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Fruit fly ( Drosophila melanogaster )
Abd-B
abd-A
Ubx
Antp
ftz
Scr
Dfd
bcd
zen
zen2
pb
lab
( Drosophila virilis )
Abd-B
abd-A
Ubx
Antp
ftz
Scr
Dfd
bcd
zen
zen2
pb
lab
?
?
?
?
( Drosophila buzzatti )
lab
Abd-B
abd-A
Ubx
Antp
ftz
Scr
Dfd
bcd
zen
zen2
pb
?
?
?
?
Amphioxus ( Branchiostoma floridae )
Hox15
Hox14
Hox13
Hox12
Hox11
Hox10
Hox9
Hox8
Hox7
Hox6
Hox5
Hox4
Hox3
Hox2
Hox1
Mouse ( Mus musculus )
Hoxa13
Hoxa11
Hoxa10
Hoxa9
Hoxa7
Hoxa6
Hoxa5
Hoxa4
Hoxa3
Hoxa2
Hoxa1
Hoxb13
Hoxb9
Hoxb8
Hoxb7
Hoxb6
Hoxb5
Hoxb4
Hoxb3
Hoxb2
Hoxb1
Hoxc13
Hoxc12
Hoxc11
Hoxc10
Hoxc9
Hoxc8
Hoxc6
Hoxc5
Hoxc4
Hoxd13
Hoxd12
Hoxd11
Hoxd10
Hoxd9
Hoxd8
Hoxd4
Hoxd3
Hoxd1
5 ¢
Figure 4.2 Schematic organization of Hox clusters in Drosophila, Amphioxus, and
mouse. Genes with a known transcriptional orientation are indicated with arrows
and genes with an unknown orientation as boxes. Orthologous genes are depicted with
the same filling. Drosophila homeobox-containing genes that are not considered as
genuine Hox genes are indicated with unfilled boxes. Genes whose location has not
yet been reported are indicated with question marks. The transcriptional directionality
of the clusters is indicated below.
3 ¢
For example, the incorrect activity of the Antennapedia gene leads to the sub-
stitution of antennas by legs ( Gehring, 1966; Schneuwly, Klemenz, &
Gehring, 1987; Struhl, 1981 ). Such homeotic transformations have been
observed whenever Drosophila and vertebrate Hox genes were inappropri-
ately activated, thus illustrating their role in organizing segmental identities
along the developing AP axis (e.g., Chisaka & Capecchi, 1991; Kessel,
Balling, & Gruss, 1990; Morata, Botas, Kerridge, & Struhl, 1983 ).
Hox genes provide these regional identities via their discrete patterns of
sustained functional activities ( Fig. 4.1 ). This is achieved either by domain-
specific transcriptional controls, like stripes in Drosophila (e.g., Akam &
Martinez-Arias, 1985; Gaunt et al., 1988; Kosman et al., 2004; Levine,
Hafen, Garber, & Gehring, 1983 ), or by using the functional dominance
of some posterior HOX proteins over more anterior ones, a feature that
turns largely overlapping expression territories into restricted functional
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