248
M. Hasebe and M. Ito
3 Divergence Time of MADS Genes
The MADS genes with the K box have been cloned from angiosperms, gymnosperms, ferns, and a moss (Purugganan et al. 1995; Theissen et al. 1996; Hasebe
and Banks 1997; Purugganan 1997; Munster et al. 1997; Hasebe et al. 1998; Mikami
and Ito, unpublished; Kofuji and Hasebe, unpublished). The gene tree of representative plant MADS genes is shown in Fig. 2. Six clades (AGL2, AGL6, AGL14,
and AG groups) include both angiosperm and gymnosperm MADS genes, indicating that these clades were already diverged from other clades when angiosperms
and living gymnosperms diverged. The relationships between fern, moss, and seed
plant clades were not well resolved with any statistical confidence, and the divergence time among these clades could not be estimated based on the tree topology.
Purugganan et al. (1995) postulated the monocot/dicot divergence date as 135 million years ago and calculated the evolutionary rate of MADS genes. The evolutionary clock was used to infer the "average" divergence time between the seed plant
MADS gene clades as about 340 million years ago (Purugganan et al. 1995). As
ferns and seed plants are estimated to have diverged approximately 400 million
years ago based on paleobotanical studies (Stewart and Rothwell 1993), the number
of seed plant MADS genes is likely to have increased after the divergence of ferns
and seed plants due to gene duplication in the common ancestor of seed plants.
Recently, Purugganan (1997) used more data than in her previous study (Purugganan
et al. 1995) and estimated the time of divergence between the AG clade and the
clade including AGL2, AGL6, and API (the AGL2-AGL6-AP1 clade) to be approximately 480 million years ago, which is before the time of divergence between
ferns and seed plants. Most genes in the AG and CRM6 clades encode proteins
with additional amino-terminal amino acids to the MADS domain, while the initiation codons of other MADS genes are located adjacent to the 5' end of the sequences encoding the MADS domain (Hasebe and Banks 1997). Therefore, the AG
and CRM6 groups are likely to form a sister relationship (Hasebe et al. 1998),
supporting the estimate by Purugganan (1997). This suggests that the
Trimerophytophyta, the common ancestor of ferns and seed plants, had at least two
MADS genes (ancestral genes to the AGL2-AGL6-API and AG clades), and that
other seed plant MADS genes later diverged from these genes around 340 million
years ago (Fig. 3).
4 Recruitment of Ancestral MADS Genes for
Reproductive Organ Development
Most of the angiosperm MADS gene groups have their own patterns of expression
and functions. For example, the API, AP3, PI, and AG groups in Fig. 2 correspond
to the A-, B- and C-function genes of floral organ development, and they are expressed in specific floral organ primordia. On the other hand, most of the fern
M. Hasebe and M. Ito
3 Divergence Time of MADS Genes
The MADS genes with the K box have been cloned from angiosperms, gymnosperms, ferns, and a moss (Purugganan et al. 1995; Theissen et al. 1996; Hasebe
and Banks 1997; Purugganan 1997; Munster et al. 1997; Hasebe et al. 1998; Mikami
and Ito, unpublished; Kofuji and Hasebe, unpublished). The gene tree of representative plant MADS genes is shown in Fig. 2. Six clades (AGL2, AGL6, AGL14,
and AG groups) include both angiosperm and gymnosperm MADS genes, indicating that these clades were already diverged from other clades when angiosperms
and living gymnosperms diverged. The relationships between fern, moss, and seed
plant clades were not well resolved with any statistical confidence, and the divergence time among these clades could not be estimated based on the tree topology.
Purugganan et al. (1995) postulated the monocot/dicot divergence date as 135 million years ago and calculated the evolutionary rate of MADS genes. The evolutionary clock was used to infer the "average" divergence time between the seed plant
MADS gene clades as about 340 million years ago (Purugganan et al. 1995). As
ferns and seed plants are estimated to have diverged approximately 400 million
years ago based on paleobotanical studies (Stewart and Rothwell 1993), the number
of seed plant MADS genes is likely to have increased after the divergence of ferns
and seed plants due to gene duplication in the common ancestor of seed plants.
Recently, Purugganan (1997) used more data than in her previous study (Purugganan
et al. 1995) and estimated the time of divergence between the AG clade and the
clade including AGL2, AGL6, and API (the AGL2-AGL6-AP1 clade) to be approximately 480 million years ago, which is before the time of divergence between
ferns and seed plants. Most genes in the AG and CRM6 clades encode proteins
with additional amino-terminal amino acids to the MADS domain, while the initiation codons of other MADS genes are located adjacent to the 5' end of the sequences encoding the MADS domain (Hasebe and Banks 1997). Therefore, the AG
and CRM6 groups are likely to form a sister relationship (Hasebe et al. 1998),
supporting the estimate by Purugganan (1997). This suggests that the
Trimerophytophyta, the common ancestor of ferns and seed plants, had at least two
MADS genes (ancestral genes to the AGL2-AGL6-API and AG clades), and that
other seed plant MADS genes later diverged from these genes around 340 million
years ago (Fig. 3).
4 Recruitment of Ancestral MADS Genes for
Reproductive Organ Development
Most of the angiosperm MADS gene groups have their own patterns of expression
and functions. For example, the API, AP3, PI, and AG groups in Fig. 2 correspond
to the A-, B- and C-function genes of floral organ development, and they are expressed in specific floral organ primordia. On the other hand, most of the fern