Biology Reference
In-Depth Information
Additional hybridization is now occurring between the NAN and
EAN forms of
O. novo-ulmi
in central Europe. These hybrids and their
backcrosses represent a series of active centers of evolution of
O. novo-ulmi
,
the outcome of which cannot be predicted. In addition, the potential exists
for hybridization with other forms of
Ophiostoma
, including not only
O.
himal-ulmi
but also members of the genus associated with other trees.
Another example of introduced fungal parasites of higher plants that
can interbreed to produce hybrids with new host infection capabilities is
provided by fungi of the genus
Phytophthora
. These fungi produce blight
on a wide variety of higher plants. In the British Isles, a
Phytophthora
blight
appeared on alders in the 1990s, killing many trees (Brasier et al. 1999).
DNA fingerprinting revealed that this agent is a highly diverse polyploid
resulting from hybridization of
Phytophthora
species alien to the British
Isles but not known to attack alders. Although the typical form is poly-
ploid, much of its diversity is related to variation in chromosome num-
ber. The parental forms appear to be two
Phytophthora
species that are
not European natives but that can occur together on plants of the genus
Rubus
—raspberries and blackberries.
Additional examples of hybridization between alien fungi are pro-
vided by rusts in New Zealand and North America (Frey et al. 1999;
Spiers and Hopcroft 1994). These hybrids have occurred among rusts of
the genus
Melampsora
that occur on trees of the genus
Populus
.
Hybridization Between Alien and Native Plants
Many other hybridizations involve alien and native species, with conse-
quences similar to those described for alien-alien hybridizations. Among
plants, another cordgrass example is provided by the alien, smooth cord-
grass (
Spartina alterniflora
), and the native species, California cordgrass (
S.
foliosa
), in San Francisco Bay, California (Daehler and Strong 1997a;
Antilla et al. 1998, 2000). Smooth cordgrass, native to the east coast of
North America, was introduced deliberately to San Francisco Bay in the
1970s. DNA fingerprints using the RAPD technique were made of both
species, together with plants in the field that were thought to be hybrids.
In 1995, the presence of hybrids was confirmed. Hybridization occurs
with either species as the pollen parent, but smooth cordgrass produces
much more viable pollen than does California cordgrass, so where the
two species occur in proximity, smooth cordgrass promotes heavy hybrid
seed set.
Recent work (Antilla et al. 2000) has shown that hybrid-dominated
