8
K. Sawamura
1984). In fact, no obvious shared consensus sequence has been recognized in the
region of transcription initiation of ribosomal gene from different species of eukaryotes. If the change in a single promotor sequence of this transcription unit is
complemented by a similar modification of polymerase I (or the transcription factor) DNA binding site, the factor from different species won't be able to induce
transcription of the ribosomal gene. This has been shown in in vitro transcription
systems by Grummt et al. (1982).
Another model system of nucleolar dominance is Drosophila melanogasterlD.
simulans hybrids (Durica and Krider 1977). Regulatory elements of nucleolar dominance (ND) have been identified outside the nucleolar organizer region (NOR =
bobbed gene) itself. By using chromosome aberrations Durica and Krider (1978)
could map the NDs very precisely. Several elements which regulate ribosomal gene
activity transcriptionally or replicationally have been characterized in D.
melanogaster (Procunier and Tartof 1978; Goodrich-Young and Krider 1989), and
it is intriguing to know whether any element(s) represent ND. Because low production of rRNA causes the bobbed phenotype or inviability in severe cases, nucleolar
dominance-mediated postzygotic isolation is plausible. In the cross between D.
melanogaster females and D. simulans males, hybrid males are inviable at the late
larval stage (Sturtevant 1920), and can be rescued by a mutation of D. simulans,
Lethal hybrid rescue (Lhr) (Watanabe 1979). If a D. melanogaster strain carrying
an X-linked defect of ribosomal gene regulation is employed, the hybrid males
cannot be rescued by Lhr because thc Y chromosome of D. simulans origin lacks
rDNA (Granadino et al. 1996). But it is not clear at the moment whether nucleolar
dominance is directly involved in the hybrid male inviability. Apparent examples
of nucleolar dominance-mediated hybrid inferiority are D. mullerilD. arizonae hybrids (Bicudo and Richardson 1977, 1978), D. hydeilD. neohydei hybrids (Schiifer
1979), and inter-subspecific European meadow grasshopper (Chorthippus parallelus)
hybrids (Bella et al. 1990; Virdec and Hewitt 1992).
2.5 Homeotic Transformation
Homeotic transformation in interspecific hybrids is somewhat anecdotal,e.g., flour
beetle (Tribolium) hybrids (Wade and Johnson 1994; Wade et al. 1994) and Drosophila virilis/D. lummei hybrids (Heikkinen 1991). A well-documented case is the
character of extra sex combs seen in hybrids between D. madeirensis females and
D. subobscura males. Males of the pure species have sex combs only on the first
pair of legs, but the hybrid males have extra sex combs on the second and the third
pairs of legs. The character is similar to several homeotic mutations of D.
melanogaster, e. g., extra sex combs (Slifer 1942). This character is caused by
multi-genes, not by a single major homeotic gene (Papaceit et a1. 1991; Khadem
and Krimbas 1991, 1993, 1997). The genes might epistatically interact with some
homeotic genes. It is well-known in a homeotic gene, Ultrabithorax, that such
"modifiers" exist all over the genome of D. melanogaster (Gibson and Hogness
1996; Gibson and van HeIden 1997).
K. Sawamura
1984). In fact, no obvious shared consensus sequence has been recognized in the
region of transcription initiation of ribosomal gene from different species of eukaryotes. If the change in a single promotor sequence of this transcription unit is
complemented by a similar modification of polymerase I (or the transcription factor) DNA binding site, the factor from different species won't be able to induce
transcription of the ribosomal gene. This has been shown in in vitro transcription
systems by Grummt et al. (1982).
Another model system of nucleolar dominance is Drosophila melanogasterlD.
simulans hybrids (Durica and Krider 1977). Regulatory elements of nucleolar dominance (ND) have been identified outside the nucleolar organizer region (NOR =
bobbed gene) itself. By using chromosome aberrations Durica and Krider (1978)
could map the NDs very precisely. Several elements which regulate ribosomal gene
activity transcriptionally or replicationally have been characterized in D.
melanogaster (Procunier and Tartof 1978; Goodrich-Young and Krider 1989), and
it is intriguing to know whether any element(s) represent ND. Because low production of rRNA causes the bobbed phenotype or inviability in severe cases, nucleolar
dominance-mediated postzygotic isolation is plausible. In the cross between D.
melanogaster females and D. simulans males, hybrid males are inviable at the late
larval stage (Sturtevant 1920), and can be rescued by a mutation of D. simulans,
Lethal hybrid rescue (Lhr) (Watanabe 1979). If a D. melanogaster strain carrying
an X-linked defect of ribosomal gene regulation is employed, the hybrid males
cannot be rescued by Lhr because thc Y chromosome of D. simulans origin lacks
rDNA (Granadino et al. 1996). But it is not clear at the moment whether nucleolar
dominance is directly involved in the hybrid male inviability. Apparent examples
of nucleolar dominance-mediated hybrid inferiority are D. mullerilD. arizonae hybrids (Bicudo and Richardson 1977, 1978), D. hydeilD. neohydei hybrids (Schiifer
1979), and inter-subspecific European meadow grasshopper (Chorthippus parallelus)
hybrids (Bella et al. 1990; Virdec and Hewitt 1992).
2.5 Homeotic Transformation
Homeotic transformation in interspecific hybrids is somewhat anecdotal,e.g., flour
beetle (Tribolium) hybrids (Wade and Johnson 1994; Wade et al. 1994) and Drosophila virilis/D. lummei hybrids (Heikkinen 1991). A well-documented case is the
character of extra sex combs seen in hybrids between D. madeirensis females and
D. subobscura males. Males of the pure species have sex combs only on the first
pair of legs, but the hybrid males have extra sex combs on the second and the third
pairs of legs. The character is similar to several homeotic mutations of D.
melanogaster, e. g., extra sex combs (Slifer 1942). This character is caused by
multi-genes, not by a single major homeotic gene (Papaceit et a1. 1991; Khadem
and Krimbas 1991, 1993, 1997). The genes might epistatically interact with some
homeotic genes. It is well-known in a homeotic gene, Ultrabithorax, that such
"modifiers" exist all over the genome of D. melanogaster (Gibson and Hogness
1996; Gibson and van HeIden 1997).