Natural Inter-subgeneric Hybridization Between Eucalyptus acmenoides Schauer and Eucalyptus cloeziana F. Muell (Myrtaceae) in Southeast Queensland

Eucalyptus acmenoides Schauer (white mahogany) and E. cloeziana F. Muell (Gympie messmate) are largely allopatric but have several sympatric populations across their range in eastern Queensland, Australia. In this study, verification of interspecific hybridization at the site of one such sympatric population was carried out using morphological and molecular (nuclear and cytoplasmic) characters. Capsule morphology was intermediate in hybrids, particularly with respect to the number of valves per capsule. Hybridization was found to occur in both directions with maternal inheritance of a taxon-specific polymorphism in the inverted repeat of the J_LA region of the chloroplast genome. However, E. acmenoides acted as the maternal parent in all but one hybrid individual, possibly due to protandry of flowering. High levels of polymorphism in microsatellite loci existed between E. acmenoides and E. cloeziana in the hybrid zone. Formation of hybrids was most likely limited to F₁ mating and was restricted in both numbers and dispersal within the hybrid zone. Current taxonomy places E. cloeziana and E. acmenoides into different subgenera, Idiogenes and Monocalyptus respectively. This is significant as it is the only example of natural hybridization between subgenera within the Eucalyptus genus, suggesting that the taxonomic classification of E. acmenoides and E. cloeziana should be investigated further.