisms need ways to stick their cells together and to pass signals from one cell to another. Animals use adhesion molecules called cadherins and a variety of signalling molecules such as tyrosine kinases. But did these molecules originate from the dawn of multicellular animals, or did the first primitive animals simply co-opt molecules that were already around?
To answer this question, Nicole King, a biologist at the University of Wisconsin-Madison, focused on a group of water-dwelling protozoans called choanoflagellates, which are generally agreed to be the closest living single-celled relative of multicellular animals. King and her colleagues sequenced more than 5000 fragments of active genes from two choanoflagellate species, then searched gene-sequence databases of higher organisms to identify those which also occur in animals but not in more distantly related groups such as plants and fungi.
Advertisement
They came up with a rich haul of adhesion and signalling molecules, including tyrosine kinases and two cadherins. This showed that these key molecules for animal development were present in the common ancestor of animals and choanoflagellates shortly before multicellular animals evolved more than half a billion years ago (Science, vol 301, p 361).
But what were they doing there? 鈥淲e know the ancestor was unicellular, so it didn鈥檛 need to have the elaborate types of adhesion and cell-to-cell communication,鈥� says King. She suggests that the molecules may have been involved in sensing and responding to the organism鈥檚 surroundings, and so were well suited to their similar job in animal tissue. Whatever the molecules鈥� function, it is clearly important, as choanoflagellates grow poorly when their tyrosine kinases are knocked out.
The next step will be to look in choanoflagellates for other genes that multicellular organisms need, especially the regulatory genes that govern how cells specialise into different tissues.
King hopes that a choanoflagellate genome project will get under way soon. 鈥淚 think this will spur the sequencing of the entire genome,鈥� agrees Mike Levine, a biologist at the University of California, Berkeley. 鈥淥nly with a whole genome can we be confident of what鈥檚 not there 鈥� and what鈥檚 not there is just as interesting as what is there.鈥�
