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| A plant escapes its enemies for 150+ years. What
happens to it during that time, and what happens when its arch enemy catches up with it? |
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Are the answers to those questions held by herbarium specimens? It seems
they are. Wild parsnips were brought to North America by
European colonists as a source of food and were in widespread cultivation by 1609.
What the colonists did not import was the parsnip webworm,
the parsnip nemesis that consumes the reproductive parts of the plant,...at least until
1869, when the insect was first recorded in Ontario Canada. Thus, parsnips enjoyed
roughly 250 years without this pest. It turns out that the furanocoumarins,
which are important defenses against webworms, are phenomenally stable over time, allowing
us to reconstruct changes in defense that occurred as the parsnip was reunited with
webworms. Even the physical interaction between plant and insect is preserved in
herbarium specimens. Presence of webworms can easily be confirmed by the presence of
characteristic webbing and feeding damage. Indeed, on rare occasions, the perpetrator
itself is preserved in the specimen as shown in this picture of parsnip collected in 1958. |
| None of the specimens collected between 1889 and 1909 showed evidence of
webworm activity. After that, a significant increase in webworm attacks was
observed. Furanocoumarin content of seeds increased dramatically with the
appearance of webworms. Two furanocoumarins in particular, sphondin and bergapten,
which are implicated in webworm resistance, increased. In
comparison with samples from Europe (EUROPEUROPE),
where webworms are native, North American samples were significantly lower (*) in both
furanocoumarins prior to establishment of webworms.
Absent webworms for such a long period of time reduced selection for furanocoumarin
defense. On reassociation with its nemesis, parsnips evolved increased
furanocoumarin content. |
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