Natural Hybridisation Between Eucalyptus regnans F. Muell. And E. macrorhyncha F. Muell. in the Cathedral Range, Victoria

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Abstract
Intermediates between E. regnans and E. macrorhyncha occur in E. macrorhyncha forests on the Cathedral Range sandstones up to 5 km from the nearest stands of E. regnans. Such intermediates are regarded as F1 hybrids, primarily because of their low variability. Except for one site adjacent to E. regnans, no introgression to E. macrorhyncha is found, suggesting that hybridization is a rare event. The presence of hybrids is likely to be a result of a 'third order reaction' requiring heavy synchronous flowering, attraction of suitable pollinators and the occurrence of bushfires within the retention time of capsules in the canopies. Intermediate trees exhibit water relation characteristics and essential oil contents similar to those of E. macrorhyncha but morphological features closer to those of E. regnans. Progeny of intermediates display wide variability, both morphologically and physiologically, between the putative parent species. Such rare, widely dispersed hybrid events may eventually lead to increased local variation of E. macrorhyncha and as such may have implications for evolution of eucalypt taxa in diverse habitats.