Gene duplication promoted evolution of bony fish
A gene duplication allowed the freshwater bony fish to conquer the sea. Two biologists from the University of Bergen have discovered that the mutation changed the composition of the yolk in fish eggs so that they could survive in s alt water.
The actual bony fish (Teleostei) originally developed in fresh water perhaps 250 million years ago. Only towards the end of the Cretaceous did they penetrate the sea, where they split into numerous species in the Eocene 55 million years ago. This adaptive radiation of teleosts has long remained a mystery to evolutionary biologists.
Roderick Nigel Finn and Børge Kristoffersen see the cause of this success in a change in the gene for the yolk precursor protein vitellogenin: through a doubling - which probably occurred when the first bony fish ventured into the sea - the fish eggs produce more protein, the can in turn be broken down into amino acids. The free amino acids increase the osmotic pressure within the eggs and thus prevent the water loss that would be triggered by the high s alt concentration in seawater. Conversely, water even penetrates the eggs, allowing them to float in the water column.
The sea habitat requires considerable adaptation efforts for the animal world living here. Since the ion concentration inside the cells of marine bony fish is lower than in the surrounding seawater, the animals are constantly losing water. They must therefore drink large quantities of seawater and actively excrete the s alt through their gills. (aj)
