The potential use of restriction fragment length polymorphisms (RFLPs) of chloroplast DNA to determine relationships at higher taxonomic levels in the genus Eucalptus was examined. Chloroplast DNA from 24 species, encompassing representatives of all the subgenera of Eucalyptus as well as one representative of the genus Angophora, was analysed using four 6-base restriction endonucleases. Eighty-four polymorphisms were obtained (twenty-eight autopomorphic) and the data matrix analysed using both cladistic and phenetic approaches. Results provided relatively good congruence with taxonomic perceptions based on morphological traits. Eucalyptus subgenera Blakella and Corymbia appear to be genetically similar to each other and to Angophora, although their phylogenetic relationships are not resolved in this study. Using Angophora alone, or together with the bloodwoods Blakella and Corynthia, as the outgroup for cladistic analysis, the two representatives of Eudesmia examined form a distinctly separate monophyletic group, which appears to be the sister taxon to Idiogenes, Gaubaen, Monocalyptus and Symphyomyrtus. The results provide some support for the close association of Idiogenes, Gauhaea and Monocalyptus and the hypothesis that they are the sister group of Symphyomyrtus. Taxonomically problematic species Eucalyptus guilfoylei, E. microcorys and E. deglupta were included in the study and it was found that E. guilfoylei appeared to diverge prior to the rest of the Symphyomyrtus, E. microcorys near the root of this clade, while the Telocalyptus representative E. deglupta fell within Symplgvontyrtus. The results obtained from the chloroplast DNA data provided independent support for previous morphological studies while generating new hypotheses and highlighting areas requiring closer examination.
Random amplified polymorphic DNA (RAPD) studies of a natural hybrid swarm between Eucalyptus amygdalina Labill. and E. risdonii Hook.f. and nearby allopatric stands revealed that, despite clear morphological differences, all bands were shared between species. However, frequency differences revealed genetic divergence between species, populations within species, and individuals within populations. Variation was greatest between individuals within populations and lowest between species. For both species, the direction of variation which distinguished the two populations was in a different direction to that which separated the two species, suggesting population differences were not due to introgression but were the result of genetic isolation and/or strong localised selection. Several morphologically typical individuals with intermediate RAPD profiles were detected in the hybrid swarm and nearby allopatric samples of both species, suggesting that some cryptic introgression may be occurring. Controlled F1 crosses generally had closer genetic affinity to E. risdonii, raising the possibility that some parents used may have been advanced generation hybrids. While natural hybrids selected for their intermediate leaf phenotype were usually also intermediate between the two species using RAPD markers, some deviated markedly toward E. risdonii. The study suggests that morphological appearance does not necessarily reflect genetic (RAPD) status and in some cases detectable RAPD differences between spatially close populations of the same species may be as great or greater than the differences between species.
Chloroplast DNA based phylogenetic studies using PCR-amplification and digestion (40 species) combined with Southern blotting (23 species) suggest that Angophora and some representatives of the Eucalyptus subgenera Corymbia and Blakella form a monophyletic group. The subgenera Eudesmia, Monocalyptus, Gaubaea, Idiogenes, Symphyoinyrtus and Telocalyptus appear to form a well defined monophyletic group encompassing most of Eucalyptus, but the exact sister taxon to this group remains unresolved. The results suggest that subgenera Corymbia and Blakella are paraphyletic and that Telocalyptus should be submerged within the subgenus Symnphyoinyrtus.
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