F-1 hybrid inviability in Eucalyptus: the case of E-ovata x E-globulus

The impact of inbreeding and hybridization on fitness was compared in the t wo co-occurring forest tree species, Eucalyptus ovata and E. globulus, aime d at explaining the rarity of their hybrids in nature. The success of selfi ng, open-pollination and outcrossing of both species and interspecific hybr idization was monitored from seed-set to 10-year's growth in a field trial. There was a unilateral barrier to hybridization with seed-set obtained onl y with E. ovata females. The F-1 hybrids exhibited reduced viability compar ed to intraspecific cross-types at virtually all stages of the life cycle a nd are clearly at a selective disadvantage compared with their open-pollina ted E. ovata half-sibs with which they would directly compete in nature. Eu calyptus ovata and E. globulus overlap in their flowering time but the F1 h ybrids flowered later with virtually no overlap with either species. The as ynchronous flowering and reduced reproductive fitness of F-1 hybrids would markedly limit the opportunity for advanced generation hybridization. Inbre eding similarly had a deleterious effect on the fitness of both species, an d the F-1 hybrids were most competitive with the E. ovata selfs. It is argu ed that changes in inbreeding levels of parental populations may be a key f actor affecting the relative fitness of hybrids and their potential to impa ct on the pure species gene pool. Reduced fitness of the pure species throu gh inbreeding may result in hybridization having its greatest evolutionary impact in small founder or relict populations.