Landscape genetics

Landscape genetics is a discipline that combines the fields of population genetics and landscape ecology to facilitate understanding of how geographical and environmental features structure genetic variation. Its analysis involves detection of genetic discontinuities and the correlation of these discontinuities with landscape features. The idea was developed about 11 years ago, so around the same time DNA Barcoding entered the stage. In a recently published review the leading author of the initial publication states that the main objective of modern landscape genetics is to improve our understanding of the effect of global change on genetic patterns to address these two key questions:

• how has recent global change (i.e., land use and land cover as well as climate change) affected patterns of neutral and adaptive genetic variation; 
• are species likely to adapt to ongoing global change on an ecological time scale?

The Asian tiger mosquito (Aedes albopictus), which is native to Southeast Asia, was spotted in Houston in 1985. By 1986 it had already reached states like Missouri or Florida, both not bordering Texas. Today it can be found in all of the southern states and as far north as Maine. The mosquito arrived in the U.S. in a shipment of used tires from Japan. Aedes albopictus lays eggs that can survive even if any water evaporates, so they're very easy to transport.

This little beast is a very potent vector as it is known be able to carry more than 50 different viruses among those Dengue or Chikungunya. It is an aggressive daytime biter with an affinity for humans. 

Looking at a map of the current range of Aedes albopictus in the U.S., it is impossible to know how the mosquito spread from its point of introduction, although it could hardly have been by wing power alone, since an adult mosquito flies less than a kilometer in its lifetime. In order to find out how the tiger mosquito spread from the point of introduction, a group of US researchers used landscape genetics as it provides a way to rigorously test competing hypotheses for dispersal.

As a first step they had to establish the genetic structure of the U.S. population. As a so called container mosquito, Aedes albopictus lays its eggs just above the waterline in old tires, flower pot saucers, water bowls, bird baths, and most importantly for this study in cemetery flower vases. To sample the mosquito population the colleagues collected larvae from abandoned flower vases in cemeteries both on the edge and within the core of the mosquitoes' U.S. range in both rural and urban areas.

The immature mosquitoes were raised to adults in the laboratory so the species could be accurately identified (well, DNA Barcoding might have helped to speed this up). Subsequently, DNA was extracted from clipped legs and genotyped at nine different microsatellite locations. The genetic structure retrieved by the microsatellite analysis was then compared to those predicted by 52 different models of mosquito dispersal that variously took into consideration habitat and highways.

It turned out that gene flow over long distances was correlated with highways and bodies of water. People had carried mosquitoes from the core of their range to its edge along highways, likely by semi-trailers or in cars. Wetlands and lakes were important, not because they are breeding sites, but because they tend to occur in areas where frequent rainfall refills artificial containers and supports mosquito growth.

The scientists also looked more closely at what was happening at the range edge. Because Aedes albopictus lays eggs in treeholes and is often found resting at forest edges, they expected forests at the northern edge of the mosquitoes' range to act as natural corridors for dispersal. However, it turned out forests were barriers rather than corridors, perhaps because Aedes albopictus had not been able to displace the native treehole mosquito, Aedes triseriatus.

Our results revealed a combined role of natural and human-aided dispersal throughout the range of Aedes albopictus only two decades after its initial introduction into the USA. Naturalized populations have become sufficiently dense that dispersal and recolonization are now naturally sustained, but long-distance dispersal, particularly between range-core and range-edge sites, is ongoing. A similar trend may be expected for introductions of other exotic species, particularly those that spread rapidly via human-aided transport and then establish dense, naturally connected sites. Our results affirm the importance of denying entry of exotic species as well as rapid responses to eradicate soon after introduction. In addition, multiple introductions can increase genetic diversity and adaptive potential for some established invasive species; potential ports of introduction should remain vigilant to continued importation of Aedes albopictus in used tires and other shipments with the potential to contain eggs and/or larvae from other continents where Aedes albopictus is now established. Finally, the spread of disease is often linked to human-aided transport, land-use change and climate-change ; recent advances in genetic and geographic techniques may improve the utility of landscape genetics as a viable assessment tool for mitigating disease risk, including disease vectors such as Aedes albopictus, at a global scale.