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Species differentiation and gene flow in the Blackbutts (genus Eucalyptus subgenus Eucalyptus section Pseudophloius)

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Abstract

The significance of the taxonomic distinction of two species of Blackbutt was studied by analysing patterns of genetic (microsatellite markers; n = 13) and phenetic (capsule morphology) differentiation. Analysis of genetic structure using a Bayesian modelling approach on range-wide samples of both taxa (n = 457) showed the major division was within the more widely distributed species, Eucalyptus pilularis, and not aligned with taxonomy. Comparisons of intra- and inter-taxon genetic differentiation in paired-samples of taxa from each of four locations spanning the distribution of the more restricted E. pyrocarpa, showed that around twice as much variation was found among locations within taxa, than between taxa. Despite the lack of differentiation at effectively neutral microsatellite markers, significant phenetic differences (including capsule size) were evident between taxa at most sites. A landscape mosaic of taxa, coincident with changes in elevation, vegetation and soil types, suggested some phenetic differences were probably adaptive and spatial differentiation was stabilised by environmental factors. An absence of morphological intermediates and a lack of correlation in the rankings of locus inter-taxon differentiation (PhiBT) across locations, was consistent with parapatric origins for E. pyrocarpa. We conclude the taxa are at the lower end of the speciation spectrum and might best be viewed as ecotypes, divergent in evolutionary potential, but with genomes broadly permeable to inter-taxa gene flow. Gene exchange between plantings of E. pilularis and nearby E. pyrocarpa forest is likely as the two taxa appear to have few barriers to reproduction.

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Acknowledgements

This research was funding by the CRC Forestry. The authors thank M. Henson, D. Thomas and S. Boyton and Forests NSW staff and contractors for foliage collections and T. Sexton for development of genic-SSR markers. We also thank K. Northcott for laboratory assistance with capsule measurements and microsatellite genotyping, G. Luker for preparation of Fig. 1, J. Grant for assistance with soil typing, J. Ochieng, T. Sexton, M. Henson, D. Johnstone, A. Specht, and I. Bevege for helpful discussions on earlier versions of the manuscript.

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Shepherd, M., Raymond, C. Species differentiation and gene flow in the Blackbutts (genus Eucalyptus subgenus Eucalyptus section Pseudophloius). Conserv Genet 11, 1965–1978 (2010). https://doi.org/10.1007/s10592-010-0086-8

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  • DOI: https://doi.org/10.1007/s10592-010-0086-8

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