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New Zealand

Coevolution between a herbivore and its host plant
The interaction between wild parsnips, Pastinaca sativa, and the parsnip webworm, Depressaria pastinacella, is mediated by a group of toxic compounds called furanocoumarins.   These compounds are found throughout the parsnip plant, including its reproductive parts, which are the favorite food of the parsnip webworm.  Few herbivores are capable of coping with the high concentrations of furanocoumarins found in wild parsnips.   Thanks to a highly efficient detoxification system involving cytochrome P450s, webworms are well adapted to coping with these compounds.  For coevolution to progress in this pair of interacting species, several conditions must be satisfied:

WB01539_.gif (682 bytes) there must be genetic variation for characters in both plant and insect that influence the interaction between the species,

WB01539_.gif (682 bytes) each species must be a selective force on the other species (i.e., affect the other's fitness), and

WB01539_.gif (682 bytes) there must a response to selection in each species.

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Genetic Variation
In order to determine whether there is genetic variation for furanocoumarin production in a parsnip population and in detoxification capacity in an interacting population of webworms, we employed quantitative genetic analyses.  Quantitative genetics rests on the principle that relatives resemble one another.  Carefully designed studies can partition the amount of phenotypic variation in a population into genetic and non-genetic (environmental) components.  The heritability is then the proportion of phenotypic variation controlled genetically.  One method by which heritabilities may be estimated is called offspring-parent regression.The heritabilities for the furanocoumarins xanthotoxin and sphondin in parsnip seeds was determined in the example at left by offspring-parent regression.  Concentrations of these compounds in the seeds of parents were regressed against the concentrations measured in their maternal parent.   If there is a significant slope to the regression line, there is genetic variation for the amounts of these compounds in parsnips.  The heritability for sphondin is quite high--a heritability of 0.834 suggests that 83.4% of the variation in sphondin content among plants is under genetic control.

 

Heritabilities

Using various methods of estimating heritabilities for furanocoumarin content in parsnips and detoxification capacity in webworms, we found ample genetic variation in most of these traits.  One notable exception was the nearly complete lack of genetic variation in the ability of webworms to detoxify sphondin. Most of these traits, then, potentially responsive to natural selection.

 

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Do webworms and parsnips influence each other's fitness (are they selective agents)?

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Webworms eat the parsnip's reproductive parts, they can devastate the fitness of this monocarpic plant..  In the photograph at left, broad yellow umbels are transformed into ugly masses of silk and frass.

 

 

 
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The graphs at left show the numbers of seeds produced in the primary umbel of wild parsnips when webworms are not present (as in the the greenhouse in this case) and when they are present (in the field). The same families of plants are displayed in both graphs.  It can readily be seen that variations in seed production are small when webworms are absent but are very high when they are present.  These data suggest that webworms, which feed on the buds, flowers and unripe fruits can impact the parsnip's fitness.

We can estimate the intensity of directional selection by the webworm acting on the furanocoumarin chemistry of the parsnip by regressing the fitness of each family against levels of its furanocoumarins.  This was accomplished by multiple regression:

FITNESS = bTRAIT1 + bTRAIT2 +bTRAIT3......+bTRAITn

where the unique contributions of traits 1 - n to fitness are estimated.  The bs (regression coefficients) are the directional selection gradients.  

 

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Significant positive selection gradients were found for bergapten and sphondin , indicating that webworms select for increased bergapten and sphondin (shown at left) content in their host plant.  In this way, webworms exert a selective influence on their parsnip hosts.


The degree to which these two species act as selective agents on each other is reflected in the extraordinary precision with which plant and insect phenotypes match in different populations (see matching).

Further evidence of the potent effect that webworms have historically had on the chemical evolution of wild parsnips can be seen in herbarium specimens

last updated 3/8/2000