​​Landscape effects on population genetic structure of the reintroduced Asiatic wild ass (Equus hemionus)

Gueta Tomer, Alan Templeton and Bar-David Shirli 

Understanding the role of landscape features in shaping a population’s genetic structure can provide valuable insights into processes such as gene flow, genetic drift and movement. Landscape features can affect connectivity between populations by influencing individual movements and, therefore, can indirectly affect the gene flow among them. By combining principles from landscape-ecology, population-genetics and spatial-statistics, the gene flow within a species and the landscape features affecting this process can be explored.
Of particular interest is the effect of landscape features on the development of the population genetic structure in colonizing populations, such as invasive and reintroduced populations. Landscape features affect the process of range expansion through dispersal and home-range establishment, and this process should be reflected in the population's genetic structure. Inferring the connectivity between subpopulations across landscapes is important for evaluating the population's structure and persistence, especially for species of conservation concern.
The Asiatic wild ass (Equus hemionus) was reintroduced into the Negev Desert in Israel, between 1982-1993 in Makhtesh-Ramon and Wadi Paran. DNA samples of the source population (‘founders’) were collected and preserved. Currently the wild population is estimated at about 200 individuals, distributed throughout the Negev.
The goal of this study was to explore the effect of landscape features on the genetic structure of the reintroduced wild ass in the Negev, 30 years after reintroduction onset, by combining genetic data and landscape analyses. I predicted that the population is genetically structured and that a subpopulation in an isolated area (in terms of landscape connectivity) will differ genetically from the rest of the population.
393 fecal samples, collected in different sites across the Negev, and 30 blood samples of the founders were genetically analyzed using mitochondrial DNA (mtDNA) genetic markers. The genetic diversity of the founders was compared to the wild population; the comparison revealed that although the three mtDNA haplotypes of the founders have been maintained in the wild populations, their frequency changed (P=0.009).
The samples collected in the Negev were delimitated to subpopulations, and the genetic structuring of the population was tested using Pairwise Fst tests, Spatial AMOVA and a Barrier analysis. All three methods revealed a significant genetic difference between the "East" subpopulation (in the Arava) and the rest of the wild ass population (P=0.009, 0.04).
Landscape analyses were conducted using 'landscape connectivity indices' in FRAGSTATS and GIS tools (ArcGIS). They indicated that the "East" area was characterized by high quality habitat patches (high vegetation cover), which were relatively disconnected from the high quality patches in the rest of the study zone. This relative landscape isolation of the "East subpopulation" can explain its genetic differentiation from the rest of the population. I suggest that the "East" subpopulation was initiated following a founder-effect of individuals who dispersed from the release site area and remained in the 'new area', due to the high quality of the habitat.
Long-term genetic monitoring of the spatially expanding population is needed as a basis for developing conservation and management measures for the wild ass, as well as for exploring the dynamics of this unique system.