Bumblebees hug shooting stars

 



By Jeff Mitton

If pressed about the most beautiful of our native flowers, I could not name a single species, but I could come up with a short list and it would certainly include shooting star, Dodecatheon pulchellum.

It also stands out for its phenomenally broad geographic distribution and its unusual mode of pollination.

Locally, shooting stars grow from the lower montane to above tree line on Niwot Ridge, a vertical distribution of at least 4,000 feet. But its entire geographic range is more remarkable, from New Mexico to Alaska. Within that range it grows in the Great Basin, the Mojave Desert, at sea level in Oregon and to 12,000 feet in the Rocky Mountains.

This remarkably wide range of habitats is at least partially attributable to unavoidable ambiguity concerning the identity of this species.

Shooting star flowers do not produce nectar, but pollination is accomplished by bumblebees foraging for pollen and using a specialized method called 'buzz pollination.'”To trace the history of the formal name of shooting star leads to a snarl of names at the level of species, subspecies and variety. A recent paper on genetics of shooting stars in British Columbia identified two subspecies, one of them with three varieties. A broader geographic study would certainly turn up many more names.

The genetics study examined chromosome numbers, and it revealed an entire layer of variability that was not concordant with either subspecies or varieties. Among the plants in the study the number of chromosomes could be 22, 44 or 66.

Chromosome number was only loosely associated with environment— for example, those with 66 were more common at the shore than in other environments. But chromosome numbers were shared among subspecies and varieties. In other species, different numbers of chromosomes create barriers to crossing, separating chromosomal types on independent evolutionary trajectories.

I conclude that the systematics of this species is a mess, but this is a wonderful puzzle and no justification to impugn systematists. When a broadly distributed species is in the process of adapting to a wide range of environments, evolutionary biologists do not expect the variation to fit into neat little boxes. There is a process underway, and it is not presently possible to identify all of the independently evolving lineages that will ultimately be recognized as separate species. The diversity of lineages currently referred to as shooting star is a work in progress.

When I first looked carefully at a shooting star flower, I could not figure out what was what, for it has an unusual design. The petals are tightly reflexed, or bent back, at the base so the petals stick up from a pendant flower (hangs down). Most of the petals are a lovely light magenta, but toward the base a fringe of white blends into gold. A squiggly line of burgundy marks the spot where the petal bends.

A conical column hangs below the petals — gold at the top, then a burgundy line and the rest is a dark color, perhaps purple, maybe brown. The conical column is formed by a fusion of the anthers. Extending from the bottom of the conical column is the stigma, the female portion of the flower.

Shooting star flowers do not produce nectar, but pollination is accomplished by bumblebees foraging for pollen and using a specialized method called "buzz pollination." A bumblebee arrives at the flower, grasps the fused anthers with its feet and mandibles and hangs upside down. It then buzzes its wings at a specific frequency, causing the anthers to vibrate, showering pollen onto the bee's thorax.

The pollination story is made more bizarre by the fact that shooting stars grow intermixed with the purple-flowered elephant's head, Pedicularis groenlandica, which is buzz-pollinated by the same bumblebees attending the shooting stars.

Behavioral studies report that a bee can move from an elephant's head to a shooting star, or vice versa, without causing hybridization, because the pollen of elephant's head is carried on the bee's abdomen, separate from the shooting star pollen on the thorax.

So as a bumblebee moves from elephant's head to shooting star, it may appear that it is bumbling, but in fact it is efficiently multitasking.

Jeff Mitton, mitton@colorado.edu, is a professor in the Department of Ecology and Evolutionary Biology at the University of Colorado. This column originally appeared in the Boulder Camera.

Dec. 19, 2014