Head and body lice splice their identical genes differently

                 

These lice will have you scratching your head (Image: Steve Gschmeissner/SPL)
You’d struggle to tell them apart. Human head and body lice look similar, appear to share the same genome and are believed to be the same species. But they’ve never been seen to interbreed outside of the lab.
What’s more, while head lice (Pediculus humanus capitis) are known to be harmless, body lice (P. humanus corporis) transmit bacteria that cause typhus and trench fever.

“Something has to be different between them,” says Araxi Urrutia of the University of Bath, UK.
Now Urrutia and her team have evidence that, although their genes seem to be the same, the two lice read them differently. They do so through a process called alternative splicing.
Many genes are split into segments of coding and non-coding DNA. When these genes are expressed to make proteins, they are first copied into RNA. At this point the non-coding segments are spliced out and the important pieces of information are stitched together.
But sometimes non-coding segments are left in, or seemingly important regions are spliced out. By varying what RNA regions are stitched together, the same gene can provide multiple templates for slightly altered proteins.

Adapting to clothes

Urrutia’s team identified more than 3500 examples where genes were spliced differently in the two types of lice. Effectively, they are reading from the same genetic book, but copying out its protein instructions differently.
This might provide an explanation for the difference between them, says Ben Blencowe of the University of Toronto, Canada.
Head lice live within our hair – a feature our species has always possessed. Body lice live on clothes, which are a relatively recent evolutionary innovation – they probably became widespread less than half a million years ago.
It might be that head lice gave rise to body lice through alternative splicing, and perhaps this happens repeatedly, whenever the right conditions occur. Coupled with bad hygiene, a person’s clothes might provide a tempting new ecological niche.
“Body lice have to adapt to a different feeding pattern, because they are not necessarily in 24-hour contact with their host,” says Urratia.
This could explain why she found evidence of alternative splicing particularly in genes involved in the salivary glands and upper feeding tract of body lice.

Evolution or experiment?

Alternative splicing could be acting as an early adaptation mechanism, which might later be supported by more familiar kinds of genetic evolution, such as changes to the actual DNA genome sequences, says Urratia. In this way, it might be the starting point for two new species with different environments and behaviours to begin forming.
“Alternative splicing could be an important mechanism for rapid adaptation and speciation, which has been little studied,” says Leslie Turner at the Max Planck Institute for Evolutionary Biology in Plön, Germany.
So far, research into evolutionary mechanisms has focused on changes in DNA sequences and how active genes are, but cheaper RNA sequencing will mean we will soon start finding out more about the importance of alternative splicing, Turner says.
For now, it remains unclear whether the lice are likely to evolve into two completely separate species. “It will depend on the stability of the body lice environment,” says Urratia.
While clothes are a relatively recent invention, high standards of bodily hygiene are even newer, and could lead to a decline in the habitat for body lice. “It is possible that this is just a failed experiment for head lice to diversify into another environment,” she says.