Isolation and Lack of Potential Mates may Threaten an Endangered Arid-Zone Acacia

Clonality may provide reproductive assurance for many threatened plants while limiting sexual reproductive success either through energetic tradeoffs or because clones are self-incompatible. Most stands of the Australian arid-zone plant Acacia carneorum, flower annually but low seed set and an absence of sexual recruitment now suggest that this species and other, important arid-zone ecosystem engineers may have low genotypic diversity. Indeed, our recent landscape-scale genetic study revealed that stands are typically monoclonal, with genets usually separated by kilometers. An inability to set sexually produced seed or a lack of genetically diverse mates may explain almost system-wide reproductive failure. Here, using microsatellite markers, we genotyped 100 seeds from a rare fruiting stand (Middle-Camp), together with all adult plants within it and its 4 neighboring stands (up to 5 km distant). As expected, all stands surveyed were monoclonal. However, the Middle-Camp seeds were generated sexually. Comparing seed genotypes with the single Middle-Camp genotype and those of genets from neighboring and other regional stands (n = 26), revealed that 73 seeds were sired by the Middle-Camp genet. Within these Middle-Camp seeds we detected 19 genotypes in proportions consistent with self-fertilization of that genet. For the remaining 27 seeds, comprising 8 different genotypes, paternity was assigned to the nearest neighboring stands Mallee and Mallee-West, approximately 1 km distant. Ironically, given this species' vast geographic range, a small number of stands with reproductively compatible near neighbors may provide the only sources of novel genotypes.