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M. Hasebe and M. Ito
same down-stream genes in Arabidopsis as the angiosperm C-function gene can. If
the gymnosperm orthologs of down-regulated genes of angiosperm C-function genes
are coded in the gymnosperm genome, and if the gymnosperm AG orthologs can
regulate the down-stream orthologs in gymnosperms, it is highly probable that the
gene cascade regulated by the C-function gene was established in the common
ancestor of angiosperms and living gymnosperms.
As for the latter possibility, recent studies give us some insights. The A-, B- and
C-function MADS genes are positively regulated by the LEAFY (LFY) gene in the
floral meristem of Arabidopsis (Weigel and Meyerowitz 1993). The interactions
among MADS genes, LFY, and some additional genes characterize the patterns of
expression of the A-, B- and C-function genes in each whorl (Parcy et al. 1998).
The A-function gene APl is directly induced by LFY in all four of the whorls and
repressed in the inner two whorls by the C-function gene AG. Consequently, APl
expression is confined to the outer two whorls. The B-function gene is likely to be
activated by the combination of LFY and other factors, such as the UNUSUAL
FLOWER (UFO) gene. While the LFY gene is ubiquitously expressed in all four
whorls, the UFO expression is restricted to the second and third whorls, suggesting
that UFO regulates the region where B-function genes are expressed. The activation of the C function-gene AG in the inner two whorls seems to rely on a combination of LFY and other unknown factors.
LFY orthologs have been reported from some gymnosperms (Frohlich and
Meyerowitz 1997; Mouradov et al. 1998b; Mellerowicz et al. 1998). The gymnosperm LFY genes show global sequence similarity to LFY and its angiosperm
orthologs, although the gymnosperm LFY genes lack the proline-rich and acidic
motifs well conserved in angiosperm orthologs. The transformation of Pinus radiata LFY gene, NEEDLY (NL Y), connected to the Arabidopsis LFY promoter could
complement the Arabidopsis flY mutant which has lost the LFY function (Mouradov
et al. 1998b), indicating that the function of LFY orthologs is conserved between
Arabidopsis and Pinus. The over-drive phenotypes of NLYin Arabidopsis also mimicked those of LFY in Arabidopsis, supporting the conservation of their functions
between the two taxa (Mouradov et al. 1998b). Therefore, LFY orthologs are not
likely to be related to the evolution of region-specific expression of MADS genes in
the angiosperm lineage. If so, interactions between A- and C-function genes, or
regulation of B-function gene by UFO, or regulation of C-function genes by unknown genes have possibly evolved in the angiosperm lineage. The gymnosperm
C-function gene is likely to have similar functions to the angiosperm C-function
genes, because transformed Arabidopsis with ectopic expression of the conifer AG
ortholog mimics the phenotype of the transformed Arabidopsis ectopically expressing angiosperm AG (Rutledge et al. 1998; Tandre et al. 1998). This suggests that
the present function of angiosperm C-function genes was already established in the
most recent common ancestor of angiosperms and living gymnosperms. The UFO
orthologs have not been reported from gymnosperms, and the future characterization of these genes should be informative in revealing the genetic modifications
related to the evolution of flowers.
M. Hasebe and M. Ito
same down-stream genes in Arabidopsis as the angiosperm C-function gene can. If
the gymnosperm orthologs of down-regulated genes of angiosperm C-function genes
are coded in the gymnosperm genome, and if the gymnosperm AG orthologs can
regulate the down-stream orthologs in gymnosperms, it is highly probable that the
gene cascade regulated by the C-function gene was established in the common
ancestor of angiosperms and living gymnosperms.
As for the latter possibility, recent studies give us some insights. The A-, B- and
C-function MADS genes are positively regulated by the LEAFY (LFY) gene in the
floral meristem of Arabidopsis (Weigel and Meyerowitz 1993). The interactions
among MADS genes, LFY, and some additional genes characterize the patterns of
expression of the A-, B- and C-function genes in each whorl (Parcy et al. 1998).
The A-function gene APl is directly induced by LFY in all four of the whorls and
repressed in the inner two whorls by the C-function gene AG. Consequently, APl
expression is confined to the outer two whorls. The B-function gene is likely to be
activated by the combination of LFY and other factors, such as the UNUSUAL
FLOWER (UFO) gene. While the LFY gene is ubiquitously expressed in all four
whorls, the UFO expression is restricted to the second and third whorls, suggesting
that UFO regulates the region where B-function genes are expressed. The activation of the C function-gene AG in the inner two whorls seems to rely on a combination of LFY and other unknown factors.
LFY orthologs have been reported from some gymnosperms (Frohlich and
Meyerowitz 1997; Mouradov et al. 1998b; Mellerowicz et al. 1998). The gymnosperm LFY genes show global sequence similarity to LFY and its angiosperm
orthologs, although the gymnosperm LFY genes lack the proline-rich and acidic
motifs well conserved in angiosperm orthologs. The transformation of Pinus radiata LFY gene, NEEDLY (NL Y), connected to the Arabidopsis LFY promoter could
complement the Arabidopsis flY mutant which has lost the LFY function (Mouradov
et al. 1998b), indicating that the function of LFY orthologs is conserved between
Arabidopsis and Pinus. The over-drive phenotypes of NLYin Arabidopsis also mimicked those of LFY in Arabidopsis, supporting the conservation of their functions
between the two taxa (Mouradov et al. 1998b). Therefore, LFY orthologs are not
likely to be related to the evolution of region-specific expression of MADS genes in
the angiosperm lineage. If so, interactions between A- and C-function genes, or
regulation of B-function gene by UFO, or regulation of C-function genes by unknown genes have possibly evolved in the angiosperm lineage. The gymnosperm
C-function gene is likely to have similar functions to the angiosperm C-function
genes, because transformed Arabidopsis with ectopic expression of the conifer AG
ortholog mimics the phenotype of the transformed Arabidopsis ectopically expressing angiosperm AG (Rutledge et al. 1998; Tandre et al. 1998). This suggests that
the present function of angiosperm C-function genes was already established in the
most recent common ancestor of angiosperms and living gymnosperms. The UFO
orthologs have not been reported from gymnosperms, and the future characterization of these genes should be informative in revealing the genetic modifications
related to the evolution of flowers.