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S. Gleissberg et al.
perianthia, in Arabidopsis which alters symmetry of the flower and also changes
floral organ number. The perianthia mutant has flowers that are radially symmetrical, with five sepals, petals, and stamens, and two carpels. This pentamerous pattern resembles flowers of other dicot species, i.e, Prunus and Vinca. Genetic analysis shows that PERIANTHIA acts downstream of floral meristem identity genes, but
independent from flower organ identity genes and meristem size genes such as
CLA VATA, FASCIATA and REVOLUTA. It will be interesting to see how bilateral
symmetry in the Brassicaceae evolved from pentamerous flowers and the role genes
like PERIANTHIA may have played in this process.
11 Asymmetry in Other Organisms
The establishment of asymmetry has been shown to be important for later developmental processes in animals, too. Dorsoventrality has been well studied in Drosophila. Wing development in Drosophila and leaf development in plants share several similarities. The wing imaginal disc in Drosophila is symmetrical at the early
stage of development. However, this imaginal disc establishes clear dorsoventrality
in later stages, producing an expanded wing along the region where dorsal and
ventral cell fates encounter each other. This situation is very similar to that seen
during leaf development in plants because a symmetric leaf primordium acquires
ab-adaxiality, leading to the formation of leaf laminae along the region where abaxial
and adaxial cells meet. The expression of APTEROUS (a LIM homeodomaill protein) first establishes dorsoventrality in the Drosophila wing imaginal disc (Cohen
et al. 1992). The role of APTEROUS in establishing dorsal cell fate is similar to
that of PHANTASTICA in establishing adaxial cell fate in a leaf primordium. APTEROUS activity in the dorsal region activates FRINGE and SERRATE in the dorsal
region. In the ventral region of the imaginal disc, DELTA is activated by SERRATE
and APTEROUS. The activity of FRINGE on one hand to activate DELTA which in
turn activates NOTCH in the dorsal region, and on the other hand to activate SERRATE which in turn activates NOTCH in the ventral region, establishes NOTCH
expression in the boundary between the dorsal and ventral region (Klein and Arias
1998). Also, a mutual feedback regulation of NOTCH and SERRATE (or DELTA)
exaggerates the NOTCH expression along the region where dorsal and ventral cell
fates meet (Pannin et al. 1997). This NOTCH expression is narrowed down into
sharp small area, usually, 2-3 layers of cells by NUBBIN (Neumann and Cohen
1998). At a later stage, NOTCH activates WINGLESS and VESTIGIAL, leading to
formation of the wing structure in the boundary region (Neumann and Cohen 1996).
Thus, in the wing imaginal disc of Drosophila, a cascade of genes sets up a
dorsal and ventral boundary. Activation of specific genes in this boundary region
leads to wing formation. Are there any parallels between the molecular controls
involved in wing development in insects and leaf blade formation in plants? The
fact that multiple non-allelic loci in maize and tomato can lead to similar phenotypes with radially symmetric leaves suggests that a gene expression cascade may
S. Gleissberg et al.
perianthia, in Arabidopsis which alters symmetry of the flower and also changes
floral organ number. The perianthia mutant has flowers that are radially symmetrical, with five sepals, petals, and stamens, and two carpels. This pentamerous pattern resembles flowers of other dicot species, i.e, Prunus and Vinca. Genetic analysis shows that PERIANTHIA acts downstream of floral meristem identity genes, but
independent from flower organ identity genes and meristem size genes such as
CLA VATA, FASCIATA and REVOLUTA. It will be interesting to see how bilateral
symmetry in the Brassicaceae evolved from pentamerous flowers and the role genes
like PERIANTHIA may have played in this process.
11 Asymmetry in Other Organisms
The establishment of asymmetry has been shown to be important for later developmental processes in animals, too. Dorsoventrality has been well studied in Drosophila. Wing development in Drosophila and leaf development in plants share several similarities. The wing imaginal disc in Drosophila is symmetrical at the early
stage of development. However, this imaginal disc establishes clear dorsoventrality
in later stages, producing an expanded wing along the region where dorsal and
ventral cell fates encounter each other. This situation is very similar to that seen
during leaf development in plants because a symmetric leaf primordium acquires
ab-adaxiality, leading to the formation of leaf laminae along the region where abaxial
and adaxial cells meet. The expression of APTEROUS (a LIM homeodomaill protein) first establishes dorsoventrality in the Drosophila wing imaginal disc (Cohen
et al. 1992). The role of APTEROUS in establishing dorsal cell fate is similar to
that of PHANTASTICA in establishing adaxial cell fate in a leaf primordium. APTEROUS activity in the dorsal region activates FRINGE and SERRATE in the dorsal
region. In the ventral region of the imaginal disc, DELTA is activated by SERRATE
and APTEROUS. The activity of FRINGE on one hand to activate DELTA which in
turn activates NOTCH in the dorsal region, and on the other hand to activate SERRATE which in turn activates NOTCH in the ventral region, establishes NOTCH
expression in the boundary between the dorsal and ventral region (Klein and Arias
1998). Also, a mutual feedback regulation of NOTCH and SERRATE (or DELTA)
exaggerates the NOTCH expression along the region where dorsal and ventral cell
fates meet (Pannin et al. 1997). This NOTCH expression is narrowed down into
sharp small area, usually, 2-3 layers of cells by NUBBIN (Neumann and Cohen
1998). At a later stage, NOTCH activates WINGLESS and VESTIGIAL, leading to
formation of the wing structure in the boundary region (Neumann and Cohen 1996).
Thus, in the wing imaginal disc of Drosophila, a cascade of genes sets up a
dorsal and ventral boundary. Activation of specific genes in this boundary region
leads to wing formation. Are there any parallels between the molecular controls
involved in wing development in insects and leaf blade formation in plants? The
fact that multiple non-allelic loci in maize and tomato can lead to similar phenotypes with radially symmetric leaves suggests that a gene expression cascade may