Biology Reference
In-Depth Information
hybridizations to produce invasive new plants is also great. In a survey of
invasive plant taxa apparently derived from interspecific hybridization,
Ellstrand and Schierenbeck (2000) identified 27 cases in which hybridiza-
tions involving one or both alien members have given rise to invasive
forms.
Hybridization Between Alien Plants
One of the most striking examples of hybridization and its consequences
involves plants of the composite family, genus
Tr agopogon
, known com-
monly as salsify or goat's beard. In eastern Washington, hybridizations
occur among all three combinations of members of the genus:
Tr agopogon
pratensis
,
T. dubius
, and
T. porrifolius
(Owenby 1950). Hybrids of
T. praten-
sis
and
T. dubius
have given rise by polyploidy to a fertile form considered
to be a new species:
T. miscellus
. Hybrids of
T. pratensis
and
T. porrifolius
have similarly given rise to a second new species:
T. mirus
. Although these
new species have not proven to be noxious or highly invasive, they have
spread well beyond their points of origin (Novak et al. 1991b).
The plant genus to which willows belong (
Salix
) is notorious for
hybridization among species that have been planted in new locations.
Many hybrids involving both native and introduced species occur in
North America. Numerous species and varieties of willows also have been
introduced to Australia, where the genus is entirely alien (Cremer et al.
1995). These forms have produced at least nine hybrids, some between
two and some among three species. Several of these hybrids have proven
to be invasive.
The complexity of potential hybridizations among alien plants is illus-
trated by knotweeds of the genus
Fallopia
in Britain and continental
Europe (Bailey 1999).Two forms of
Fallopia japonica
,varieties
japonica
and
compacta
,have been introduced to Britain. Variety
japonica
, an octoploid
with a chromosome number of 88, is male sterile but spreads vegetatively
by rhizomes and is a very aggressive invader of disturbed habitats.Variety
compacta
,a tetraploid (n = 44), is a much less invasive form but has male-
fertile as well as male-sterile plants.Another widely introduced knotweed,
Fallopia sachalinensis
,a tetraploid (n = 44), also has both male-fertile and
male-sterile forms. A third introduced species,
Fallopia baldschuanica
, has a
chromosome number of 20 and produces fertile pollen. Male sterile vari-
eties of
Fallopia japonica
are frequently pollinated by
sachalinensis
or
bald-
schuanica
and in many cases produce viable seed. Hybrids of
japonica
with
both other species, varying in chromosome number from 44 to 88, have
