Climatic niche modeling reveals divergence between cytotypes in Eutrema edwardsii (Brassicaceae)

Polyploidy among plants is most frequent in the arctic, where glaciation cycles put selective pressures on populations by repeated fragmentation and fluctuation in climate. Polyploids should have been more fit in the novel habitats created as glaciers receded because of increased genetic material and novel gene products, which results in phenotypic plasticity and rapid adaptation. Higher ploidy is then expected to confer a broader tolerance of environmental conditions. Eutrema edwardsii R. Br. (Brassicaceae) is an arctic-alpine mustard with a near circumpolar distribution that occurs as a tetraploid, hexaploid, and octaploid. We used flow cytometry to document the distribution of polyploid cytotypes using herbarium tissue, and modeled the niche of each cytotype to test for niche differentiation. Flow cytometry revealed four cytotypes among 85 individuals. Notably, 60% of the herbarium tissue assays were successful using tissue up to 50 years old. Principle components analysis was performed on 20 climatic variables, of which, the first four axes were used as environmental variables for niche modeling. Niche models were created for tetraploid and hexaploid populations and used to calculate niche overlap (Shoener's D). Overlap between tetraploid and hexaploid models (D = 0.534) is lower than the null distribution (D = 0.681-0.944) supporting the hypothesis of niche divergence.Key words: allopolyploidy, flow cytometry, Maxent, cryptic variation, herbarium tissue.La polyploidie chez les plantes est plus frequente dans l'Arctique ou les cycles de glaciation exercent des pressions selectives sur les populations par des fragmentations et des fluctuations repetees sur le climat. Les individus polyploides devraient etre mieux adaptes aux nouveaux habitats lors de la recession des glaciers a cause d'un materiel genetique plus riche et de produits genetiques nouveaux, ce qui donne lieu a une plasticite phenotypique et une adaptation rapide. Une ploidie plus elevee est ainsi presumee conferer une tolerance plus etendue aux conditions environnementales. Eutrema edwardsii R. Br. (Brassicaceae) est une moutarde alpine arctique dont la distribution se situe pres du circumpolaire, qui se presente sous formes tetraploide, hexaploide et octaploide. Les auteurs ont utilise la cytometrie en flux pour documenter la distribution des cytotypes polyploides a partirde tissu d'herbier etils ont modelise laniche de chaque cytotype afin de tester la differenciation des niches. La cytometrie en flux a revele l'existence de quatre cytotypes parmi les 85 individus. Fait notable, 60 % des tests realises avec les tissus d'herbier etaient fructueux, meme sur des tissus ayant jusqu'a 50 ans d'age. L'analyse en composantes principales a ete realisee sur 20 variables climatiques, parmi lesquelles les quatre premiers axes ont ete utilises comme variables environnementales pour la modelisation de la niche. Des modeles de niche ont ete crees pour les populations tetraploides et hexaploides et utilises pour calculer le chevauchement des niches (indice D de Shoener). Le chevauchement entre les modeles tetraploides et hexaploides (D = 0,534) est plus faible que la distribution nulle (D = 0,681-0,944), appuyant l'hypothese de divergence des niches. [Traduit par la Redaction]Mots-cles : alloploidie, cytometrie en flux, Maxent, variation cryptique, tissu d'herbier.
IntroductionPolyploidy is ubiquitous among vascular plants and is especially prevalent in the arctic where it characterizes up to 80% of plant species (Soltis and Soltis 2000; Abbott and Brochmann 2003), [...]