Chloroplast DNA evidence for reticulate evolution in Eucalyptus (Myrtaceae)

Four highly differentiated chloroplast DNA (cpDNA) lineages were identified in the forest tree species Eucalyptus globulus Labill. (Myrtaceae) in Aust ralia using restriction site polymorphisms from Southern analysis. The cpDN A variation did not conform with ssp. boundaries, yet there was a strong ge ographical pattern to the distribution of the lineages. One lineage (C) was geographically central and widespread, whereas the other three lineages we re found in peripheral populations: Western (W), Northern (N) and Southern (S). Thirteen haplotypes were detected in E. globulus, seven of which belon ged to clade C. At least three of the cpDNA lineages (C, N and S) were shar ed extensively with other species. On the east coast of the island of Tasma nia, there was a major north-south difference in cpDNA in the virtually con tinuous distribution of E. globulus. Northern populations harboured haploty pes from clade C while southeastern populations harboured a single haplotyp e from clade S. This difference was also reflected in several co-occurring endemic species. It is argued that the extensive cpDNA differentiation with in E. globulus is likely to originate from interspecific hybridization and 'chloroplast capture' from different species in different parts of its rang e. Superficially, this hybridization is not evident in taxonomic traits; ho wever, large-scale common garden experiments have revealed a steep dine in quantitative genetic variation that coincides with the haplotype transition in Tasmania. Our cpDNA results provide the strongest evidence to date that hybridization has had a widespread impact on a eucalypt species and indica te that reticulate evolution may be occurring on an unappreciated scale in Eucalyptus.