Evolution and temporal diversification of western European polyploid species complexes in Dactylorhiza (Orchidaceae)

Abstract

Patterns of polyploid evolution in the taxonomically controversial Dactylorhiza incarnata/maculata groups were inferred genetically by analyzing 399 individuals from 177 localities for (1) four polymorphic plastid regions yielding aggregate haplotypes and (2) nuclear ribosomal ITS allele frequencies. Concordance between patterns observed in distributions of plastid haplotypes and ITS alleles renders ancestral polymorphism an unlikely cause of genetic variation in diploids and allopolyploids. Combining the degree of concerted evolution in ITS alleles (thought to reflect gene conversion) with inferred parentage provides support for a quadripartite classification of western European allopolyploid dactylorchids according to their respective parentage and relative dates of origin. The older allotetraploids that generally exhibit only one parental ITS allele can be divided into those derived via hybridization between the divergent complexes we now call D. incarnata s.l. and D. fuchsii (e.g., D. majalis) and those derived via hybridization between D. incarnata s.l. and D. maculata (e.g., D. elata). Similarly, the younger allotetraploids that maintain evidence of both parental ITS alleles can be divided into those derived from hybridization between D. incarnata s.l. and D. fuchsii, or perhaps in some cases a diploid species resembling D. saccifera (e.g., D. praetermissa, D. purpurella, D. traunsteineri s.l., D. baltica), and those derived from hybridization between the D. incarnata s.l. and D. maculata groups (e.g., D. occidentalis, D. sphagnicola). Older allotetraploids are inferred to have passed through glacially induced migration bottlenecks in southern Eurasia, whereas at least some younger allotetraploids now occupying northern Europe are inferred to have originated post‐glacially and remain sympatric with their parents, a scenario that is largely in agreement with the morphology and ecology of these allotetraploids. ITS conversion is in most cases biased toward the maternal parent, eventually obscuring evidence of the original allopolyploidization event because plastid haplotypes also reflect the maternal contribution. Gene flow appears unexpectedly low among allotetraploids relative to diploids, whereas several mechanisms may assist the gene flow observed across ploidy levels. There is good concordance between (1) the genetically delimited species that are required to accurately represent the inferred evolutionary events and processes and (2) morphologically based species recognized in certain moderately conservative morphological classifications previously proposed for the genus. Further research will seek to improve sampling, especially in eastern Eurasia, and to develop more sensitive markers for distinguishing different lineages within (1) the remarkably genetically uniform D. incarnata group (diploids) and (2) locally differentiated populations of (in some cases unnamed) allotetraploids.

Clarified relationship between Dactylorhiza viridis and Dactylorhiza iberica renders obsolete the former genus Coeloglossum (Orchidaceae: Orchidinae)

“…Analysis of a sample of Dactylorhiza iberica collected in the Pindos Mountains of northern Greece was conducted too late for its inclusion in the published tree; the authors simply reported in the text its position as near-basal within the genus. Identical topologies, differing only in bootstrap support values, were later produced from ITS matrices by Pillon et al (2007), Tang et al (2015) and Bateman et al (in review).…”

“…A year later, Pillon et al (2007, their Appendix 1) published a considerably expanded dataset that included ten geographically disparate samples of Dactylorhiza viridis and two samples of D. iberica; sample 960 from Cyprus was finally joined by sample "Hedrén 98078" from northwest Turkey. Nonetheless, sample 960 -presumably by now re-sequenced -was once again selected to represent D. iberica in their ITS tree.…”

“…1). Pillon et al (2007) also analysed microsatellites in the plastid regions trnS-trnG, trnL-F and trnL intron, yielding a good range of plastid haplotypes that proved useful for distinguishing dactylorchid taxa. Unsurprisingly, both D. viridis and sample 960 of D. iberica proved to have unique and distinct haplotypes, the latter reputedly yielding ribotype XI and haplotype J.…”

“…targeting particular genes for Sanger sequencing) have added little to the story (Bateman 2012a). When yet again exploring phylogeny across tribe Orchideae, Inda et al (2010) effectively demonstrated the widely recognised chronic weakness of mitochondrial coxI data for reconstructing plant rather than animal phylogenies, but this did not dissuade them from later adding those mitochondrial data to the matrices developed by previous authors for nuclear ITS Bateman et al 2003;Pillon et al 2007) and plastid rpl16 (Pillon et al 2007). The three disparate datasets were for the first time analysed simultaneously, using Bayesian methods, thereby yielding a topology for Dactylorhiza that received strong statistical support (their Fig.…”

“…However, both D. viridis and especially D. iberica have received less research attention than other, more derived taxa within the genus that have become a wellknown model system for the study of polyploidy events (e.g. Hedrén et al 2001;Pillon et al 2007;Paun et al 2010Paun et al , 2011Hedrén et al 2011).…”