Disentangling Reticulate Evolution in an Arctic-Alpine Polyploid Complex

Although polyploidy plays a fundamental role in plant evolution, the elucidation of polyploid origins is fraught with methodological challenges. For example, allopolyploid species may confound phylogenetic reconstruction because commonly used methods are designed to trace divergent, rather than reticulate patterns. Recently developed techniques of phylogenetic network estimation allow for a more effective identification of incongruence among trees. However, incongruence can also be caused by incomplete lineage sorting, paralogy, concerted evolution, and recombination. Thus, initial hypotheses of hybridization need to be examined via additional sources of evidence, including the partitioning of infraspecific genetic polymorphisms, morphological characteristics, chromosome numbers, crossing experiments, and distributional patterns. Primula sect. Aleuritia subsect. Aleuritia (Aleuritia) represents an ideal case study to examine reticulation because specific hypotheses have been derived from morphology, karyology, interfertility, and distribution to explain the observed variation of ploidy levels, ranging from diploidy to 14-ploidy. Sequences from 5 chloroplast and 1 nuclear ribosomal DNA (nrDNA) markers were analyzed to generate the respective phylogenies and consensus networks. Furthermore, extensive cloning of the nrDNA marker allowed for the identification of shared nucleotides at polymorphic sites, investigation of infraspecific genetic polymorphisms via principal coordinate analyses PCoAs, and detection of recombination between putative progenitor sequences. The results suggest that most surveyed polyploids originated via hybridization and that 2 taxonomic species formed recurrently from different progenitors, findings that are congruent with the expectations of speciation via secondary contact. Overall, the study highlights the importance of using multiple experimental and analytical approaches to disentangle complex patterns of reticulation.