Biology Reference
In-Depth Information
populations of cordgrass are rapidly pushing California cordgrass toward
extirpation in south San Francisco Bay. Whereas smooth cordgrass flow-
ers later than California cordgrass, hybrids have an intermediate flower-
ing period that overlaps for a longer period with California cordgrass.The
various cordgrass forms have the same chromosome numbers and are
interfertile. Hybrid cordgrass, which possesses the higher pollen produc-
tion of its smooth cordgrass parent, thus backcrosses extensively with Cal-
ifornia cordgrass.
Like
Spartina anglica
in the British Isles, smooth cordgrass and its
hybrids with California cordgrass are able to invade open mudflats at tidal
levels lower than those occupied by California cordgrass. Thus, the eco-
logical impact of smooth cordgrass and the hybrid forms may be great. It
also appears likely that smooth cordgrass or hybrid cordgrass forms will
eventually become established outside San Francisco Bay.
In California, extensive hybridization occurs between two species of
sea figs,
Carpobrotus chilensis
,a rose-flowered native, and
C. edulis
,a yellow-
flowered alien (Albert et al. 1997; Gallagher et al. 1997). Most fitness char-
acteristics of hybrids were similar to or intermediate between those of the
two parents (Vilà and D'Antonio 1998). Overall estimates of seed and
seedling survival showed that survival probabilities were highest for
C.
edulis
, second highest for hybrids, and lowest for
C. chilensis
.The fact that
74% of plants examined in a series of coastal transects were hybrids, how-
ever, suggests that additional factors may give hybrids some advantage
over both parents.
Many other cases of hybridization between alien and native plants
exist. In Florida and the Bahama Islands, the alien
Lantana camara
hybridizes with the native
L. depressa
(Sanders 1987). On Madagascar, the
invasive bramble
Rubus alceifolius
has hybridized with the native
R. roridus
,
yielding an apomictic form. In turn, this form has invaded La Réunion
and other Indian Ocean islands (Amsellem et al. 2001). In Germany, the
alien cress
Rorippa austriaca
has hybridized with the native
R. sylvestris
to
give rise to a complex of invasive hybrid forms (Bleeker 2003).
R. austri-
aca
has a diploid chromosome number of 16, whereas
R. sylvestris
is a
hexaploid with a chromosome number of 48. Only some of the hybrids,
with ploidy levels ranging from 3X to 5X, reproduce sexually, but their
populations are spreading into areas where neither parent occurs.
Artificial hybrids created by crop and horticultural plant breeding
must also be considered as forms that can encourage the flow of foreign
genes into native plant species. More than 100 such crop hybrids and
thousands of horticultural hybrids have been created (Daehler and Carino
