Allopolyploid speciation and ongoing backcrossing between diploid progenitor and tetraploid progeny lineages in the Achillea millefolium species complex: analyses of single-copy nuclear genes and genomic AFLP
1 Ministry of Education Key Laboratory for Biodiversity Science and Ecological Engineering, and College of Life Sciences, Beijing Normal University, Beijing 100875, China
2 College of Biological Sciences and Biotechnology, Beijing Forestry University, Beijing, China
3 Institute of Animal Breeding and Genetics, University of Veterinary Medicine in Vienna, A-1210 Vienna, Austria
4 Department of Systematic and Evolutionary Botany, Faculty of Life Sciences, University of Vienna, A-1030 Vienna, Rennweg 14, Austria
5 Beijing Engineering Research Center for Hybrid Wheat, Beijing 100097, China
BMC Evolutionary Biology 2010, 10:100 doi:10.1186/1471-2148-10-100Published: 13 April 2010
In the flowering plants, many polyploid species complexes display evolutionary radiation. This could be facilitated by gene flow between otherwise separate evolutionary lineages in contact zones. Achillea collina is a widespread tetraploid species within the Achillea millefolium polyploid complex (Asteraceae-Anthemideae). It is morphologically intermediate between the relic diploids, A. setacea-2x in xeric and A. asplenifolia-2x in humid habitats, and often grows in close contact with either of them. By analyzing DNA sequences of two single-copy nuclear genes and the genomic AFLP data, we assess the allopolyploid origin of A. collina-4x from ancestors corresponding to A. setacea-2x and A. asplenifolia-2x, and the ongoing backcross introgression between these diploid progenitor and tetraploid progeny lineages.
In both the ncpGS and the PgiC gene tree, haplotype sequences of the diploid A. setacea-2x and A. asplenifolia-2x group into two clades corresponding to the two species, though lineage sorting seems incomplete for the PgiC gene. In contrast, A. collina-4x and its suspected backcross plants show homeologous gene copies: sequences from the same tetraploid individual plant are placed in both diploid clades. Semi-congruent splits of an AFLP Neighbor Net link not only A. collina-4x to both diploid species, but some 4x individuals in a polymorphic population with mixed ploidy levels to A. setacea-2x on one hand and to A. collina-4x on the other, indicating allopolyploid speciation as well as hybridization across ploidal levels.
The findings of this study clearly demonstrate the hybrid origin of Achillea collina-4x, the ongoing backcrossing between the diploid progenitor and their tetraploid progeny lineages. Such repeated hybridizations are likely the cause of the great genetic and phenotypic variation and ecological differentiation of the polyploid taxa in Achillea millefolium agg.