Biology Reference
In-Depth Information
Perhaps not surprisingly, genetic analyses sometimes raise as yet unan-
swered questions about the origin of alien species.The round goby (
Neso-
gobius melanostomus
), an invasive fish native to the Black and Caspian seas
and major inflowing rivers, appeared in the North American Great Lakes
in 1990 and has now established populations in all five lakes. The goby
also appeared in the Gulf of Gdansk, Poland, in the same year. Analyses of
mtDNA from these alien populations did not closely match those of gob-
ies from the northern Black Sea, nor did the Polish and North American
gobies show strong similarity.Thus, the exact source of the founding fish
for these alien populations is still unknown (Dillon and Stepien 2001).
Source Areas of Species with Multiple Introductions
Genetic analyses can also reveal information about the number of intro-
ductions of individual alien species. For many aliens, multiple introduc-
tions are the rule. The numerous opportunities for transport of both
aquatic and terrestrial plants and animals make multiple introductions
increasingly frequent. In Australia, for example, the discovery of three new
populations of the invasive marine alga
Caulerpa taxifolia
in New South
Wales, Australia, led to a series of analyses of ribosomal DNA to try to
determine their sources (Schaffelke et al. 2002). One was confirmed as
originating from populations along the coast of Queensland.The remain-
ing two populations, which are different from each other, apparently came
from other sources, as yet unknown.
One of the best examples of multiple introductions of an alien plant
comes from ongoing studies of cheatgrass (
Bromus tectorum
). As noted in
chapter 3, cheatgrass has a wide native distribution in the Old World.
Novak and Mack (2001) examined samples from 164 localities through-
out the species' native and introduced ranges, using allozyme elec-
trophoresis.These studies led to the identification of a number of genetic
markers that are specific for locations in the species' native range. Using
these markers, they are able to infer the source areas of populations in
regions to which the species has been introduced.
In North America, at least seven or eight independent introductions
of cheatgrass have occurred (Novak and Mack 2001; Bartlett et al. 2002).
Most of these seem likely to have been direct introductions to western
North America, since the genetic markers do not occur in the East.Two
may have been to eastern North America, with subsequent spread west-
ward to the Pacific Northwest. In other areas of the world, at least two
introductions, from the Iberian peninsula and Morocco, appear to have
