Cell size, genome size and the selfish gene

Classic theories such as the ‘selfish gene hypothesis’ are not always easy to test. If true, then the theory proposes that cells accumulate extra DNA over time even when they provide no benefit to the organism – selfish genes. This means individuals with more DNA relative to their cell size should have reduced fitness. What is more, because the accumulation of DNA creates a burden on the organism, a good evolutionary strategy should be to reduce the amount of redundant DNA over time.

In one of the first tests of the theory using a single species, Martino Malerba, Giulia Ghedini and Dustin Marshall have found direct evidence that reduced amounts of total DNA (genome size) is associated with fitness benefits in a species of marine microalgae.

Scientists face a problem when they compare cell size and genome size between different species. It is hard to separate whether the slower rates of metabolism, development and growth are a result of genome size or the fact that larger organisms tend to have a slower pace of life anyway.

Martino and his colleagues were able to overcome this issue by using their evolved lines of the marine phytoplankton Dunaliella tertiolecta where cells have been artificially selected to be large or small. They were able to use a staining technique to measure the amount of DNA content within cells of different sizes but, importantly, of the same species and the same evolutionary age.

They weren’t surprised to find larger cells had more DNA. But they also found that, in cells of the same size, the cells with smaller genomes grew faster and accumulated more biomass; that is, had greater fitness.

So, this first finding was consistent with the prediction that reducing the amount of DNA in a cell can have positive effects for the fitness of a species. But having demonstrated that minimizing DNA can improve fitness, the team then wanted to test the prediction that species should decrease their DNA content as they evolve. Again, they were able to test this by using the Dunaliella and monitoring the evolving lines for a year or approximately100 generations.

Their results confirmed that cells decreased their DNA content by up to 11% across 100 generations of evolution. However, they also found that cells with already low amounts of DNA showed no change over time, which suggests the existence of an absolute lower limit in the DNA content of this species.

Overall, they have direct evidence for fitness benefits associated with reduced relative genome size consistent with the selfish gene hypothesis and as well as a minimum genome size below which an organism can’t maintain functionality.

This figure looks at how genome size or cell DNA content changes with cell size. The continuous black line shows the relationship between the two for the large, small and control algae after 350 generations of evolution. The broken line shows the relationship after 450 generations of evolution – 1 year later. Notice how the cell DNA content has reduced after 1 year of evolution.

This research was published in the journal Current Biology.