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C.R. Largiadèr
of the world’s known crayfish species, of which about 30 % are threatened or
endangered (Lodge et al. 2000). Indeed, the rusty crayfish (Orconectes rusticus)
has been shown to be hybridizing with, and displacing native O. propinquus
(Perry et al. 2001). O. rusticus is native to southwestern Ohio, and has
been introduced widely as fish bait throughout the United States, where it has
become a serious pest over the last 35 years. A detailed study by Perry et al.
(2001) revealed a quite interesting situation. Patterns of cytonuclear disequilibrium
between allozymes and mtDNA suggested that the majority of F1-
hybrids were offspring of matings between O. rusticus females and O. propinquus
males, although O. rusticus males were expected to outcompete the
native O. propinquus males, which are smaller in size than their introduced
congeners. Compared to both parental forms, these authors also found no
reduction of fecundity and early survivorship, but rather a competitive superiority
in hybrids. They stated that these results, at first sight, seemed to be at
odds with the expectation that introgressive hybridization would enhance the
displacement of the native gene pools by the invading genes, since the asymmetrical
gene flow of native nuclear genes into the invaders gene pool should
operate rather in the opposite direction. However, by assuming that the higher
competitive ability of early-generation hybrids translates into higher relative
fitness, and using a simple one-locus model, they estimated that introgressive
hybridization would accelerate the elimination of pure O. propinquus by
4.8–36.3 % above that due to the previously documented ecological interaction.
Among birds, several duck species have been intensively studied in the
context of anthropogenic hybridization. These studies provide impressive
examples on how habitat modifications and transplantations, followed by
introgressive hybridization, have led to declines of several native taxa
throughout the world (Kulikova et al. 2004; Williams et al. 2005). A nearly
complete genetic mixing of the New Zealand grey duck (Anas s. superciliosa)
with introduced mallards (A. platyrhynchos) threatens the native taxa to
become extinct, and to be replaced by a new mongrel species (Rhymer et al.
1994). An especially noteworthy example concerning duck species involves
the American black duck (A. rubripes) and A. platyrhynchos. These two
species have been primarily allopatric prior to the settlement by Europeans
of North America. Habitat alteration and game-farm mallard releases during
the 20th century enabled mallards to extend their range, and to come into
contact and interbreed with the morphologically and behaviorally similar
black duck (Mank et al. 2004). Molecular data based on modern specimens
suggested the black duck to be a recent evolutionary derivative of a more
broadly distributed mallard–black duck ancestor (Avise et al. 1990). However,
the genetic analysis of modern and museum specimens clearly showed
a dramatic decrease in genetic differentiation between the two taxa, indicating
that the present-day genetic similarity is the consequence of gene flow
through introgressive hybridization (Mank et al. 2004). This study is a nice