Promiscuous Plants

Naturalists have long been aware of the greater tendency for plants than for animals to create viable interspecies hybrids. This is attributable not only (as some might expect) to a higher likelihood of passive plants whose mating is mediated by pollen-hungry insects, or the wind, to hybridize more often, but rather to a greater ability of plants, with the simpler design of their anatomies, successfully to build a functioning organism with a Gemisch of genes from parents of different species. Such hybrids occur naturally, and are often reported in regional floras. Further, the advent of modern techniques for characterizing DNA has revealed that hybridizations of yore have given rise to numerous species, and higher lineages, in plants, in fungi, and to a lesser extent in animals.

My recent wanderings in quest of fall flora photos at Shaw Nature Reserve really brought this phenomenon of admixture of species to mind as I was examining populations of the three Gentiana species that live at the reserve. All three are fairly recent introductions at SNR, added to the flora in several locations in our prairie and wetland habitat reconstruction program. Hybridization among these gentian populations was first brought home in my observation over the last three years of increasing numbers of purplish and bluish and outright blue individuals in a population that was originally pure white gentian – Gentiana alba. This population was sowed in the mid-1990s as part of a mesic prairie reconstruction in the watershed of our wetland complex.

Gentiana alba, G. andrewsii and their lavender tinted hybrid growing side by side at Shaw Nature Reserve.

Pale or white bottle gentian, in "pure" form.

A few years later, 50 or so meters distant, separated by a dense row of trees and shrubs, and in a much wetter habitat in which water pools after every rain and seeps subsurficially much of the year, blue bottle gentian – G. andrewsii — was sowed into a wet prairie / sedge meadow reconstruction.

The rich blue flowers of the blue bottle gentian, Gentiana andrewsii

At first the two populations grew independently and remained separate, but what I surmise was a combination of water borne seed transport (along the shore of a pond whose edge both populations are near), and bumblebee borne pollen transport, conspired to bring gametes of the two species together, creating what population geneticists call a hybrid swarm, and what taxonomists call a — well, I can’t write it in polite company such as my readers.

Observe in the sequence of images above how a bumblebee gyne (a potential queen of one of next year’s annual bumblebee colonies) pries open a bottle gentian flower and dives in for a long drink of nectar at the base of the large vessel. Apparently the nectar is copious, because bumblebees may remain in a single gentian flower for up to a minute.

The result of pollen transport among pale and blue bottle gentians, a hybrid of intermediate characteristics.

While there are other populations of both species on the reserve (one hopes, out of bumblebee range from each other) that may retain their genetic integrity, the rampantness of the admixture at this site does give me pause.

And it gets worse! — On drier ground up the slope, among a dense planting dominated by prairie dropseed and little bluestem grasses,  a third gentian known as downy or prairie gentian – Gentiana puberulenta – was established from a seed mix sowed 10 years ago to convert the watershed of the reserve’s wetlands to prairie vegetation.

Unlike the two previously mentioned species and their hybrids, the downy gentian's petals open wide at anthesis, admitting entry to small bees and even to spindly-legged potential pollinators such as syrphid flies.

And now those perverse bumblebees have gone and defied the laws of speciesness and created what appear to be hybrids of this third gentian species with the other two. Honestly, I don’t know whether to feel that I have done some sort of wrong by creating the situation that allowed this to happen … or simply to be intrigued by this unforeseen outcome of my work, and to wonder what will come of it after I’m gone?

The gentian in the upper photo appears to be the offspring of a cross between white and downy gentian parents, while the one in the lower photo appears to be the result of a cross between blue bottle and downy gentian.

22 thoughts on “Promiscuous Plants

  1. Thanks for the fascinating post. Speciation and hybrid impacts on species is an amazingly interesting topic. I knew that hybrids were thought to be more common in plants but I had not seriously considered why. In your post you said “a higher ability of plants, with the simpler design of their anatomies, successfully to build a functioning organism with a Gemisch of genes from parents of different species.” I don’t suppose that you have any idea where I might go to learn more about this idea or how it has been determined. Either way thanks for the great post!

  2. Hi James

    Thanks for this post. Several years ago I bought what was supposed to be a bottle gentian (Gentiana anndrewsii) from a local botanical garden, but what I got was a mystery gentian with open flowers. I’ve been assuming incompetence at the garden, but now I have an alternative hypothesis, and even less chance of eventually putting a name to it.

    Good example of why botanists were quick to drop the Biological Species Concept. Last estimate I saw of allopolyploid species in grasses was 70% – not surprising when you cast your male fitness to the wind. There’s nothing wrong with hybrids, even in animals, they are perfectly ‘natural’. So I wouldn’t worry about your gentians. If you’d ever tried to identify a hillside of manzanitas to species, you’d have given up worrying about species purity.

    • This is high praise coming from you, Alex. Thanks. Hand-held, actually. I guess I’m still not all that old and shaky, yet. 🙂 It seems my equipment is fine for this size range – It’s those pesky ants that elude sharp imagery!

  3. Nice post James. I find this really fascinating. Have other sites reported similar hybridization? What isolates these gentians in nature? Distance between ecosystems? General rarity?

    • Hi Malinda:

      Answer to above, yes. Oh yeah, one of the questions requires more:

      There may be some isolation by pollinator syndrome in nature, as the bottles require bumblebees, while downy gentians don’t open until the sun shines on them and small bees are flying — e.g. Hylaeus, Agapostemon, seen this very morning, may usually get to them first.
      I had an interesting conversation this week with a land manager up near Chicago whose site has six species of gentians, the three I talk about here and two in another genus, that he says largely (but not perfectly) remain separate. This would be a situation worth studying for a pollination biologist.

      Also, one wonders how widely separated populations were in the past, before the habitat alteration / destruction of the last 10-20 decades took place?

  4. The people I write about live on the prairie. They have to fight for it, because it is the most fragile of ecosystems.

    No room for error. But obviously, plenty of room for tiny beauties.

  5. As a haiku novice, I am a ‘sucker’ for anything written about flowers, bees, all of nature. Names of species, descriptions of behavior like the bumblebee gyne (how fascinating!), and how it pries the bottle gentian sound more poetic than I could ever try to re-create in poetry. I’m glad yours is a featured blog I couldn’t resist to read. Thanks!

  6. very interesting, even to us non-entomologists! Too bad to lose the vibrant blue and the pure white! Though the inbetween has it’s virtues….nice photos, too. I love bugs…

  7. Pingback: Promiscuous Plants (via Beetles In The Bush) « Joys Blog

  8. Pingback: Capsule Blog


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