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Background  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.    Results  In both the ncp GS  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.    Conclusions  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.

bmc evolutionary biology

Allopolyploid speciation and ongoing backcrossing between diploid progenitor and tetraploid progeny lineages in the Achillea millefoliumspecies complex: analyses of single-copy nuclear genes and genomic AFLP