Landscape genetic structure of Olea europaea subsp. cuspidata in Ethiopian highland forest fragments

Understanding patterns of genetic diversity at the landscape scale will enhance conservation and management of natural populations. Here we analyzed the genetic diversity, population connectivity, and spatial genetic structure among subpopulations and age groups of Olea europaea subsp. cuspidata, a cornerstone species of the Afromontane highlands. The study was conducted at the landscape level within a radius of approximately 4 km, as well as on a fine scale (intensive study plot) of less than 300 m radius. In total 542 samples from four natural subpopulations in northwestern Ethiopia were analyzed using ten nuclear microsatellite markers. Inbreeding was higher in smaller populations. No genetic difference was detected among cohorts of different tree sizes in the intensive studied plot. Average population differentiation was low but significant (F_ST = 0.016). Landscape genetic analysis inferred two groups: the most distant subpopulation WE located less than 4 kms from the other three subpopulations formed a separate group. Sixty-four percent of the total migrants were shared among the three latter subpopulations, which are spatially clustered. Immigrants were non-randomly distributed inside of the intensive study plot. Significant spatial genetic structure (SGS) was found both at the landscape scale and in the intensive study plot, and adults showed stronger SGS than young trees. An indirect estimate of 220 m as mean gene dispersal distance was obtained. We conclude that even under fragmentation migration is not disrupted in wild olive trees and that large protected populations at church forests are very important to conserve genetic resources. However, the higher level of inbreeding and evidence for population bottlenecks in the small populations, as well as the persisting heavy pressure on most remaining populations, warrants quick action to maintain genetic diversity of wild olive in the Ethiopian highlands.