Versatile gene expression helps trail-blazing house sparrows adapt

Pioneers face many challenges when venturing into new terrain: novel landscapes, unfamiliar foods and a host of fresh infections for their immune systems to combat. Yet some super-adaptors are champions of colonization, and the humble house sparrow is in a league of its own. Setting up home on almost every continent, the ubiquitous bird seems capable of taking to almost any environment. Intrigued by the bird's versatility, Haley Hanson and Marty Martin from the University of South Florida, USA, with Aaron Schrey from Georgia Southern University, USA, wondered whether the birds’ resourcefulness may have something to do with their adaptable DNA. Many creatures are able to fine-tune the expression of specific genes by chemically altering minute segments of DNA – a cytosine (C) followed by a guanine (G) – to cope better with changes in their environment, so the trio wondered whether this DNA modification mechanism could allow trail-blazing sparrows to flexibly alter expression of their genes to give their immune systems a boost when pushing the boundaries.

‘House sparrows arrived in Florida around the 1870s, although they are surprisingly hard to come by in this area’, says Hanson, who went mist netting the mischievous birds with Cedric Zimmer and undergraduate Bilal Koussayer around Tampa Bay, Florida. Back in the lab, the researchers gave some of the birds a fake infection by injecting them with some bacteria-mimicking sugars – to trick the birds’ immune systems into action – while others received an injection of harmless salt water. Then they collected minute samples of the birds’ blood over the course of several hours as the immune systems of the birds that received the mimic infection sprang into action. In addition, the team collected samples of the birds’ livers and spleens, which also play a role in fighting infection. Next, Dylan Maddox from the Field Museum of Natural History, USA, began painstakingly revealing how adaptable each individual was – by sequencing a specific region of the birds’ DNA in search of the tell-tale CG steps – while Zimmer, Koussayer and Hanson tracked the expression of a gene [Toll-like receptor 4 (TLR4)] that senses infection levels in the birds’ blood, liver and spleen, to figure out how hard the birds were fighting the simulated infection. ‘Preparing the samples to measure gene expression was the most time-consuming part, but we were fortunate to have some fantastic undergraduate researchers assisting us.’ says Hanson.

After 2 months of patiently searching the birds’ DNA and tracking their immune response, the team was impressed to see that the birds that had the largest numbers of CG steps (7 or 8) in the portion of DNA that triggers expression of TLR4 had the highest levels of expression in the blood relative to birds with fewer CG steps. However, the fake infection failed to trigger a surge in the gene's expression as the birds’ immune systems sprang into action. And, when they compared expression of TLR4 in the liver and spleen, it was lower in the birds with high numbers of CG steps. Yet, when they compared the expression of the gene in the female's blood with that of the males, the females deactivated the infection-sensing gene several hours after receiving the fake infection injection, while the males did not.

So, the audacious sparrows’ genetic flexibility – known also as epigenetic potential – may have enabled them to adapt to novel environments where other interlopers would not have thrived, and Hanson and colleagues are keen to find out more about the role of these versatile segments of DNA and how they contributed to the bird's prodigious success.