The rare silver gum, Eucalyptus cordata , is leaving its trace in the organellar gene pool of Eucalyptus globulus

The process of genetic assimilation of rare species by hybridizing congeners has beendocumented in a number of plant genera. This raises the possibility that some of the geneticdiversity found in phylogeographical studies of widespread species has been acquiredthrough hybridization with species that are now rare or extinct. In this fine-scale phylo-geographical analysis, we show that a rare eucalypt species is leaving its trace in the chloroplastgenome of a more abundant congener. The heart-leafed silver gum, Eucalyptus cordata , isa rare endemic of south-eastern Tasmania. Its populations are scattered amidst populationsof more abundant related species, including the Tasmanian blue gum, Eucalyptus globulus .Using 339 samples from across the full range of both species, we compared chloroplast (cp)DNA haplotype phylogeography in E. globulus and E. cordata . The genealogy and distribu-tion of chloroplast haplotypes suggest that E. globulus has acquired cpDNA from E. cordata in at least four different mixed populations. Shared haplotypes are highest in E. globulus sampled within 2 km of known E. cordata populations and drop to zero at a distance of25 km from the nearest known E. cordata population. Localized haplotype sharing occurs inthe absence of obvious hybrid zones or locally shared nuclear ribosomal DNA sequences.Given that the future loss of E. cordata from some mixed populations is likely, these findingsindicate that phylogeographical analyses of organellar DNA should consider the possibilityof introgression, even from species that have been eliminated from the sites of interest