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Hybridization and Introgression Between Native and Alien Species 279
microsatellites (Pritchard et al. 2000; Anderson and Thompson 2002), have
already been widely applied to the study of hybridization and introgression in
natural and man-induced cases (e.g., Beaumont et al. 2001; Barilani et al. 2005;
Williams et al. 2005; Lecis et al. 2006).
The simultaneous use of cytoplasmic (mitochondrial and chloroplast
DNA) and nuclear genetic markers has become an important standard in
studies of introgressive hybridization. The combination of these two marker
classes allow us to gain very detailed insight into the processes of hybridization
and introgression (as reviewed, e.g., for aquatic organisms by Avise
2000).
These studies take advantage of the fact that these cytoplasmic genomes
are usually maternally inherited, and thus show a pattern of inheritance different
to that of recombining nuclear markers. A joint use of these marker
classes provides information that cannot be obtained by using either marker
class alone. For example, if only one sex of the invaded taxon hybridizes, then
the maternally inherited markers will introgress asymmetrically in relation to
nuclear makers, which are inherited by both sexes equally. The direction of
the asymmetry will give us information on which combinations of mating
occur in the interbreeding of the two parental taxa. If the interbreeding is
restricted to males of the invading taxon with females of the native taxon,
then we would observe that hybrid individuals always carry a mitochondrial
genotype of the native taxon, while having a mixture of alleles of the two
parental taxa at the nuclear markers. Thus, measures of association between
specific alleles at nuclear markers and cytoplasmic genotypes can be used to
formulate hypotheses of factors involved in hybrid formation, and the rate
and direction of genetic introgression in hybridization events. This development
of a cytonuclear theory and of statistical models provided an important
framework for hypothesis testing using empirical data in hybridizing taxa
(Asmussen et al. 1987; Scribner et al. 2000).
16.3 Basic Types of Anthropogenic Hybridization:
Empirical Examples
16.3.1 Hybridization Without Introgression
Hybridization may contribute to the decline of native species even if F1-
hybrids are all completely sterile. In this case, although hybridization is not a
threat through genetic mixing, part of the reproductive effort will be wasted
in the production of sterile hybrids (Rhymer and Simberloff 1996; Allendorf
et al. 2001). A genetic model suggests that hybridization with sterile hybrids
has little effect on the dynamics of the displacement (Huxel 1999), which pri-