Gene flow between introduced and native Eucalyptus species: crossability of native Tasmanian species with exotic E. nitens

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Abstract
Eucalyptus nitens (Deane & Maiden) Maiden has been extensively introduced to the island of Tasmania for plantation purposes. Natural hybridisation with two native species has already been confirmed and this study aimed to determine which other Tasmanian native species could potentially hybridise with E. nitens. Controlled and supplementary pollinations with E. nitens pollen were undertaken on all Tasmanian native species that are potentially at risk of exotic gene flow and hence genetic pollution. Across the seven species tested by using controlled pollinations, seed set per flower, following E. nitens pollinations, was significantly less than for intraspecific outcross pollinations. No significant differences were evident in the percentage of seed that germinated or the percentage of germinants that grew into healthy seedlings in the glasshouse. Hybridity was verified by morphometric analyses and F1 hybrid seedlings were clearly differentiated from parental species and generally intermediate in morphology. Supplementary E. nitens pollination of open-pollinated native flowers was conducted to simulate natural pollination where pollen competition would occur. Seven of the fifteen species tested produced F1 hybrids in this case; however, further crossing is required to verify failed cross combinations. Although E. nitens can potentially hybridise with many native species, the results from both supplementary and controlled pollinations suggest the presence of post-pollination barriers of varying strength that need to be considered in assessing the risk of exotic gene flow from plantations.