Life on the edge – High levels of genetic diversity in a cliff population of 'Bertya ingramii' are attributed to 'B. rosmarinifolia' (Euphorbiaceae)

Wind-facilitated migration of new genotypes into small, geographically disjunct populations should buffer them against local extinction. 'Bertya ingramii', a monoecious, wind-pollinated shrub, is restricted to three populations in a 4 km² area in eastern Australia. Populations are separated by deeply dissected gorges where it is unlikely that seeds are exchanged but where wind movement may facilitate pollen dispersal. Using 156 highly polymorphic ISSR markers, we found moderate genetic variation within and among populations of 'B. ingramii' and less genetic diversity in a nearby and small population of the widespread 'Bertya rosmarinifolia'. The smallest population of 'B. ingramii' (<30 plants) had the highest genetic variation (65% polymorphic markers, Shannon Information Index = 0.30). AMOVA and a Bayesian analysis showed that molecular variance was equally distributed within and among populations suggesting that gene flow is as limited within as in among populations. Genetic distances between populations were only weakly explained by their relative geographic distances (mantel test, R² = 0.21, P = 0.001) but the distribution of private bands, the departure from Hardy–Weinberg equilibrium, and a UPGMA tree showed that the smallest population of 'B. ingramii' was generally more similar to an upstream population of 'B. rosmarinifolia'. Thirty-eight percent of bands in this small population of 'B. ingramii' were exclusively shared with 'B. rosmarinifolia'. This covert hybridisation may have been an ancient event but may be responsible for contemporary declines in germination and establishment in 'B. ingramii'. The conservation implications are amplified by the endangered status of 'B. ingramii'.