Hemiparasitism:a way station to holoparasitism

or an evolutionary stable

strategy?

Prof. Joseph E. ArmstrongIllinois State University

Behavior, Evolution, Ecology, and Systematics SectionDepartment of Biological Sciences

Normal, IL 61790-4120 USA<jearmstr@ilstu.edu>

Destination: Holoparasitism

You are here: Hemiparasitism

Way back the other way: Autotrophy

Is this the ultimate destination of hemiparasitism?

Orobanchaceae

Holoparasitism has evolved from hemiparasitism 7-10 times in this clade.

Why be a hemiparasite?

• Q. Why should an obligate parasite retain its photosynthetic ability?

• A. It must benefit by doing so.

• Q. When causes hemiparasitism to begin the journey to holoparasitism?

• A. When the cost of maintaining its PS ability is greater than the benefit received from PS.

Under what circumstances does PS benefit a hemiparasite?

• A1. In a fluctuating environment PS and parasitism may be functional tradeoffs such that hemiparasites enhance their gains via one or the other at different times.

• A2. Hemiparasites might alter their environment in such a way that they enhance their gain via PS and parasitism at the same time.

Pedicularis canadensis

• Prairie grasslands & eastern deciduous forest

• Obligate hemiparasite• Wide range of host

species• “Fully” photosynthetic• Root parasite – water,

nitrogen, some carbs seasonally.

• Short stature, but not a spring ephemeral

A nutrient-limited grassland

Pedicularis canadensis has a profound impact on the prairie community.

• It significantly reduces the height of the prairie canopy,

• which enhances light reaching the prairie under story.

• It differentially affects host species thereby altering the community structure.

Presence of Pedicularis canadensis (Pc) reduces prairie canopy height by ~36%.

Amount of light reaching Pedicularis canadensis doubles as prairie canopy height decreases

Presence of Pedicularis canadensis (Pc) alters the relative composition of the prairie

community.

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10

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60

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Control Pc+

% R

ela

tiv

e C

ov

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Bare Ground

Big Bluestem

Indian Grass

Rich soil prairie community

• Canopy height and community structure not altered by presence of hemiparasite.

• At height of 20 cm, hemiparasite is in dense shade & higher humidity.

• Pedicularis canadensis barely persists on tiny “islands” of poor soil (pioneer cemetery-prairie remnant).

No evidence of functional tradeoffs between PS and parasitism.

Nutrient/water limited Not so limited

PS & Parasitism Enhanced

PS & Parasitism Reduced

Nutrient limited

• Increased light in under story enhances PS.

• Open canopy enhances transpiration.

• Thus increasing gains via both PS and parasitism.

• In this environment hemiparasitism is an ESS.

Not (as) limited

• Light levels remain low in under story

• Transpiration is reduced in shade and higher humidity

• Thus reducing gains via both PS and parasitism.

• Hemiparasite does not persist.

What circumstances would favor a shift toward parasitism?

• Loss of nutrients to parasite must not overly affect host.

• Hemiparasite must be able to obtain adequate resources without significantly altering community structure.

• Benefit received from parasitism must be greater than benefit from PS.

In a deciduous forest community

• Pedicularis canadensis also occurs in eastern deciduous forest communities where trees are among its primary hosts.

• Much greater size difference between host and parasite means little impact on trees.

• Presence of parasite does not alter forest canopy or under story light regime. PS only significant in spring, but Pedicularis is not a spring ephemeral.

Prediction

• In forest communities, Pedicularis canadensis no longer can alter its environment to enhance PS, so PS capacity should decline.

• Much larger host size means parasite can “take more” without unduly harming host, and woody hosts may provide more carbohydrates in xylem stream, especially in the spring, reducing need for PS.

• Forest environment shifts the balance toward holoparasitism. (Where did other holoparasites evolve?)

Test by determining if Pedicularis canadensis has reduced

photosynthetic capacity in eastern deciduous forest communities.

• Measurements in the field.

• Glass house experimental conditions.

Acknowledgements

• My colleague: Dr. Victoria Borowicz

• My students: J. Sunley, D. White, A. Hedberg.

• McLean County Parks (study sites) Reference: Hedberg, A., V. Borowicz, and J. Armstrong.

2005. Interactions between a hemiparasitic plant, Pedicularis canadensis L. (Orobanchaceae) and members of a tall grass prairie community. J. Torrey Bot. Soc. 132: in press.