Hybridization between Eucalyptus obliqua L'Herit. and E. pulchella Desf

The present study provides the first evidence for the occurrence of natural hybrids between the wide-ranging species E. obliqua (series Obliquae) and the restricted Tasmanian endemic E. pulchella (series Piperitae). The progeny from one putative hybrid is shown to be intermediate between the progeny from the two parental species for virtually all characters examined. Principal component and likelihood analyses support this intermediacy of the hybrid progeny and suggest that no other species are involved in this hybrid system. The hybrid progeny shows greater variability than progeny from either parental species. Further, the hybrid tree has a lower seed output per capsule than trees of either parental species and therefore the hybrid tree appears to be at an evolutionary disadvantage.

The two adult hybrids were found on the edge of shallow gullies on E. pulchella-dominated, north-facing slopes away from pure stands of E. obliqua. Some natural saplings outside the phenotypic range of either species were also found in the gullies. These results are most easily interpreted as suggesting that the intermediate trees examined originated from recent hybridization (possibly F₁) following the migration of E. obliqua pollen into the range of E. pulchella. It is suggested that long-distance pollen migration of this type may be of evolutionary importance in allowing the colonization of suitable sites (in this case, small gullies) by advantageous genes or gene combinations from a distant species. The gene combinations of the pollen parent may be reselected from a segregating hybrid swarm.