A morphological study with univariate and principal component analyses of mature tree and open-pollinated seedling populations shows that natural hybridization is occurring between Eucalyptus
melanophloia and E. crebra, and E. populnea and E. crebra (subgenus Symphyomyrtus, section Adnataria).
The level of successful hybridization in these two systems, where fundamental genetic (reproductive) and
ecological (habitat and spatial) barriers are absent, is examined.
Within the ecotone, hybrids between E. melanophloia and E. crebra are about one-fifth as frequent as
those between E. populnea and E. crebra. Analyses of the phenotype/genotype compositions of the hybrid
populations, the patterns of segregation, and outcrossing frequencies between the species pairs indicate the
level of development of the hybrid populations. All E. melanophloia × E. crebra hybrids appear to be F1
hybrids. In contrast, the E.populnea x E. crebra hybrid population attains a much higher level of development
in terms of both hybrid number and genotypic complexity.
Flowering phenology is a major barrier to E. melanophloia × E. crebra hybrid formation compared with
the other hybrids, although it is considered an incomplete barrier and alone may not explain the diversity of
hybrid success which occurs.A theory of plant hybridization which incorporates the concept of hybridization success is presented and
discussed in relation to the two hybridizing systems, as a basis for further investigating the mechanisms
involved.
Padovan and Perrin hybrid number identities
