Patterns of Natural and Manipulated Hybridisation in the Genus Eucalyptus L'hérit. ̵1 A Review

Abstract

A database of herbarium specimens of Eucalyptus, together with published and unpublished literature, was searched for records of natural or manipulated interspecific hybrids. The database was also used in conjunction with the informal classification of Pryor and Johnson to generate a list of all pairwise combinations of species within each subgenus, indexed according to tazionomic affinity and degree of coincidence of geographic distribution. The frequency of recorded interspecific hybrids in relation to the total numbers of species pairs in each index category provided a basis for exploration of patterns of hybridisation within the genus. The different subgenera are reproductively isolated under both natural and manipulated conditions. Within subgenera, current geographic distribution is a major determinant of natural hybridisation, The frequency of natural hybridisation in general reflects the hierarchy of taxonomic affinities, although important exceptions were noted in Monomlyptus and Corymbia, and there is considerable variation in rates of inter and intrasectional hybridisation within Symphyomyrtus. Tammmic revision may be indicated in such cases. Across the genus, natural hybridisation is a rather restricted phenomenon. Only 15% of combin- ations expected on geographic/taxonomic grounds have been recorded, and 37% of 'these are known from only a single herbarium record. Most records of manipulated hybrids derive from the wmmercidy important subgenus Symphyomyrtus. Combinations between geographically isolated species are Frequent and successfd crosses have been made between species in different sections, although an increased frequency of viability problems was noted in some cases. Implications for tree breeding are discussed.