High nuclear genetic diversity, high levels of outcrossing and low differentiation among remnant populations of Quercus petraea at the margin of its range in Ireland

• Background and Aims Quercus petraea colonized Irelandafter the last glaciation from refugia on mainland Europe. Deforestation,however, beginning in Neolithic times, has resulted in small,scattered forest fragments, now covering less than 12 000 ha. • Methods Plastid (three fragments) and microsatellitevariation (13 loci) were characterized in seven Irish populationssampled along a north–south gradient. Using Bayesian approachesand Wright’s F-statistics, the effects of colonizationand fragmentation on the genetic structure and mating patternsof extant oak populations were investigated. • Key Results All populations possessed cytotypes commonto the Iberian Peninsula. Despite the distance from the refugialcore and the extensive deforestation in Ireland, nuclear geneticvariation was high and comparable to mainland Europe. Low populationdifferentiation was observed within Ireland and populationsshowed no evidence for isolation by distance. As expected ofa marker with an effective population size of one-quarter relativeto the nuclear genome, plastid variation indicated higher differentiation.Individual inbreeding coefficients indicated high levels ofoutcrossing. • Conclusions Consistent with a large effective populationsize in the historical migrant gene pool and/or with high geneflow among populations, high within-population diversity andlow population differentiation was observed within Ireland.It is proposed that native Q. petraea populations in Irelandshare a common phylogeographic history and that the presentgenetic structure does not reflect founder effects.