Imagine trying to photocopy a pile of papers, only for one of the copied sheets to magically jump back into the queue. It gets duplicated again. When the photocopier is finished, you’re left with two sets of papers and three copies of the mysteriously mobile sheet.
The same thing happens in the cells of a fly. Every time a cell divides, it duplicates its entire genome so the two daughter cells each have a copy. But some genes aren’t content to be duplicated just once. A selfish gene called a P-element has the ability to jump around its native genome. Like the paper jumping back into the photocopier queue, the P-element lands in parts of the fly genome that haven’t been copied yet. This ability allows it to spread throughout a genome, and even around the world.
Replicating a genome takes time, and it doesn’t all happen in one burst. Rather than feeding one large stack of paper into one photocopier, it’s all divided up and given to different machines. Each burst of replication begins at fixed locations called origins. A group of six proteins, collectively known as the Origin Recognition Complex or ORC, arrives at the origins and recruits other proteins that being the copying process. Once this happens, that origin has “fired” and it only does so once in every round of replication. This ensures that every bit of DNA gets copied once and only once. But P-elements have a way of getting seconds.
Allan Spradling from the Carnegie Institution for Science has found that P-elements prefer to jump into DNA origins. By analysing over 100,000 jumps, he found that these genes are attracted to the same places that the ORC is drawn to.
Once in place, Spradling thinks that the P-elements somehow wait to jump until the origins have fired, perhaps by detecting the presence of the ORC or by sensing the structure of the local DNA. Once the origin fires, the P-element is among the first genes to be copied. Then, one of the duplicates jumps to another unfired origin, where it can be copied yet again. This could explain why the sections of animal genomes that are replicated last tend to be rife with jumping genes.
It could also explain how these jumping genes spread so furiously. P-elements jump by cutting themselves out of the surrounding DNA, and then pasting themselves in somewhere else. This cut-and-paste system means that the number of P-elements should stay the same, but that’s certainly not the case.
A single P-element can quickly spread through a group of captive flies. In the wild, these jumping genes invaded the fruit fly Drosophila melanogaster from a related species around 80 years ago. Since then, they have spread through all natural populations, and there are many copies. Individuals can carry anywhere between 30 to 50 P-elements each.
P-elements are just one of several groups of jumping genes or ‘transposons’ hopping about our genomes. We’re used to the idea of bacteria and viruses jumping from host to host and making copies of themselves. But genes can do the same thing, and some of them have evolved strategies as ingenious as those of any microbe.
Reference: Spradling, Bellen & Hoskins. 2011. Drosophila P elements preferentially transpose to replication origins. PNAS http://dx.doi.org/10.1073/pnas.1112960108
Image by Asiatic League