Phenotype matching between wild parsnip and parsnip
Wild parsnip plants can be characterized by
their furanocoumarin production, and parsnip webworms can be characterized by their
ability to metabolize those furanocoumarins. Each plant and each insect can then be
assigned a phenotype. We collected both seeds and pupae from four Midwestern
populations. The seeds were analyzed for concentrations of four
furanocoumarins--xanthotoxin, bergapten, isopimpinellin, and sphondin. The larval
offspring of the adults collected as pupae were analyzed for their capacity to metabolize
those same four furanocoumarins. By adjusting each value (furanocoumarin production
or furanocoumarin metabolism) by the average across populations, we able to combine both
plants and insects in a cluster analysis.
|| Cluster analysis was used to group
individual plants and insects into four clusters that can be considered multivariate
phenotypes. Each cluster contains insects and plants from each of the four
populations. This means that all of the variation found in these populations is
represented in each population. However, the frequencies of each phenotype differs
The frequencies of the four phenotypes in each population of plants was compared to the
frequency of insect phenotypes. An extraordinary degree of matching was discovered
in three of the populations (Winona, Peotone, and Charleston). The only population
in which the frequency of plant phenotypes differed significantly from that of the insects
was the one in Urbana. We suspect that the failure of the Urbana populations to
match may be due to a difference in sampling methodology. In each of the three
matching populations, we collected both plants and insects within well-defined boundaries.
In Urbana, the plants were collected from a well-defined area, but the
insects were collected from a much larger area but still within the same continuous
parsnip population. These results suggest that coevolution between this plant and its
herbivore is very tight.
|In order to
determine what factors might influence matching, a large study
investigating 20 populations occuring in Illinois and Wisconsin was undertaken.
Among the notable findings were that matching can occur within a single growing
season. We know this because populations of wild parsnip from which webworms were
eradicated the previous year by insecticide were recolonized the following year and
matched their plant populations. For this to have happened so quickly, differential
mortality of maladapted insect phenotypes must have occurred to produce the matching.
The alternative explanations involving differential mortality of plant phenotypes
and selective oviposition of female webworms are highly doubtful. In the case of the
plants, webworms rarely kill parsnips outright, although they can severely reduce the
plant's seed output, and the female webworms lay eggs before the plant parts that their
offspring feed on even exist, so it is unlikely they can predict what their offspring will
encounter. A transplant experiment of larvae that managed to make it to fifth instar
on parsnips in the field showed that indeed larvae are more likely to die if moved to
another plant than if moved to a different location on their natal plant.
However, not all populations in the study exhibited matching and the factors found to
contribute to mismatching were the presence of an alternative hostplant, cow parsnip (Heracleum
lanatum), or the presence of particularly high concentration furanocoumarin plants (the plant and insect data sets are now available as excel files).
Does this kind of matching make any
difference? Don't larvae adjust metabolism to meet demands anyway?