Biology Reference
In-Depth Information
A clear example of a shift from sexual to asexual reproduction is pro-
vided by bulbous bluegrass (
Poa bulbosa
).This grass is widespread in west-
ern Europe, where it reproduces primarily by sexually produced seed
(Novak and Welfley 1997). Bulbous bluegrass has become established
widely in North America, but here its primary reproductive mode is by
asexually produced bulblets. Many of the individual grass florets develop
directly into these bulblets, and others develop into flowers that normally
do not set seed. Some sexual reproduction may occur, however, since this
species maintains a high level of genetic variation in North America.
In some cases, a shift from sexual to asexual reproduction may only
occur in small, isolated populations of alien plants.Yellow starthistle (
Cen-
taurea solsticialis
), as noted earlier, is an aggressive forb invader of range-
lands in much of western North America. Most populations have
remained strongly outcrossing. A small population in San Diego County,
California, however, shows a high level of inbreeding (Sun and Ritland
1998). A strong genetic bottleneck during the establishment of this pop-
ulation may have eliminated alleles, leading to inbreeding depression and
a reduction in the frequency of genotypes among which outcrossing
could occur.
Examples of a shift toward outcrossing are less frequent. An increased
frequency of outcrossing has been seen in weedy grasses in areas to which
they have been introduced (Brown and Marshall 1981). Slender wild oat
(
Avena barbata
) and soft brome (
Bromis mollis
) exhibit less than 1% out-
crossing in European populations. Slender wild oat outcrosses at 2-7% in
California and soft brome at 10% in Australia.
Other basic life history patterns, such as a shift from annual to peren-
nial habit, may also occur as a result of strong selection in the alien envi-
ronment. In annual bluegrass (
Poa annua
), for example, perennial ecotypes
tend to develop in areas of intensively mowed turf (McElroy et al. 2002).
This species, native to Europe, is a problem grass in lawns and golf
courses.
Alien plants, both perennial and annual, have shown a variety of pat-
terns of adaptation to physical habitat conditions of their new environ-
ment. Rose clover (
Tr ifolium hirtum
), for example, was introduced to Cal-
ifornia from the Mediterranean region of Eurasia in the late 1940s as a
forage plant (Jain and Martins 1979). From pasture plantings, it has spread,
in some places, to roadsides and other disturbed habitats in central and
northern California. Comparison of the morphology and population
dynamics of pasture and roadside populations showed that several differ-
ences existed between these populations. In roadside populations, the
