by Tamsin Jones.
We share about 70% of our genes with fruit flies. Those 70% are old, ancient genes, that were present in the common ancestor of flies and humans. We share many, many genes with most animals, and these conserved genes are thought to be the ones controlling all those really important processes necessary for life. What about the other 30% though? Are they necessary for life? Could we live without them?
An article published in Science at the end of last year explores that very question – are newly evolved genes essential or dispensable?
New genes are often thought of as performing relatively minor functions. It makes sense, really – the ancient genes that we all share, that all animals share, must be the really important ones that we can’t do without, like the engine in your car; while the newly-evolved ones might just be for more superficial purposes, like the colour of your paintwork, or those fluffy dice you hang from the rear-view mirror.
Some scientists at the University of Chicago examined the dispensability of newly evolved genes in the fruit fly Drosophila melanogaster. By comparing the genome of D. melanogaster with those of other closely related fruit flies, they identified a bunch of newly evolved genes. They then knocked down the functions of these genes one by one, and observed the effect these knockdowns had on the flies, and whether they survived and were normal.
The somewhat surprising result was that a whopping 30% of newly evolved genes were essential for the flies’ viability. What do I mean by viability? If the genes were knocked out, the flies died.
How does this compare to ‘old’ genes? The researchers performed the same experiment using ancient genes, and found that 35% were essential. That’s not much different from 30%, suggesting that new genes are just as important as old genes for survival!
How can a newly evolved gene become essential for life? Well, most ‘new’ genes aren’t that new at all, they arise from gene duplication events, where a gene is copied and then the two separate copies go on to evolve independently. A duplicated gene can very quickly evolve a new function by gathering a bunch of mutations, especially when the animal population is large. If the ‘parent’ gene is lost, the ‘new’ gene could also become essential that way, because it is now solely responsible for performing the function that the original gene was.
Pretty interesting results so far! The next thing the scientists looked at was the actual functions of new genes: why where those new genes essential for survival?
Before I go on I will quickly outline the fruit fly life cycle. Fruit flies have a few different developmental stages before they become adults. First, they are small embryos, which then grow into larvae (think tiny maggots!) and eat as much food as they can, before pupating so that they can morph, and then hatch as an adult (similar to how butterflies morph from catterpillars to butterflies).
The researchers examined where and when essential genes were being used during fruit fly development, and found that most (80%) of the newly evolved essential genes were necessary for larval development and pupation, while a few (10%) were turned on earlier on during embryonic development.
By comparison, only about 50% of old genes are needed for pupation, while 44% are needed for earlier embryonic development.
So the scientists found that newer genes are often essential for larval and pupal development- ie, later stages of development; whereas old genes are more often essential for early embryonic development.
Perhaps this reflects a need to keep the genetic control early development as tightly regulated as possible, with little room for change, while later stages of development are more flexible and can allow new genes to become incorporated into developmental processes.
Reference: Chen, S., Zhang, Y., & Long, M. (2010). New Genes in Drosophila Quickly Become Essential Science, 330 (6011), 1682-1685 DOI: 10.1126/science.1196380
Want to learn more about gene evolution? Here are some good links: