Habitat differentiation, hybridization and gene flow patterns in mixed populations of diploid and autotetraploid Dactylorhiza maculata s.l. (Orchidaceae)

Abstract

Detailed ecological, morphological and molecular analyses were performed in mixed populations of diploid and autotetraploid Dactylorhiza maculata s.l. in Scandinavia. Comparisons were made with pure populations of either diploid ssp. fuchsii or tetraploid ssp. maculata. It was shown that mixed populations are the result of secondary contact between ssp. fuchsii and ssp. maculata. No patterns of recent and local autopolyploidization were found. Morphology and nuclear DNA markers (internal transcribed spacers of nuclear ribosomal DNA) showed that diploids and tetraploids from mixed populations have similar levels of differentiation to diploids and tetraploids from pure populations. Vegetation analyses, as well as analyses of environmental variables, revealed that diploid and tetraploid individuals in mixed populations are ecologically well differentiated on a microhabitat level. Diploids and tetraploids in pure populations have wider ecological amplitudes than they do in mixed populations. Triploid hybrids grew in intermediate microhabitats between diploids and tetraploids in the mixed populations. Plastid DNA markers indicated that both diploids and tetraploids may act as the maternal parent. Based on morphology and nuclear markers triploids are more similar to tetraploids than to diploids. There were indications of introgressive gene flow between ploidy levels. Plastid markers indicated that gene flow from diploid to tetraploid level is most common, but nuclear markers suggested that gene flow in opposite direction also may occur. Similar patterns of differentiation and gene flow appeared in localities that represented contrasting biogeographic regions. Disturbance and topography may explain why hybridization was slightly more common and the differentiation patterns somewhat less clear in the Scandinavian mountains than in the coastal lowland.

Genetic variation and phylogenetic relationships of a pantropical species group in Polystachya (Orchidaceae)

“…This is a comparable situation to Dactylorhiza (Orchidaceae) in Europe, another group in which allotetraploid complexes with multiple independent origins are known to occur (Hedrén, 1996;Hedrén et al, 2001;Pillon et al, 2007). In Dactylorhiza, co-occurring species (diploid or tetraploid) tend to inhabit different microhabitats when they co-occur (Ståhlberg, 2009) and, although hybrid zones exist, the parental species retain their genetic identity. In this case, occasional introgression has probably been an important factor allowing rapid colonization of new habitats in northern Europe following the last glacial maximum (Hedrén, 2003).…”

Pure polyploidy: Closing the gaps in autopolyploid research

“…This process can enhance the genetic diversity of a nascent polyploid population and potentially mitigate the consequences of inbreeding among a small number of autopolyploids (Bretagnolle & Thompson, 1995). The evolutionary importance of gene flow between diploids and tetraploids was noted by Stebbins (1971), and the contributions of intercytotype gene flow on genetic diversity have since been reported in autopolyploid Dactylorhiza maculata (Ståhlberg, 2009), Houstonia (Glennon & Church, 2015), and Larrea tridentata (Laport et al, 2016). Selection on new autopolyploids may therefore be complex, with potentially opposing pressures-complete reproductive isolation from diploid progenitors (through prezygotic or postzygotic mechanisms) will counteract the deleterious effects of MCE by preventing often-unsuccessful intercytotype mating, but maintaining reproductive overlap with diploids may provide the benefit of increased genetic diversity through rare introgression via unreduced gametes.…”

When polyploidy and hybridization produce a fuzzy taxon: the complex origin of the insular neoendemicNeotinea commutata(Orchidaceae)

“…However, species distribution, particularly at the range limits, is influenced by dispersion capacity and adaptive potential to the progressively increasing changes in peripheral habitat (Pearson, 2006). In Dactylorhiza, diploids and tetraploids in mixed populations have been found to be ecologically differentiated on a microhabitat level, suggesting that genome doubling and hybridogenic genetic variation can act in concert to promote novel adaptive potential in allopolyploid taxa relative to their parental diploids (Ståhlberg & Hedrén, 2009;Paun et al, 2010).…”