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 populations 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 introduced 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 containing the native crayfish O. propinquus and O. virilis. In Trout Lake, Wisconsin, 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 rusticus 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 population 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.

Among insects, the spread of African honey bees (Apis mellifera scutel-lata) in the New World following their introduction to Brazil in the late 1950s has involved hybridization between this form and domesticated European subspecies. The African subspecies has spread through tropical and subtropical regions as feral populations that interbreed with the European forms in managed apiaries (Hall and McMichael 2001).The representation of European genetic markers declines gradually in apiary populations, and in time, European genotypes are expected to disappear completely (Clarke et al. 2001).

A recent concern among insects is the possibility of invasion of North America by the Asian form of the gypsy moth (Lymantria dispar). In the Asian form, the female is able to fly, rather than being flightless, as in the case of the established European strain (Liebhold et al. 1996).The Asian and European strains hybridize freely, and the consequences of such an interaction are difficult to predict but could likely lead to a greatly increased rate of spread of this destructive insect. Several incipient populations of the Asian form have been detected in North America, all of which have either failed or been eradicated.

Examples of hybridization between aliens and natives are numerous among fish (Leary et al. 1995). Most of this result from the deliberate stocking of alien species for the supposed improvement of sportfishing. In these cases, much of the conservation concern also relates to genetic extinction of the native (see chapter 16). In addition, substantial evidence now exists that the stocking of hatchery-reared fish into wild populations of the same species can reduce fitness of wild populations.The hatchery environment selects for several morphological and behavioral characteristics in Atlantic salmon (Salmo salar) and Pacific salmon (Oncorhynchus spp.) that confer reduced fitness in the wild (Fleming and Einum 1997;Reisen-bichler and Rubin 1999).

Through hybridization between fish of hatchery and wild origin, the fitness of some populations spawning in the wild is also reduced. In Denmark, using DNA microsatellite analysis, Hansen (2002) found that wild brown trout (Salmo trutta) populations were in some cases highly intro-gressed and others only weakly introgressed by heavy stocking of hatchery fish. Hatchery trout appeared to be poorly adapted as anadromous fish, as opposed to permanent residents in freshwater, so interbreeding with wild fish may have reduced fitness of the anadromous wild population. In British Columbia, Canada, chinook salmon (Oncorhynchus tshawytscha) captured in the wild are spawned in aquacultural facilities, and the resulting fry are released back into streams to supplement natural reproduction.This procedure tends to select for high fecundity and small egg size (Heath et al. 2003). Offspring from small eggs have been shown to have lower survival than those from large eggs. In rivers that have received heavy supplementation with fry from hatchery spawning, significant reductions in egg size have been noted in the wild population.

Few cases of hybridization between aliens and natives are known among terrestrial vertebrates. Among mammals, hybridization occasionally occurs between domestic dogs (C. familiaris) and other canids, such as the gray wolf (Canis lupus) and the Ethiopian wolf (Canis simiensis) (see, e.g., Andersone et al. 2002; Gottelli et al. 1994). Feral domestic cats (Felis catus), derived from the North African race of the wildcat (Felis sylvestris), have also hybridized with the Scottish race of the wildcat, which is the last remaining wild race of the species (Beaumont et al. 2001).

More interesting is the interaction between the coyote (C latrans) and the gray wolf. In eastern North America, coyotes expanded their range eastward and northward during the 1900s (Thurber and Peterson 1991). Animals in the eastern areas of range expansion also tend to be larger and heavier, especially in parts of New England.This difference is probably at least in part genetic. We shall consider this example in more detail in chapter 10.

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