Thursday, 11th March 2010
Analysis of stability genes
The first group of genes I've analysed are the stability genes. Below shows the value of the five stability genes in the top organism after four separate runs of evolution. The horizontal line indicates the value of the gene in the ancestor organism. The colour of the bar gives an indication of the fitness of the top organism relative to the other runs of evolution (the darker the bar, the faster the filament of bacteria grew).
I'm part way through looking at detail at these genes and have been developing a way to visualise how genes change in the population over time, which I'll add to this analysis later.
The stability of Photosystem II in the ancestral organism was much lower than the stability of the other enzymes. There were two reasons for this. First, the amount of Photosystem II was capped at 2.7 units, representing the amount of enzyme that could fit in the plastid membranes. Second, the reaction catalysed by Photosystem II produces oxygen, so it is vital that is enzyme is quickly removed from cells that are transforming into heterocysts.
The evolution of the nitrogenase stability gene is simplest to understand. In all four runs of evolution, the fastest organism has a mutation that increases the stability of nitrogenase to the maximum value of 0.96. In each case, this mutation appeared early on in evolution and quickly spread through the population. This is because the availability of fixed nitrogen is the limiting factor for cell growth, so the more that can be produced, the faster a filament of cells will grow.
The stability of the catabolism enzyme is also the same in all four runs of evolution, however, in this case, the optimum value appears to be the original value of 0.9. At first glance, this is surprising, since the rate of catabolism directly controls the rate of cell growth, so we might predict that the more enzyme the better.
I think the reason that a higher stability enzyme wasn't selected for is that this enzyme uses up fixed carbon and nitrogen. More enzyme will therefore reduce the amount of fixed carbon available to heterocysts to produce energy. In addition, the more stable the catabolism enzyme, the greater the chance that a heterocyst will replicate, which will result in two heterocysts next to one another, which is inefficient for filament growth. It is still surprising that the optimum value exactly 0.9, the value in the ancestral organism.