|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:
there must be genetic variation for
characters in both plant and insect that influence the interaction between the species,
each species must be a selective force on
the other species (i.e., affect the other's fitness), and
there must a response to selection in each species.
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.
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.
Do webworms and parsnips influence each other's fitness (are they
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.
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
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.
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