250
M. Hasebe and M. Ito
When did the specification of mRNA expression evolve? A recent report on
gymnosperm MADS genes in Pinus radiata (Mouradov et al. 1998a) gives a partial
answer to the question. A female strobilus, a pine cone, is composed of numerous
modified shoots. The shoot comprises two ovules, an ovuliferous scale, and a bract.
The ovule is covered with an integument. A male strobilus is made of microsporophylls bearing microsporangia. The PrMADSI gene of the AGL2 group and the
PrMADS2 gene of the AGL6 group were not expressed in any vegetative organs
examined, but similarly expressed in the primordia of ovules and ovuliferous scales
in the seed cone and sporogenous cells in the pollen cone (Mouradov et al. 1998a).
This means that specification of MADS gene expression into reproductive organs
occurred in gymnosperms. Therefore, the recruitment of MADS genes as reproductive-organ specific genes took place in the common ancestor of seed plants or in
parallel in gymnosperm and angiosperm lineages.
5 A-, B- and C-Function Genes Already
Diversified in the Common Ancestor of
Gymnosperms and Angiosperms
Gymnosperms have more complex reproductive organs than ferns, but simpler organs than in angiosperms (Fig. 1). The common ancestor of gymnosperms and
angiosperms diverged from ferns 400 million years ago, and gymnosperms and
angiosperms diverged from each other 200 million years ago. Recently, more than
ten MADS genes were cloned from conifers (Mouradov et al. 1998a), Ginkgo
(Mikami and Ito, unpublished), and Gnetum (Shin do and Hasebe, unpublished). In
the MADS gene tree (Fig. 2), the AGL2, AGL6, AGL14, and AG groups include
both angiosperm and gymnosperm MADS genes. In other words, these groups had
already diverged in the most recent common ancestor of gymnosperms and angiosperms (Fig. 3). Monophyly of the AGL2, AGL6, and API groups is supported
with a high bootstrap value (91 %) and all fern MADS genes cluster outside of the
AGL2-AGL6-API clade, suggesting that the common ancestor of gymnosperms
and angiosperms is also likely to have had the ancestral gene of the API group.
This means that the orthologous gene of the API clade is likely to have been lost in
the gymnosperm lineage, although it is possible that gymnosperm API orthologs
may be found by conducting further studies. The sister relationship between PI and
AP3 groups is supported with moderately high bootstrap values (66%). The divergence time of these two groups is likely to be subsequent to the divergence of gymnosperm and angiosperm lineages, because one gene cloned from Ginkgo clustered
basal to AP3 and PI groups with moderately high bootstrap values (M. Ito, data not
shown). Other angiosperm MADS groups (AGL12, AGL15, AGL17) clustered
outside of the AGL2-AGL6-API and AP3-PI clades but the branching order among
these groups and other MADS groups is not well resolved with any statistical confidence. A study using the molecular clock (Purugganan et al. 1995) suggests that
M. Hasebe and M. Ito
When did the specification of mRNA expression evolve? A recent report on
gymnosperm MADS genes in Pinus radiata (Mouradov et al. 1998a) gives a partial
answer to the question. A female strobilus, a pine cone, is composed of numerous
modified shoots. The shoot comprises two ovules, an ovuliferous scale, and a bract.
The ovule is covered with an integument. A male strobilus is made of microsporophylls bearing microsporangia. The PrMADSI gene of the AGL2 group and the
PrMADS2 gene of the AGL6 group were not expressed in any vegetative organs
examined, but similarly expressed in the primordia of ovules and ovuliferous scales
in the seed cone and sporogenous cells in the pollen cone (Mouradov et al. 1998a).
This means that specification of MADS gene expression into reproductive organs
occurred in gymnosperms. Therefore, the recruitment of MADS genes as reproductive-organ specific genes took place in the common ancestor of seed plants or in
parallel in gymnosperm and angiosperm lineages.
5 A-, B- and C-Function Genes Already
Diversified in the Common Ancestor of
Gymnosperms and Angiosperms
Gymnosperms have more complex reproductive organs than ferns, but simpler organs than in angiosperms (Fig. 1). The common ancestor of gymnosperms and
angiosperms diverged from ferns 400 million years ago, and gymnosperms and
angiosperms diverged from each other 200 million years ago. Recently, more than
ten MADS genes were cloned from conifers (Mouradov et al. 1998a), Ginkgo
(Mikami and Ito, unpublished), and Gnetum (Shin do and Hasebe, unpublished). In
the MADS gene tree (Fig. 2), the AGL2, AGL6, AGL14, and AG groups include
both angiosperm and gymnosperm MADS genes. In other words, these groups had
already diverged in the most recent common ancestor of gymnosperms and angiosperms (Fig. 3). Monophyly of the AGL2, AGL6, and API groups is supported
with a high bootstrap value (91 %) and all fern MADS genes cluster outside of the
AGL2-AGL6-API clade, suggesting that the common ancestor of gymnosperms
and angiosperms is also likely to have had the ancestral gene of the API group.
This means that the orthologous gene of the API clade is likely to have been lost in
the gymnosperm lineage, although it is possible that gymnosperm API orthologs
may be found by conducting further studies. The sister relationship between PI and
AP3 groups is supported with moderately high bootstrap values (66%). The divergence time of these two groups is likely to be subsequent to the divergence of gymnosperm and angiosperm lineages, because one gene cloned from Ginkgo clustered
basal to AP3 and PI groups with moderately high bootstrap values (M. Ito, data not
shown). Other angiosperm MADS groups (AGL12, AGL15, AGL17) clustered
outside of the AGL2-AGL6-API and AP3-PI clades but the branching order among
these groups and other MADS groups is not well resolved with any statistical confidence. A study using the molecular clock (Purugganan et al. 1995) suggests that