Abstract
BackgroundHybridization and polyploidy are potent forces that have regularly stimulated plant evolution and adaptation. Dactylorhiza majalis s.s., D. traunsteineri s.l. and D. ebudensis are three allopolyploid species of a polyploid complex formed through unidirectional (and, in the first two cases, recurrent) hybridization between the widespread diploids D. fuchsii and D. incarnata. Differing considerably in geographical extent and ecological tolerance, the three allopolyploids together provide a useful system to explore genomic responses to allopolyploidization and reveal their role in adaptation to contrasting environments.ResultsAnalyses of cDNA-AFLPs show a significant increase in the range of gene expression of these allopolyploid lineages, demonstrating higher potential for phenotypic plasticity than is shown by either parent. Moreover, allopolyploid individuals express significantly more gene variants (including novel alleles) than their parents, providing clear evidence of increased biological complexity following allopolyploidization. More genetic mutations seem to have accumulated in the older D. majalis compared with the younger D. traunsteineri since their respective formation.ConclusionsMultiple origins of the polyploids contribute to differential patterns of gene expression with a distinct geographic structure. However, several transcripts conserved within each allopolyploid taxon differ between taxa, indicating that habitat preferences shape similar expression patterns in these independently formed tetraploids. Statistical signals separate several transcripts - some of them novel in allopolyploids - that appear correlated with adaptive traits and seem to play a role favouring the persistence of individuals in their native environments. In addition to stabilizing the allopolyploid genome, genetic and epigenetic alterations are key determinants of adaptive success of the new polyploid species after recurrent allopolyploidization events, potentially triggering reproductive isolation between the resulting lineages.
Highlights
Hybridization and polyploidy are potent forces that have regularly stimulated plant evolution and adaptation
Gene expression in diploid and polyploid Dactylorhiza The 27 AFLP primer combinations yielded 305 unambiguous cDNA fragments; of these, 32% were monomorphic among all individuals analyzed
Across the cDNA-AFLP dataset there was a remarkably similar number of fragments characteristic of either parent: markers were specific for D. fuchsii and for D. incarnata, which contributed to a total transcript differentiation between the diploid parental species of 58% (Table 2, Figure 2)
Summary
Hybridization and polyploidy are potent forces that have regularly stimulated plant evolution and adaptation. Recent genomic investigations have uncovered signals of past whole-genome duplications (WGD) across angiosperms, indicating that polyploidy is a common mechanism of genome evolution in flowering plants [1,2]. The prevalence of WGD across the history of flowering clustering of genome duplications around the Cretaceous-Tertiary (KT) boundary in independent angiosperm lineages indicates that polyploid lineages may be better able to radiate if they are fortunate enough to survive the randomness inherent in mass extinction events [9,10,11]. Ramsey & Schemske [12] reported that unreduced gametes are produced at rates on average ca 50 times higher in hybrids than in non-hybrid lineages. Autopolyploids are widespread in nature [17,18], and many remain phenotypically hidden within their diploid parent
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