Biology Reference
In-Depth Information
that epistasis is an important component of adaptation by the soapberry
bug (
Jadera haematoloma
) to new alien host plants. We shall examine this
case in detail in chapter 13.
Genetic Variability Among Alien Plants
Levels of genetic variability seen in populations of alien plant species vary
greatly. Almost all alien plants that are dioecious or monoecious but
which reproduce by obligatory outcrossing show levels of genetic vari-
ability equal to or only slightly below that of their source populations.
Some of these species, such as tree of heaven (
Ailanthus altissima
), are
highly invasive. A few, however, exhibit somewhat reduced variability,
reflecting small founding populations or genetic bottlenecking following
introduction.
Many introduced plants that are self-fertilizing, apomictic, or vegeta-
tively reproducing show lower genetic variability than do populations in
their native regions. White bryony (
Bryonia alba
), for example, is an
apomictic herbaceous vine that was introduced from central and eastern
Europe to parts of the northwestern United States in the mid- to late-
twentieth century (Novak and Mack 1995). Populations have become
established in three somewhat separate areas: southeastern Washington and
adjacent Idaho, western Montana, and southeastern Idaho and northern
Utah. If the species had been introduced as a small number of plants from
a single Old World source, one would expect that the population in the
United States should show little genetic variation. Allozyme analysis
showed that populations in the two northern areas were polymorphic in
about 19-20% of the loci examined, whereas the southern population is
polymorphic in only slightly more than 7% of loci. Strong differences
were evident among populations, however, and it is likely that at least two
and possibly three independent introductions, of white bryony occurred
in the United States.
Populations of alien plants that are self-fertilizing, and thus do not
experience genetic recombination, tend to show low genetic variability if
they originate from a small founder population. Witchweed (
Striga asiat-
ica
), a self-fertilizing plant parasitic on grains such as sorghum and maize,
is a relatively recent introduction to the eastern United States. In this case,
allozyme analysis found that plants from two populations in North and
South Carolina showed no allele differences at any of 32 enzyme loci,
although this plant is quite variable in many characteristics in different
parts of Asia and Africa (Werth et al. 1984).
