Biology Reference
In-Depth Information
been found in nature. These hybrids are very widely distributed in
Britain. Viable seeds with a chromosome number of 32 have also been
produced from other crosses or backcrosses. Similar patterns of hybridiza-
tion also occur in continental Europe. The frequency and diversity of
these patterns of hybridization gives a considerable evolutionary potential
to this plant complex. New, fully fertile knotweeds that are better adapted
to European climates and soils might well appear.
Fallopia japonica
has also
been introduced to other world regions and is a widespread invasive plant
in Canada and the United States (Sieger 1997).
Weedy forms have also arisen by hybridization between aliens in many
other plant genera, including
Carduus
(Warwick et al. 1989),
Centaurea
(Talbott-Roché and Roché 1991),
Lonicera
(Harrington et al. 1989),
Rhamnus
(Gil-ad and Reznicek 1997),
Rhododendron
(Milne and Abbott
2000),
Tamarix
(Gaskin and Schall 2002), and
Onopordum
(O'Hanlon et al.
2000). Johnsongrass (
Sorghum halepense
), one of the world's most noxious
agricultural weeds, is also suspected to have arisen by hybridization, in this
case between two species of wild sorghums in Africa (Paterson et al.
1995). Soft brome (
Bromus hordaceus
), another invasive grass of regions of
Mediterranean climate, is a tetraploid derivative of hybridization between
two other Old World grasses (Ainouche et al. 1999).
Many hybridizations involve introduced crop plants and alien relatives.
In northern California, for example, a weedy form of the genus
Secale
has
evolved by hybridization of cultivated rye (
Secale cereale
) with
S. mon-
tanum
,a grass that was introduced to an experimental station in Washing-
ton.The hybrid has become established in the wild in the Pacific North-
west (Sun and Corke 1992). Many other such cases are known. Enhanced
weediness has resulted from gene flow from crops to weedy relatives for
seven of the world's most important crop species (Ellstrand et al. 1999).
Hybridization Between Alien Animals
Alien animal hybridizations also produce new forms that can become
aggressive invaders. Many of these involve aquatic organisms, especially
fish. Fish hybrids, many of artificial origin, are common among tilapias,
salmonids, centrarchids, esocids, and various ornamental fish.
The major commercial species of tilapia include the Mozambique
tilapia (
Oreochromis mossambicus
), blue tilapia (
O. aureus
), Nile tilapia (
O.
niloticus
),Warm River tilapia (
O. urolepis hornorum
), and the redbelly tilapia
(
Tilapia zillii
) (Chapman 1992).Tilapia are now farmed worldwide. In the
United States, commercial tilapia culture is concentrated in Arizona, Cal-
