The brown tree snake, Boiga irregularis
Photo: USGS
Biological invasions present evolutionary biologists with an apparent paradox. A newly established population may begin with only a handful of individuals and therefore contain only a small sample of the genetic variation present in its parent population. The resulting population bottleneck should increase inbreeding, reduce its capacity to adapt and expose harmful recessive mutations to natural selection. In extreme cases, the population should succumb to what population geneticists call an “extinction vortex”, in which declining genetic diversity and declining population size reinforce one another.
And yet some introduced species do precisely the opposite. They survive, proliferate and overwhelm the ecosystems into which they have been introduced. Few examples are more spectacular—or more ecologically destructive—than the brown tree snake, Boiga irregularis, on the Pacific island of Guam.
Native to Australia, Papua New Guinea and other parts of the western Pacific, brown tree snakes probably reached Guam as accidental stowaways in military cargo sometime after the Second World War. The invading population is believed to have been founded by only a handful of snakes. Nevertheless, their descendants multiplied until, in some parts of the island, densities reached an extraordinary 30,000 snakes per square mile.
The consequences for Guam’s wildlife have been catastrophic. As nocturnal, generalist predators encountering abundant prey that had evolved without such a snake, they devastated the island’s native birds, causing several species to disappear from the wild on Guam. They have also disrupted food webs, preyed upon reptiles and small mammals and caused hundreds of power cuts by climbing electricity infrastructure.
But how did a population founded by so few individuals avoid collapsing under the burden of inbreeding?
A new paper in Science Advances by Christopher A. Osborne of the University at Buffalo and colleagues from Buffalo and the US Geological Survey suggests that the answer lies partly in a form of genetic variation that earlier sequencing methods tended to miss.
The snakes did not, in fact, avoid the bottleneck. Their genomes retain compelling evidence of it. The researchers found that roughly half of each snake’s genome consisted of long “runs of homozygosity”—regions in which the copies inherited from the mother and father were almost identical. This is a genomic signature of intense inbreeding, comparable with that seen in some endangered species of conservation concern.
However, genetic diversity is not confined to changes in individual DNA letters. Using long-read sequencing, which can detect much larger differences, the researchers found nearly 19,000 structural variants. These included sections of DNA that had been duplicated, deleted, inserted or rearranged. Together, these structural variants affected almost eight times as much of the genome as the single-nucleotide variants upon which conventional estimates of genetic diversity have largely relied.
More importantly, the surviving variation was not distributed randomly. Structural variants were especially abundant around gene promoters and in genes associated with adaptive immunity and olfaction. Variation in immune genes could help the snakes respond to pathogens, while variation affecting their highly developed sense of smell could influence their ability to recognise prey, exploit different foods and distinguish closely related snakes from potential prey. In other words, a population can be genetically impoverished overall while retaining disproportionately important variation in precisely those parts of its genome that influence survival and adaptability.


































