Biology Reference
In-Depth Information
ology, plant-microbe relationships, and plant-herbivore interactions. Many
of the herbivorous insects with which it now interacts are alien species in
either Europe or North America. Many ecotypes of
A. thaliana
have been
obtained from locations around the world. The genome of two of the
commonly used ecotypes has been sequenced, and it is apparent that great
variation exists at the DNA sequence level.
Jander et al. (2001) examined feeding on
A. thaliana
by the cabbage
looper (
Tr ichoplusia ni
), a generalist lepidopteran native to North America
but introduced to Europe.They found that ecotypes of
A. thaliana
varied
greatly in resistance to the cabbage looper, with some being virtually
untouched whereas others were eaten to the ground. Ecotypes from cen-
tral Asia tended to be highly resistant to cabbage loopers. In addition,
some portion of the resistance appeared to be induced, as demonstrated
by the loopers' tendency to prefer undamaged leaves over those that had
been experimentally damaged. Resistance is apparently controlled by an
allele at a locus on Chromosome 1 of
A. thaliana
,but the physiological
basis of this resistance is still unknown.
Mauricio and Rausher (1997), also working with
A. thaliana
, exam-
ined selection for two constitutive traits of the annual plant. They noted
that ecotypes of this plant vary genetically in two characters: concentra-
tion of glucosinolates and density of trichomes. Glucosinolates are organic
compounds found in many plants of the cabbage family (Brassicaceae).
These compounds are unpalatable or toxic to many arthropods and ver-
tebrates. In a field experiment,
A. thaliana
plants from different families
were grown in locations where they were exposed to or protected from
herbivorous arthropods and fungal pathogens. Results showed that natu-
ral selection influenced both defensive characteristics. In plants that were
protected from natural enemies, glucosinolate concentration and tri-
chome density tended to decline. In plants exposed to enemies, both con-
stitutive traits were maintained.Thus, it is likely that natural selection acts
quickly in natural populations of
A. thaliana
to adjust these defenses to the
intensity of influence of natural enemies. This experiment thus strongly
supported the concept that constitutive defenses have an appreciable cost.
Similarly, Shonle and Bergelson (2000), working with jimson weed
(
Datura stramonium
) in Illinois, found that the concentrations of two alka-
loids in leaf tissues were influenced by selection by a combination of spe-
cialist and generalist herbivores. This plant, native to Mexico, has appar-
ently spread northward into the United States since European settlement
as an agricultural weed. In Illinois, it is attacked by specialist insects asso-
ciated with it in Mexico and with several generalist insect herbivores,
