Biology Reference
In-Depth Information
western North America, whereas the unnamed form showed greater vari-
ability. These results suggested that
destructiva
is an alien species that
entered North America and has spread rapidly. The unnamed form, in
contrast, appears likely to be a native fungus that has coevolved closely
with North American dogwood and is less virulent.The native region of
destructiva
is not yet known, but the fungus could have been imported on
horticultural stock, such as Japanese dogwood (
Cornus kousa
), which is
native to Japan and Korea.
Source Areas Revealed by Genetic Analyses
For many other alien species, including plants, animal, and microorgan-
isms, genetic analyses have helped to pinpoint source areas. Source areas
are often those suspected from other evidence, but many unexpected pat-
terns of origin are also revealed.
Hydrilla (
Hydrilla verticillata
) is one of the most troublesome aquatic
weeds in the southern United States. Two forms occur in the United
States. One, a dioecious plant, was introduced near Tampa, Florida, in the
early 1950s and has spread through many southern states and west to
California. The other, a monoecious form introduced to the Potomac
River near Washington, D.C., in the 1970s (Steward et al. 1984), is dis-
tributed from South Carolina northward in coastal states to Connecticut
and in California and Washington (Madeira et al. 2000). Hydrilla is wide-
spread in Australia, New Zealand, eastern Asia, and islands of the south-
west Pacific and also occurs in localized areas of Europe and Africa. Con-
siderable effort has been made to find biological control agents for this
weed, so source areas of the North American invasive forms are of great
interest.
Using DNA fingerprinting by the random amplified polymorphic
DNA analysis (RAPD) technique, Madeira et al. (1997) examined the
relationships of 44 hydrilla samples from most of the world regions where
it occurs. The dioecious form in the United States was most similar to a
sample from Bangalore, a city in southern India.This agrees with an ear-
lier report that hydrilla was introduced to Florida by a plant dealer who
obtained the material from Sri Lanka, an island country close to the
southern tip of India. The monoecious form closely matches a sample
from Seoul, Korea, which perhaps accounts for its adaptation to more
northern areas in the United States. The hydrilla sample from New
Zealand, where it first appeared in 1963, appears most similar to forms
from Australia, where hydrilla is widespread and has long been present.
