Biology Reference
In-Depth Information
Garlic mustard (
Alliaria petiolata
) is an invasive alien plant in forests of
eastern North America. This species has a wide distribution in the Old
World. Its original distribution extends from England and western conti-
nental Europe eastward to Russia and Asia Minor and southward to
North Africa.This invasive herb was introduced to eastern North Amer-
ica in the 1860s and now occurs in at least 30 U.S. states and six Cana-
dian provinces. DNA fingerprinting that compared North American pop-
ulations to those in Scotland and those in western continental Europe
indicated that the greatest similarity of most alien populations was to the
Scottish populations (Meekins et al. 2001). Some populations in eastern
Canada and Maryland did not show close relationships with the Scottish
material, however, and may have been introduced separately from other
European sources.
The native regions of several plant pathogens have been identified
from similar evidence. Sorghum ergot (
Claviceps africana
) is a fungal dis-
ease of plants of the genus
Sorghum
. This fungus was unknown in the
Western Hemisphere until 1995, when it appeared in Brazil, either by
introduction in contaminated seed or by long-distance dispersal of spores.
Within 2 years, however, it had spread north to the United States. This
ergot fungus is widespread in Africa, Asia, and Australia. Amplified frag-
ment length polymorphism analysis revealed that the Western Hemi-
sphere strain is most closely related to strains in Africa and also showed
that despite lacking a sexual stage, the American strain has high genetic
diversity (Tooley et al. 2002).
In other cases, the likelihood that a new plant disease is alien has been
revealed, although the source area remains in doubt. An example is but-
ternut canker (Furnier et al. 1999), which has caused widespread mortal-
ity of butternut (
Juglans cinerea
) in eastern North America. Genetic finger-
printing by the RAPD technique was used to examine 86 samples of this
fungus from throughout the range of butternut. No variability was found
in these samples. This, together with the very recent appearance of the
disease, indicates that it is a recent introduction, probably from a location
in Asia. A fir canker also showed much lower genetic variation in North
America than in a location in Sweden, suggesting that it, too, was a recent
introduction to North America (Wang et al. 1997).
Among marine invertebrates, the colonial ascidian (“sea squirt”)
Botryllus schlosseri
is now a nearly cosmopolitan marine invertebrate that
has been introduced to new coastal regions by oceanic ships.
Botryllus
is a
fouling organism that grows commonly on the hulls of ships and is
believed to be native to the Mediterranean Sea. This ascidian first
