Saturday, 16 January 2016

Uncovering A Genetic Component To The Origin Of Multicellularity

It is often argued that evolution is impossible as the probability of spontaneously creating new structures by simple mutation is astronomically low. They must have been wrought by a creator. Yet evolution does not work this way. Instead of creating from scratch, evolution simply jerry rigs pre-existing structures, gradually enrolling mutations to tweak their function. In this way fins can become legs and camera eyes evolve from simple spots of photopigments. Body plans have evolved in a similar fashion, in particular by the duplication of genes. This allows one copy to maintain original function whilst the other can be adapted for another purpose.

Top: a single choanoflagellate cell with the flagellae stained green.
Bottom: a multicellular choanoflagellate colony
The duplication of genes through time has resulted in the genomes of complex organisms being littered with gene families, all arising through duplication and then differential adaptation.

A recent study has shown how this process may have aided in the origin of multicellularity in animals. Researchers, led by Ken Prehoda from the University of Oregon, examined the evolution of a particular protein found in animals and their closest relatives, single celled protists known as choanoflagellates, and from this constructed the evolutionary history of the parent gene.

In choanoflagellates the primary function of the gene is in the creation of the flagella; a whip-like structure found in many single celled organisms used for locomotion. In choanoflagellates, however, it also plays a vital role in determining the orientation of choanoflagellate individuals in multicellular colonies.

The results of the genetic study showed that the duplication of the gene reduced the flagella's importance, resulting in its eventual loss from animal cells. The duplicated gene family and its resulting protein domain is found in all animals and their closest relatives, indicating its continued importance in multicellularity. Additionally, they found that a single mutation allowed the copies to aid in cell orientation.

'This mutation is one small change that dramatically altered the protein's function, allowing it to perform a completely different task' said Prehoda. 'You could say that animals really like these proteins because there are now over 70 of them inside of us.' 

The eventual cooperation of choanoflagellates, lacking flagellae, resulted in the first animals. These multicellular organisms were undoubtedly simple but just as with animals today, they would also have possessed the same vital set of duplicated genes required for cell to cell adhesion and communication.