Eucalyptus, a large and iconic Australian genus with upwards of 700 species, has weak reproductive barriers and is known to hybridise extensively within subgenera. Two large subgenera, subg. Symphyomyrtus and subg. Eucalyptus, occur in young, disturbed landscapes, and old, nutritionally poor landscapes, respectively. The OCBIL theory predicts that taxa growing in old, climatically buffered, infertile landscapes (OCBILs) will undergo less hybridisation than those growing in young, often disturbed, fertile landscapes (YODFELs). Furthermore, the taxa growing in old landscapes are themselves predicted to be older and therefore genetically more well-defined, leading to increased barriers to hybridisation.
In a new article published in Annals of Botany, lead author T.P. Robins and colleagues investigated whether the OCBIL theory holds true for Eucalyptus subgenera Symphyomyrtus and Eucalyptus, and how the resulting hybrids are structured genetically. The authors used single nucleotide polymorphism (SNP) markers to determine hybrid status, parentage, and hybrid class (generation) for eight putative hybrid combinations in the two subgenera, as well as linking these results to the landscapes the trees grow in.
The researchers were able to determine the parental taxa for nearly all the hybrids studied. In older landscapes, parents were well-defined genetically and produced only F1 hybrids, many of which were clonal, suggesting hybridization events were few. On younger landscapes, parental taxa were less well-defined genetically and hybrids showed high levels of introgression and extensive backcrossing with one or both parent species.
“Our data have shown considerable differences in patterns of hybridisation between the Eucalyptus subgenera Eucalyptus and Symphyomyrtus and support a hypothesis for reduced hybridisation on older landscapes as presented in OCBIL Theory,” write the authors. “While the hybrids investigated here represent only a small number of Eucalyptus species, our results highlight that clade divergence times, landscape age and clonality are important drivers of differing patterns in species divergence and hybridisation patterns in Eucalyptus.”