The risk of pollen-mediated gene flow into a vulnerable eucalypt species

Hybridisation is recognized as a threat to the genetic integrity of rare, threatened and vulnerable species. Eucalypts are widely planted for plantation forestry and many species readily hybridise making “vulnerable”, locally-occurring species potentially at risk of genetic swamping. Eucalyptus argophloia (Queensland western gum or Chinchilla white gum) is a vulnerable species with limited distribution in South Eastern Queensland, Australia. The risks to this species from gene flow from surrounding forest or plantations are poorly understood. We investigated the breeding system and hybridisation potential of E. argophloia by controlled pollinations with selected sympatric and allopatric species. Treatments included self-pollination, outcross within species and hybridisation with E. crebra, E. microcarpa and E. moluccana (from within the same section, Adnataria) and E. pellita and E. resinifera (from a more distantly related section, Latoangulatae). The aim was to determine the degree of self-pollination and gene flow into E. argophloia from closely-related and more distantly-related sympatric and allopatric species. Eucalyptus argophloia is readily self-fertile. It also hybridises with a broad range of intra-sectional and inter-sectional eucalypt species. All crosses produced some pollen tubes, capsules and seed, and open pollination produced significantly more capsules and seeds than all controlled pollinations attempted. The number of seeds per capsule pollinated was not significantly different between pollen treatments. There is a potential risk of gene flow into E. argophloia populations from adjacent natural populations of the species tested as pollen parents, or from planted Eucalyptus species. This risk of foreign gene flow could be greater with open pollination than indicated by controlled pollination in this study because of the limiting effect of bud damage incurred during controlled pollinations. The ability to self-pollinate in the small population of E. argophloia raises the potential risk of genetic swamping from foreign pollen and elevated evolutionary pressure. These results will be useful for guiding management of this vulnerable species such as limiting plantings of cross-compatible species close to existing populations