Hey, You Stole My Chloroplast!

Like all natural-history collections, the National Herbarium of Canada—a library of nearly 1 000 000 dried plant specimens at the Canadian Museum of Nature—is a powerful tool for research. Thanks to the collective effort of thousands of collectors over hundreds of years, scientists consulting the collection can examine specimens of nearly any plant species imaginable from across Canada and around the world.

In the effort to unravel botanical riddles, the herbarium is a cost-effective, low-tech way to travel through space and time. This invaluable resource has come in handy over the last few years as museum student Colin Chapman, research scientist Lynn Gillespie, and I worked to understand a curious case of hybridization in the Canadian Arctic.

Cabinets of shelves piled with herbarium sheets.

The National Herbarium of Canada contains hundreds of Pedicularis specimens from across the Canadian Arctic that were collected over the last few hundred years. Image: Paul Sokoloff © Canadian Museum of Nature

Louseworts (genus Pedicularis) are a group of parasitic plants found in many Arctic habitats. Two lousewort species, the hairy lousewort (Pedicularis hirsuta) and the Arctic lousewort (Pedicularis langsdorffii subsp. arctica), are sister species found primarily in the eastern and western Canadian Arctic respectively.

A plant in bloom.

The hairy lousewort, true to its name, is covered in woolly white hairs that keep the plant warm during the cool summer months. Image: Paul Sokoloff © Canadian Museum of Nature

Within the main part of their ranges, these species have sometimes been easily distinguished from one another based on their flowers: the hairy lousewort commonly possesses relatively small, pale pink flowers surrounded by insulating white hairs, while the Arctic lousewort has larger, darker pink flowers without these hairs and possessing two small “teeth” near the tip of each flower.

However, where these species meet in the High Arctic, such as on Axel Heiberg, Ellesmere and Devon islands, the difference between these species gets muddled, with some populations containing a mix of characters. To quote our very own A.E. Porsild in his 1957 Illustrated Flora of the Canadian Arctic Archipelago, for example: “In N.W. Greenland and in Ellesmere and Baffin Island, a form intermediate between P. hirsuta and P. arctica [P. langsdorffii subsp. arctica] is found. It has the general habit of P. hirsuta but has the larger flower and minutely dentate helmet of P. arctica“.

A flowering plant.

The Arctic lousewort has large, showy flowers that attract pollinators. Because of their movement between the flowers of the Arctic lousewort and the hairy lousewort, these insects (such as bees) are responsible for the cross-pollination, and therefore hybridization, of the two species. Image: Paul Sokoloff © Canadian Museum of Nature

By studying the morphology (shape and form) and DNA sequences from across the range of both of these species, with a focus on these overlapping regions, we found that certain populations were, in fact, hybrids, with an intermediate morphology between both species.

Importantly, we found that most hairy louseworts do, in fact, have “teeth” on the flowers, so we wrote a paper about differentiation of the species based on flower size and whether or not the style protruded from the flower.

In each of these hybrid populations, the plants that we sampled for DNA contained the nuclear DNA from the Arctic lousewort and the chloroplast DNA (which is inherited from the mother plant) of the hairy lousewort.

Interestingly, we found populations of “good” Arctic lousewort (plants clearly identifiable as such, and without hybrid characters) that also possessed the chloroplast DNA of the hairy lousewort.

This is likely the result of hybrid individuals backcrossing with the Arctic lousewort (which has showier, more-attractive-to-pollinator flowers) for several generations, until there are no more hybrid characteristics left in the resulting plants. In this way, the Arctic lousewort has “captured” the chloroplast of the hairy lousewort.

This paper, based on Colin Chapman’s Honours project at the University of Ottawa, is now available from the journal Botany.

A map showing northern Canada, Alaska, Greenland.

This map shows the location of all of the Pedicularis specimens that we DNA-sequenced:

  • Orange triangles: Arctic louseworts
  • Blue circles: Hairy louseworts
  • Red hexagons: Hybrid populations or introgressed populations (Arctic louseworts with the chloroplast-DNA genotype of the hairy lousewort).

Image: Paul Sokoloff et al. © Canadian Museum of Nature

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