Biology Reference
In-Depth Information
2001). With horticultural hybrids in particular, the parentage of derived
forms may involve species from widely different geographic regions and
is often held secret by the breeder. These hybrids in at least some cases
hold the potential for further hybridization with native species. In regions
such as the Hawaiian Islands and Florida, where thousands of horticultural
forms have been introduced, the risk of such hybridizations is high.
Hybridization Between Alien and Native Animals
Hybridization between introduced and native invertebrates is probably
more common than yet documented (Perry et al. 2002). Some of these
hybridizations are cryptic and have only been detected by molecular
genetic techniques. For example, an allozyme analysis of 64 populations
of the cladoceran
Daphnia galeata
from North America and ten popula-
tions from Europe revealed that several populations in Ontario, Canada
and New York were hybrids of North American and European forms
(Taylor and Hebert 1993).
Crayfish are a group of freshwater invertebrates that have been intro-
duced to new waters both accidentally and deliberately and both intra-
and inter-continentally. These introductions have had many effects,
including displacement of natives (see chapter 16). In northern Michigan
and Wisconsin,
Orconectes rusticus
has been introduced to many lakes con-
taining the native crayfish
O. propinquus
and
O. virilis
. In Trout Lake,Wis-
consin, where
O. rusticus
was introduced in 1979, allozyme analysis
and mitochondrial DNA fingerprinting have revealed that hybridization
has occurred with
O. propinquus
(Perry et al. 2001a). Hybridization
has not occurred with
O. virilis
, which is also present.
The hybrid crayfish in Trout Lake arise primarily from matings of
rus-
ticus
females and
propinquus
males.These hybrids have high fecundity and
survivorship but in addition show greater competitive ability than either
parent.The least competitive form,
propinquus
,however, is being displaced
rapidly, and since the female parent of most hybrids is
rusticus
, the mito-
chondrial genome of
propinquus
is disappearing rapidly. Thus, as pure
propinquus
crayfish disappear, the first-generation hybrids between the two
species, which show enhanced competitive ability, will become less fre-
quent.The genome that will eventually characterize the surviving popu-
lation may contain some component of
propinquus
genes, however.Thus,
in a sense,
O. propinquus
will have effectively been extirpated in Trout
Lake, but
O. rusticus
will have undergone some evolutionary change as a
result of genetic introgression.
