Biology Reference
In-Depth Information
Cheatgrass ( Bromus tectorum ) is another example of a successful alien
plant with low genetic variability. It is one of the most widespread, abun-
dant, and ecologically disruptive alien plants in North America (Mack
1981). A short, predominately self-fertilizing, annual grass, Linnaeus
named it tectorum (“of roofs”) because it grew commonly on sod roofs of
European houses. Cheatgrass has been recorded from every U.S. state,
including Alaska and Hawaii. It has proven to be a highly invasive plant in
open habitats throughout western North America. In semiarid regions of
the Columbia Plateau and Great Basin, cheatgrass invasion leads to
increased wildfire frequency that can transform sagebrush steppe to pure
stands of cheatgrass. One can hardly imagine a more successful invader.
Allozyme analyses of cheatgrass plants from 60 sites across North Amer-
ica reveal little overall genetic variability (Novak et al. 1991a). About 4.6%
of the loci examined were polymorphic in individual populations. Further-
more, in a sample of more than 2,000 individuals, no individual was found
to be heterozygous for any of 25 genetic loci. Most of the variability that
does exist is among, rather than within, populations, suggesting that some
adaptation to local environments may be occurring.Analyses of microsatel-
lite markers, a somewhat more sensitive indicator of variability, from four
locations in Utah and Nevada revealed 15 different genotypes (Ramakrish-
nan et al. 2001). At one site, however, all 52 individuals were identical in
their microsatellite genotype.The various genotypes also showed some ten-
dency to sort out by habitat differences in seasonal temperatures.
Nevertheless, genetic variability in cheatgrass is relatively low in North
America. Not surprisingly, in its native range, which extends from west-
ern Europe to Afghanistan, greater overall genetic variation exists (Novak
and Mack 1993).Almost twice the number of loci showed polymorphism
in Eurasia as in North America.The differences among these populations
are also greater than in North America. Surprisingly, however, the vari-
ability within individual Eurasian populations was actually less than in
North America, averaging slightly more than 2%.This suggests that North
American populations, although possessing less genetic variability overall,
are probably derived from seed originating from several parts of the
Eurasian range. This has apparently led to the greater variability within
populations in North America than in Eurasia.
Many alien plants show high levels of genetic variability, usually indi-
cating that they were introduced several or many times and, in some cases,
from different parts of their native range abroad. High genetic variability
can be shown both by species that are outcrossing and by those that are
self-fertilizing, apomictic, or clonal.
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