4
K. Sawamura
1 Introduction
Speciation has two aspects, horizontal and vertical. The nodes of a phylogenetic
tree represent the former, which increases the number of species, while internodes
represent the latter, which elaborate the form of species. Biodiversity has bcen shaped
as the sum of speciation, and it is important to shed light on both these aspects.
From the viewpoint of the biological species concept (Dobzhansky 1937; Mayr
1942), it is a fundamental question as to how reproductive isolation is acquired in
diverged populations. Although aU mechanisms preventing genetic exchange between popUlations are sometimes regarded as reproductive isolation, they are not
homogeneous. Some mechanisms perturb inter-population matings (prezygotic isolation), and others produce abnormal hybrids of mixed genomes (postzygotic isolation). Although we cannot rule out the possibility that some mechanisms developed
in order to reduce the cost of producing unadapted hybrids (reinforcement), most of
the mechanisms are supposed to be the result, not the cause, of genetic diversity
between populations. Regardless of the mode of speciation, whether sympatric or
allopatric, genetic distance between popUlations increases, resulting in reproductive isolation as byproducts (Zouros 1989; Coyne and Orr 1989, 1998; Coyne 1992;
Wu and Davis 1993; Wu and Palopoli 1994; Forejt 1996; Wu et al. 1996; Hutter
1997; Laurie 1997; Orr 1997).
In the present review, I will concentrate on mechanisms of postzygotic isolation
which is the consequence of genetic incompatibility betwecn species. This is not
because postzygotic isolation is more important than prezygotic isolation in speciation, but merely because the former has been better elucidated in this century. Because each species has its own history and may have accumulated different modifications of genetic systems of development, the coexistence of genomes in interspecific hybrids may result in genetic incompatibility. The only forcc to conserve the
conspecific genetic systems is the shared history bounded by reproduction. Any
kinds of deviation from perfect development of individuals with mixed genomes
should be regarded as incompatibility, although only inviability and sterility in F J
hybrids and in descended generations (hybrid breakdown) have been seriously treated
so far. Even defects seen in interspecific gene transformation, somatic cell hybrids,
and organ transplantation can be included in this category.
2 Case Studies
2.1 Tumorigenesis
An example of genetic incompatibility whose molecular mechanism is well documented is melanoma formation in the poeciliid fish hybrids. This phenomenon has
been well-known since classical descriptions of the hybrids [for reviews see Schwab
(1987) and Schart! (1995)]. The platifish (Xiphophorus maculatus) and the sword-
K. Sawamura
1 Introduction
Speciation has two aspects, horizontal and vertical. The nodes of a phylogenetic
tree represent the former, which increases the number of species, while internodes
represent the latter, which elaborate the form of species. Biodiversity has bcen shaped
as the sum of speciation, and it is important to shed light on both these aspects.
From the viewpoint of the biological species concept (Dobzhansky 1937; Mayr
1942), it is a fundamental question as to how reproductive isolation is acquired in
diverged populations. Although aU mechanisms preventing genetic exchange between popUlations are sometimes regarded as reproductive isolation, they are not
homogeneous. Some mechanisms perturb inter-population matings (prezygotic isolation), and others produce abnormal hybrids of mixed genomes (postzygotic isolation). Although we cannot rule out the possibility that some mechanisms developed
in order to reduce the cost of producing unadapted hybrids (reinforcement), most of
the mechanisms are supposed to be the result, not the cause, of genetic diversity
between populations. Regardless of the mode of speciation, whether sympatric or
allopatric, genetic distance between popUlations increases, resulting in reproductive isolation as byproducts (Zouros 1989; Coyne and Orr 1989, 1998; Coyne 1992;
Wu and Davis 1993; Wu and Palopoli 1994; Forejt 1996; Wu et al. 1996; Hutter
1997; Laurie 1997; Orr 1997).
In the present review, I will concentrate on mechanisms of postzygotic isolation
which is the consequence of genetic incompatibility betwecn species. This is not
because postzygotic isolation is more important than prezygotic isolation in speciation, but merely because the former has been better elucidated in this century. Because each species has its own history and may have accumulated different modifications of genetic systems of development, the coexistence of genomes in interspecific hybrids may result in genetic incompatibility. The only forcc to conserve the
conspecific genetic systems is the shared history bounded by reproduction. Any
kinds of deviation from perfect development of individuals with mixed genomes
should be regarded as incompatibility, although only inviability and sterility in F J
hybrids and in descended generations (hybrid breakdown) have been seriously treated
so far. Even defects seen in interspecific gene transformation, somatic cell hybrids,
and organ transplantation can be included in this category.
2 Case Studies
2.1 Tumorigenesis
An example of genetic incompatibility whose molecular mechanism is well documented is melanoma formation in the poeciliid fish hybrids. This phenomenon has
been well-known since classical descriptions of the hybrids [for reviews see Schwab
(1987) and Schart! (1995)]. The platifish (Xiphophorus maculatus) and the sword-