Abstract
BackgroundIn 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.ResultsIn 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.ConclusionsThe 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.
Highlights
In the flowering plants, many polyploid species complexes display evolutionary radiation
Genealogical relationships based on the nuclear gene sequences Amplifications for both the ncpGS and the PgiC locus yielded a single band from each individual sample
The PgiC gene tree does not completely correspond to the divergence of the diploid species (Fig. 2a). This can be attributed to incomplete sorting of two ancestral PgiC alleles in Achillea millefolium agg. (Fig. 2c) or to introgression
Summary
Many polyploid species complexes display evolutionary radiation. 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. Most of the Eurasian polyploids, ranging from tetra- to octoploids, are either derived from primary hybridization between diploid progenitors or may be products of secondary introgression on the same or on different ploidy levels. This has created complex genetic and phenotypic variation patterns within A. millefolium agg. The relationships of the diploid species conform to a tree structure, whereas most of the polyploid taxa exhibit complex and reticulate relationships with each other and with the diploid species [11,19]
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